One document matched: draft-rfced-info-kane-00.txt
INTERNET-DRAFT EXPIRES NOVEMBER 1997 INTERNET-DRAFT
Network Working Group L. Kane
INTERNET-DRAFT K. Dobbins
Category: Informational R. Soczewinski
Cabletron Systems Incorporated
May 1997
VLS Protocol Specification
<draft-rfced-info-kane-00.txt>
Status of this Memo
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Abstract
The Virtual LAN Link State Protocol (VLSP) is part of the
InterSwitch Message Protocol (ISMP). ISMP was designed to
facilitate interswitch communication within distributed
connection-oriented switching networks. VLSP is used to
determine and maintain a fully connected mesh topology graph of
the switch fabric. Each switch maintains an identical database
describing the topology. Call-originating switches use the
topology database to determine the path over which to route a
call connection.
VLSP provides support for equal-cost multipath routing, and
recalculates routes quickly in the face of topological changes,
utilizing a minimum of routing protocol traffic.
Table of Contents
Status of this Memo 1
Abstract 1
1. Introduction 3
1.1 Acknowledgments 3
1.2 Data Conventions 4
2. ISMP Overview 4
3. General ISMP Packet Format 5
3.1 Frame Header 5
3.2 ISMP Packet Header 6
3.3 ISMP Message Body 7
4. VLS Protocol Overview 8
4.1 Definitions of Commonly Used Terms 8
4.2 Differences Between VLSP and OSPF 10
4.2.1 Operation at the Physical Layer 10
4.2.2 All Links Treated as Broadcast 10
4.2.3 Routing Path Information 11
4.2.4 Configurable Parameters 11
4.2.5 Features Not Supported 11
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4.3 Functional Summary 12
4.3.1 Discovery Process 12
4.3.2 Synchronizing the Databases 12
4.3.3 Maintaining the Databases 12
4.3.4 Calculating the Routing Table 12
4.4 Protocol Packets 13
4.5 Protocol Data Structures 14
4.6 Basic Implementation Requirements 14
4.7 Organization of the Remainder of This Document 15
5. Interface Data Structure 15
5.1 Interface States 18
5.2 Events Causing Interface State Changes 20
5.3 Interface State Machine 22
6. Neighbor Data Structure 24
6.1 Neighbor States 26
6.2 Events Causing Neighbor State Changes 29
6.3 Neighbor State Machine 31
7. Area Data Structure 34
7.1 Adding and Deleting Link State Advertisements 35
7.2 Accessing Link State Advertisements 36
8. Routing Table 36
8.1 Routing Table Lookup 37
9. Discovery Process 38
9.1 Hello Packets 38
9.2 Bidirectional Communication 39
9.3 Designated Switch 39
9.3.1 Selecting the Designated Switch 40
9.4 Adjacencies 43
10. Synchronizing the Databases 44
10.1 Link State Advertisements 44
10.1.1 Determining Which Link State
Advertisement Is Newer 45
10.2 Database Exchange Process 46
10.2.1 Database Description Packets 46
10.2.2 Negotiating the Master/Slave Relationship 47
10.2.3 Exchanging Database Description Packets 48
10.3 Updating the Database 50
10.4 An Example 51
11. Maintaining the Databases 53
11.1 Originating Link State Advertisements 53
11.1.1 Switch Link Advertisements 54
11.1.2 Network Link Advertisements 57
11.2 Distributing Link State Advertisements 58
11.2.1 Overview 58
11.2.2 Processing an Incoming Link State
Update Packet 60
11.2.3 Forwarding a Link State Update Packet 62
11.2.4 Installing Link State Advertisements
in the Database 64
11.2.5 Retransmitting Link State Advertisements 64
11.2.6 Acknowledging Link State Advertisements 65
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11.3 Aging the Link State Database 67
11.3.1 Premature Aging of Advertisements 68
12. Calculating the Routing Table 68
13. Protocol Packets 69
13.1 Packet Processing 70
13.2 Network Layer Address Information 70
13.3 VLSP Packet Header 72
13.4 Options Field 74
13.5 Packet Formats 75
13.5.1 Hello Packets 75
13.5.2 Database Description Packets 77
13.5.3 Link State Request Packets 79
13.5.4 Link State Update Packets 80
13.5.5 Link State Acknowledgment Packets 81
14. Link State Advertisement Formats 82
14.1 Link State Advertisement Headers 82
14.2 Switch Link Advertisements 85
14.3 Network Link Advertisements 87
15. Protocol Parameters 88
15.1 Architectural Constants 88
15.2 Configurable Parameters 89
Footnotes 91
References 92
Security Considerations 92
Authors Addresses 92
1. Introduction
This memo is being distributed to members of the Internet
community in order to solicit reactions to the proposals
contained herein. While the specification discussed here may
not be directly relevant to the research problems of the
Internet, it may be of interest to researchers and
implementers.
1.1 Acknowledgments
VLSP is derived from the OSPF link-state routing protocol
described in [RFC1583], written by John Moy, formerly of
Proteon, Inc., Westborough, Massachusetts. Much of the current
RFC has been drawn from [RFC1583]. Therefore, this author
wishes to acknowledge the contribution Mr. Moy has
(unknowingly) made to this document.
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1.2 Data Conventions
The methods used in this memo to describe and picture data
adhere to the standards of Internet Protocol documentation
[RFC1700]. In particular:
The convention in the documentation of Internet
Protocols is to express numbers in decimal and to
picture data in "big-endian" order. That is, fields
are described left to right, with the most
significant octet on the left and the least
significant octet on the right.
The order of transmission of the header and data
described in this document is resolved to the octet
level. Whenever a diagram shows a group of octets,
the order of transmission of those octets is the
normal order in which they are read in English.
Whenever an octet represents a numeric quantity the
left most bit in the diagram is the high order or
most significant bit. That is, the bit labeled 0 is
the most significant bit.
Similarly, whenever a multi-octet field represents a
numeric quantity the left most bit of the whole field
is the most significant bit. When a multi-octet
quantity is transmitted the most significant octet is
transmitted first.
2. ISMP Overview
The InterSwitch Message Protocol (ISMP) is used for interswitch
communication within distributed connection-oriented switching
networks. ISMP provides the following services:
- Topology services. Each switch maintains a distributed
topology of the switch fabric by exchanging the following
interswitch messages with other switches:
- Interswitch Keepalive messages (SNDM protocol) are sent by
each switch to announce its existence to its neighboring
switches and to establish the topology of the switch
fabric.
- Interswitch Spanning Tree BPDU messages and Interswitch
Remote Blocking messages (LSMP protocol) are used to
determine and maintain a loop-free flood path between all
network switches in the fabric. This flood path is used
for all undirected interswitch messages -- that is,
messages of the ARLD, SBCD, and SFCT protocols.
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- Interswitch Link State messages (VLS protocol) are used to
determine and maintain a fully connected mesh topology
graph of the switch fabric. Call-originating switches use
the topology graph to determine the path over which to
route a call connection.
- Address resolution services. Interswitch Resolve messages
(ARLD protocol) are used to resolve a packet destination
address when the packet source and destination pair does not
match a known connection. Interswitch New User messages
(also part of the ARLD protocol) are used to provide end-
station address mobility between switches.
- Tag-based flooding. A tag-based broadcast method (SBCD
protocol) is used to restrict the broadcast of unresolved
packets to only those ports within the fabric that belong to
the same VLAN as the source.
- Call tapping services. Interswitch Tap messages (SFCT
protocol) are used to monitor traffic moving between two end
stations. Traffic can be monitored in one or both
directions along the connection path.
NOTE
This document describes the VLS protocol.
Other ISMP protocols are described in other
RFCs. See the References section for a
list of these related RFCs.
3. General ISMP Packet Format
ISMP packets are of variable length and have the following
general structure:
- Frame header
- ISMP packet header
- ISMP message body
3.1 Frame Header
ISMP packets are encapsulated within an IEEE 802-compliant
frame using a standard header as shown below:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
+ Destination address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
04 | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Source address +
08 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 | Type | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
16 | |
+ +
: :
Destination address
This 6-octet field contains the Media Access Control (MAC)
address of the multicast channel over which all switches in
the fabric receive ISMP packets. The destination address of
all ISMP packets contain a value of 01-00-1D-00-00-00.
Source address
This 6-octet field contains the physical (MAC) address of
the switch originating the ISMP packet.
Type
This 2-octet field identifies the type of data carried
within the frame. The type field of ISMP packets contains
the value 0x81FD.
3.2 ISMP Packet Header
The ISMP packet header consists of 6 octets, as shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00
|///////////////////////////////////////////////////////////////|
://////// Frame header /////////////////////////////////////////:
+//////// (14 octets) /////////+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 |///////////////////////////////| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 | ISMP message type | Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
20 | |
+ +
: :
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Frame header
This 14-octet field contains the frame header.
Version
This 2-octet field contains the version number of the
InterSwitch Message Protocol to which this ISMP packet
adheres. This document describes ISMP Version 2.0.
ISMP message type
This 2-octet field contains a value indicating which type of
ISMP message is contained within the message body. Valid
values are as follows:
1 (reserved)
2 Interswitch Keepalive messages (SNDM protocol)
3 Interswitch Link State messages (VLS protocol)
4 Interswitch Spanning Tree BPDU messages and Remote
Blocking messages (LSMP protocol)
5 Interswitch Resolve and New User messages (ARLD
protocol)
6 (reserved)
7 Tag-Based Flood messages (SBCD protocol)
8 Interswitch Tap messages (SFCT protocol)
VLS protocol messages have a message type of 3.
Sequence number
This 2-octet field contains an internally generated sequence
number used by the various protocol handlers for internal
synchronization of messages.
3.3 ISMP Message Body
The ISMP message body is a variable-length field containing the
actual data of the ISMP message. The length and content of
this field are determined by the value found in the message
type field.
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4. VLS Protocol Overview
VLSP is a dynamic routing protocol. It quickly detects
topological changes in the switch fabric (such as, switch
interface failures) and calculates new loop-free routes after a
period of convergence. This period of convergence is short and
involves a minimum of routing traffic.
All switches in the fabric run the same algorithm and maintain
identical databases describing the switch fabric topology.
This database contains each switchs local state, including its
usable interfaces and reachable neighbors. Each switch
distributes its local state throughout the switch fabric by
flooding. From the topological database, each switch
constructs a set of best path trees (using itself as the root)
that specify routes to all other switches in the fabric.
4.1 Definitions of Commonly Used Terms
This section contains a collection of definitions for terms
that have a specific meaning to the protocol and that are used
throughout the text.
Switch ID
A 10-octet value that uniquely identifies the switch within
the switch fabric. The value consists of the 6-octet base
MAC address of the switch, followed by 4 octets of zeroes.
Network link
The physical connection between two switches. A link is
associated with a switch interface.
There are two physical types of network links supported by
VLSP:
- Point-to-point links that join a single pair of switches.
A serial line is an example of a point-to-point network
link.
- Multi-access broadcast links that support the attachment
of multiple switches, along with the capability to address
a single message to all the attached switches. An
attached ethernet is an example of a multi-access
broadcast network link.
A single topology can contain both types of links. Note,
however, the current version of VLSP treats all links as if
they were multi-access.
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Interface
The port over which a switch accesses one of its links.
Interfaces are identified by their interface ID, a 10-octet
value consisting of the 6-octet base MAC address of the
switch, followed by the 4-octet local port number of the
interface.
Neighboring switches
Two switches attached to a common link. Neighbors are
dynamically discovered by the Hello protocol.
Adjacency
A relationship formed between selected neighboring switches
for the purpose of exchanging routing information. Not
every pair of neighboring switches become adjacent.
Link state advertisement
Describes the local state of a switch or a link. Each link
state advertisement is flooded throughout the switch fabric.
The collected link state advertisements of all switches and
links form the protocol's topological database.
Hello protocol
That part of VLSP used to establish and maintain neighbor
relationships.
Designated switch
Each multi-access network link has a designated switch. The
designated switch generates a link state advertisement for
the link and has other special responsibilities in the
running of the protocol.
The use of a designated switch permits a reduction in the
number of adjacencies required on multi-access links. This
in turn reduces the amount of routing protocol traffic and
the size of the topological database.
The designated switch is elected by the Hello protocol. A
designated switch is not selected for a point-to-point
network link.
Backup designated switch
Each multi-access network link has a backup designated
switch. The backup designated switch maintains adjacencies
with the same switches on the link as the designated switch.
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This optimizes the failover time when the backup designated
switch must take over for the (failed) designated switch.
The backup designated switch is elected by the Hello
protocol. A backup designated switch is not selected for a
point-to-point network link.
4.2 Differences Between VLSP and OSPF
The VLS protocol is derived from the OSPF link-state routing
protocol described in [RFC1583].
4.2.1 Operation at the Physical Layer
The primary differences between the VLS and OSPF protocols stem
from the fact that OSPF runs over the IP layer, while VLSP runs
at the physical MAC layer. This difference has the following
repercussions:
- VLSP does not support features (such as fragmentation) that
are typically provided by network layer service providers.
- Due to the unrelated nature of MAC address assignments, VLSP
provides no summarization of the address space (such as,
classical IP subnet information) or level 2 routing (such
as, IS-IS Phase V DECnet). Thus, VLSP does not support
grouping switches into areas. All switches exist in a
single area. Since a single domain exists within any switch
fabric, there is no need for VLSP to provide interdomain
reachability.
- As mentioned in Section 3.1, ISMP uses a single well-known
multicast address for all packets. However, parts of the
VLS protocol (as derived from OSPF) are dependent on certain
network layer addresses -- in particular, the AllSPFSwitches
and AllDSwitches multicast addresses that drive the
distribution of link state advertisements throughout the
switch fabric. In order to facilitate the implementation of
the protocol at the physical MAC layer, network layer
address information is encapsulated in the protocol packets
(see Section 13.2). This information is unbundled and
packets are then processed as if they had been sent or
received on that multicast address.
4.2.2 All Links Treated as Broadcast
The current version of VLSP treats all network links as multi-
access broadcast media, regardless of whether the link is
actually point-to-point or multi-access.
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4.2.3 Routing Path Information
Instead of providing the next hop to a destination, VLSP
calculates and maintains complete end-to-end path information.
On request, a list of individual port identifiers is generated
describing a complete path from the source switch to the
destination switch. If multiple equal-cost routes exist to a
destination switch, up to three paths are calculated and
returned.
4.2.4 Configurable Parameters
OSPF supports (and requires) configurable parameters. In fact,
even the default OSPF configuration requires that IP address
assignments be specified. On the other hand, no configuration
information is ever required for the VLS protocol. Switches
are uniquely identified by their base MAC addresses and ports
are uniquely identified by the base MAC address of the switch
and a port number.
While a developer is free to implement configurable parameters
for the VLS protocol, the current version of VLSP supports
configurable path metrics only. Note that this has the
following repercussions:
- All switches have a switch priority of 1. This forces the
selection of the designated switch to be based solely on
base MAC address.
- Authentication is not supported.
4.2.5 Features Not supported
In addition to those features mentioned in the previous
sections, the following OSPF features are not supported by the
current version of VLSP:
- Periodic refresh of link state advertisements. (This
optimizes performance by eliminating unnecessary traffic
between the switches.)
- Routing based on non-zero type of service (TOS).
- Use of external routing information for destinations outside
the switch fabric.
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4.3 Functional Summary
There are essentially four operational stages of the VLS
protocol.
4.3.1 Discovery Process
When a switch comes on-line, it initializes its routing data
structures. It then waits for notification from the SNDM
protocol [RFCxxxx] that its interfaces are functional.
Once the switch learns that its interfaces are functional, it
uses the Hello protocol to dynamically discover its neighbors
by sending Hello packets over its outports and receiving Hello
packets back in return. The Hello protocol is also used to
select a designated switch for each multi-access network link.
The designated switch on each link determines which switches
will become adjacent.
4.3.2 Synchronizing the Databases
Adjacencies are used to simplify and speed up the process of
synchronizing the topological database (also known as the link
state database) maintained by each switch in the fabric. Each
switch is only required to synchronize its database with those
neighbors to which it is adjacent. This reduces the amount of
routing protocol traffic across the fabric, particularly for
multi-access links with multiple switches.
4.3.3 Maintaining the Databases
Each switch advertises its state (also known as its link state)
any time its link state changes. Link state advertisements are
distributed throughout the switch fabric using a reliable
flooding algorithm that ensures that all switches in the fabric
are notified of any link state changes.
4.3.4 Calculating the Routing Table
The link state database consists of the collection of link
state advertisements received from each switch. Each switch
uses its link state database to calculate a set of best paths,
using itself as root, to all other switches in the fabric.
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4.4 Protocol Packets
In addition to the frame header and the ISMP packet header
described in Section 3, all VLS protocol packets share a common
protocol header, described in Section 13.3.
The VLSP packet types are listed below in Table 1. Their
formats are described in Section 13.5.
Type Packet Name Protocol Function
1 Hello Discover/maintain neighbors
2 Database Description Summarize database contents
3 Link State Request Database download
4 Link State Update Database update
5 Link State Ack Flooding acknowledgment
Table 1: VLSP Packet Types
The Hello packets are used to discover and maintain neighbor
relationships. The Database Description and Link State Request
packets are used to form adjacencies. Link State Update and
Link State Acknowledgment packets are used to update the
topological database.
Each Link State Update packet carries a set of link state
advertisements. A single Link State Update packet may contain
the link state advertisements of several switches. There are
two different types of link state advertisement, as shown below
in Table 2.
LS Advertisement Advertisement Description
Type Name
1 Switch link Originated by all switches. This
advertisements advertisement describes the collected
states of the switch's interfaces.
2 Network link Originated by the designated switch.
advertisements This advertisement contains the list of
switches connected to the network link.
Table 2: VLSP Link State Advertisements
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4.5 Protocol Data Structures
The VLS protocol is described in this specification in terms of
its operation on various protocol data structures. Table 3
lists the primary VLSP data structures, along with the section
in which they are described in detail.
Structure Name Description
Interface Data Structure Section 5
Neighbor Data Structure Section 6
Area Data Structure Section 7
Routing Table Section 8
Table 3: VLSP Data Structures
4.6 Basic Implementation Requirements
An implementation of the VLS protocol requires the following
pieces of system support:
Timers
Two types of timer are required. The first type, known as a
one-shot timer, expires once and triggers an event. The
second type, known as an interval timer, expires at preset
intervals. Interval timers are used to trigger events at
periodic intervals. The granularity of both types of timers
is one second.
Interval timers should be implemented in such a way as to
avoid drift. In some switch implementations, packet
processing can affect timer execution. For example, on a
multi-access link with multiple switches, regular broadcasts
can lead to undesirable synchronization of routing packets
unless the interval timers have been implemented to avoid
drift. If it is not possible to implement drift-free
timers, small random amounts of time should be added to or
subtracted from the timer interval at each firing.
List manipulation primitives
Much of the functionality of VLSP is described here in terms
of its operation on lists of link state advertisements. Any
particular advertisement may be on many such lists.
Implementation of VLSP must be able to manipulate these
lists, adding and deleting constituent advertisements as
necessary.
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Tasking support
Certain procedures described in this specification invoke
other procedures. At times, these other procedures should
be executed in-line -- that is, before the current procedure
has finished. This is indicated in the text by instructions
to "execute" a procedure. At other times, the other
procedures are to be executed only when the current
procedure has finished. This is indicated by instructions
to "schedule" a task. Implementation of VLSP must provide
these two types of tasking support.
4.7 Organization of the Remainder of This Document
The remainder of this document is organized as follows:
- Section 5 through Section 8 describe the primary data
structures used by the protocol. Note that this
specification is presented in terms of these data structures
in order to make explanations more precise. Implementations
of the protocol must support the functionality described,
but need not use the exact data structures that appear in
this specification.
- Section 9 through Section 12 describe the four operational
stages of the protocol: the discovery process,
synchronizing the databases, maintaining the databases, and
calculating the routing table.
- Section 13 describes the processing of VLSP packets and
presents detailed descriptions of their formats.
- Section 14 presents detailed descriptions of link state
advertisements.
- Section 15 summarizes the protocol parameters.
5. Interface Data Structure
The port over which a switch accesses a network link is known
as the link interface. Each switch maintains a separate
interface data structure for each network link.
The following data items are associated with each interface:
Type
The type of network to which the interface is attached --
point-to-point or broadcast (multi-access). Note that the
current version of VLSP treats all links as broadcast media.
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State
The functional level of the interface. The state of the
interface is included in all switch link advertisements
generated by the switch, and is also used to determine
whether full adjacencies are allowed on the interface. See
Section 5.1 for a complete description of interface states.
Interface identifier
A 10-octet value that uniquely identifies the interface.
This value consists of the 6-octet base MAC address of the
neighbor switch, followed by the 4-octet local port number
of the interface.
Area ID
A 4-octet value identifying the area. Since VLSP does not
support multiple areas, the value here is always zero.
HelloInterval
The interval, in seconds, at which the switch sends Hello
packets over the interface.
SwitchDeadInterval
The length of time, in seconds, that neighboring switches
will wait before declaring the local switch down once they
stop receiving Hello packets from the local switch.
InfTransDelay
The estimated number of seconds it should take to transmit a
Link State Update packet over this interface. Link state
advertisements contained in the update packet will have
their age incremented by this amount before transmission.
This value must be greater than zero and must take into
account transmission and propagation delays.
Switch priority
An 8-bit unsigned integer. When two switches attached to
the same network link contend for selection as the
designated switch, the switch with the highest priority
takes precedence. If both switches have the same priority,
the switch with the highest base MAC address becomes the
designated switch. A switch whose switch priority is set to
zero is ineligible to become the designated switch on the
attached link.
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Hello timer
The interval timer used to regulate the transmission of
Hello packets over the interface. This timer expires every
HelloInterval seconds.
Wait timer
The one-shot timer used to time the Waiting state. When
this timer expires, the interface exits the Waiting state
and begins selection of the designated switch on the link.
The length of the timer is switchDeadInterval seconds.
Neighboring switches
A list of the neighboring switches attached to this network
link. This list is created by the Hello protocol.
Adjacencies are formed to one or more of these neighbors.
The set of adjacent neighbors can be determined by examining
the states of the neighboring switches as shown in their
link state advertisements.
Designated switch
The designated switch selected for the multi-access network
link. (A designated switch is not selected for a point-to-
point link.) This data item is initialized to zero when the
switch comes on-line, indicating that no designated switch
has been chosen for the link.
Backup designated switch
The backup designated switch selected for the multi-access
network link. (A backup designated switch is not selected
for a point-to-point link.) This data item is initialized
to zero when the switch comes on-line, indicating that no
backup designated switch has been chosen for the link.
Interface output cost(s)
The cost of sending a packet over the interface. The link
cost is expressed in the link state metric and must be
greater than zero.
RxmtInterval
The number of seconds between link state advertisement
retransmissions, for adjacencies belonging to this
interface. This value is also used to time the
retransmission of Database Description and Link State
Request packets.
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5.1 Interface States
This section describes the various states of a switch
interface. The states are listed in order of progressing
functionality. For example, the inoperative state is listed
first, followed by a list of the intermediate states through
which the interface passes before attaining the final, fully
functional state. The specification makes use of this ordering
by references such as "those interfaces in state greater than
X".
Figure 1 represents the interface state machine, showing the
progression of interface state changes. The arrows on the
graph represent the events causing each state change. These
events are described in Section 5.2. The interface state
machine is described in detail in Section 5.3.
Down
This is the initial state of the interface. In this state,
the SNDM protocol [RFCxxxx] has indicated that the interface
is unusable, and no protocol traffic is sent or received on
the interface. In this state, interface parameters are set
to their initial values, all interface timers are disabled,
and no adjacencies are associated with the interface.
Loopback
In this state, the switch interface is looped back, either
in hardware or in software. The interface is unavailable
for regular data traffic.
Waiting
In this state, the switch is attempting to identify the
backup designated switch for the link by monitoring the
Hello packets it receives. The switch does not attempt to
select a designated switch or a backup designated switch
until it changes out of this state, thereby preventing
unnecessary changes of the designated switch and its backup.
Point-to-Point
In this state, the interface is operational and is connected
to a physical point-to-point link. On entering this state,
the switch attempts to form an adjacency with the
neighboring switch.
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Note
In the current version of VLSP, network
links are always considered to be multi-
access, regardless of the physical nature
of the link, and this state is not used.
+-------+ Interface +----------+ Unloop Ind +----------+
| any | -----------> | Down | <----------- | Loopback |
| state | Down +----------+ +----------+
+-------+ | ^
| Interface Up |
+----------------+ | |
| Point-to-Point | <------+ Loop Ind |
+----------------+ | |
V |
+-----------+ +-------+
| Waiting | | any |
+-----------+ | state |
| +-------+
Backup Seen |
| Wait Timer
|
|
+----------+ Neighbor V Neighbor +----------+
| DS | <------------> [ ] <------------> | DS Other |
+----------+ Change ^ Change +----------+
|
|
Neighbor Change |
|
V
+----------+
| Backup |
+----------+
Figure 1: Interface State Machine
DS Other
In this state, the interface is operational and is connected
to a link on which other switches have been selected as the
designated switch and the backup designated switch. On
entering this state, the switch attempts to form adjacencies
with both the designated switch and the backup designated
switch.
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Backup
In this state, the switch itself is the backup designated
switch on the attached link. It will be promoted to
designated switch if the current designated switch fails.
The switch establishes adjacencies with all other switches
attached to the link. (See Section 9.3 for more information
on the functions performed by the backup designated switch.)
DS
In this state, this switch itself is the designated switch
on the attached link. The switch establishes adjacencies
with all other switches attached to the link. The switch is
responsible for originating network link advertisements for
the link, containing link information for all switches
attached to the link, including the designated switch
itself. (See Section 9.3 for more information on the
functions performed by the designated switch.)
5.2 Events Causing Interface State Changes
The state of an interface changes due to an interface event.
This section describes these events.
Interface events are shown as arrows in Figure 1, the graphic
representation of the interface state machine. For more
information on the interface state machine, see Section 5.3.
Interface Up
This event is generated by the SNDM protocol [RFCxxxx] and
indicates that the interface is now operational. This event
causes the interface to change out of the Down state.
Wait Timer
This event is generated when the one-shot Wait timer
expires, triggering the end of the required waiting period
before the switch can begin the process of selecting a
designated switch and a backup designated switch.
Backup Seen
This event is generated when the switch has detected the
existence or non-existence of a backup designated switch for
the link, as determined in one of the following two ways:
- A Hello packet has been received from a neighbor that
claims to be the backup designated switch.
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- A Hello packet has been received from a neighbor that
claims to be the designated switch. In addition, the
packet indicated that there is no backup.
In either case, the interface must have bidirectional
communication with its neighbor -- that is, the local switch
must be listed in the neighbor's Hello packet.
This event signals the end of the Waiting state.
Neighbor change
This event is generated when there has been one of the
following changes in the set of bidirectional neighbors
associated with the interface. (See Section 6.1 for
information on neighbor states.)
- Bidirectional communication has been established with a
neighbor -- the state of the neighbor has changed to 2-Way
or higher.
- Bidirectional communication with a neighbor has been lost
-- the state of the neighbor has changed to Init or
lower.
- A bidirectional neighbor has just declaring itself to be
either the designated switch or the backup designated
switch, as detected by examination of that neighbor's
Hello packets.
- A bidirectional neighbor is no longer declaring itself to
be either the designated switch or the backup designated
switch, as detected by examination of that neighbor's
Hello packets.
- The advertised switch priority of a bidirectional neighbor
has changed, as detected by examination of that neighbor's
Hello packets.
When this event occurs, the designated switch and the backup
designated switch must be reselected.
Loop Ind
This event is generated when an interface enters the
Loopback state. This event can be generated by either the
network management service or by the lower-level protocols.
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Unloop Ind
This event is generated when an interface leaves the
Loopback state. This event can be generated by either the
network management service or by the lower-level protocols.
Interface Down
This event is generated by the SNDM protocol [RFCxxxx] and
indicates that the interface is no longer functional. This
event forces the interface state to Down.
5.3 Interface State Machine
This section presents a detailed description of the interface
state machine.
Interface states (see Section 5.1) change as the result of
various events (see Section 5.2). However, the effect of each
event can vary, depending on the current state of the
interface. For this reason, the state machine described in
this section is organized according to the current interface
state and the occurring event. For each state/event pair, the
new interface state is listed, along with a description of the
required processing.
Note that when the state of an interface changes, it may be
necessary to originate a new switch link advertisement. See
Section 11.1 for more information.
Some of the processing described here includes generating
events for the neighbor state machine. For example, when an
interface becomes inoperative, all neighbor connections
associated with the interface must be destroyed. For more
information on the neighbor state machine, see Section 6.3.
State(s): Down
Event: Interface Up
New state: Depends on action routine
Action:
Start the Hello interval timer, enabling the periodic
sending of Hello packets over the interface. If the
interface is attached to a physical point-to-point link, the
interface state is set to Point-to-Point. Otherwise, the
attached link is a multi-access link. If the switch is not
eligible to become the designated switch, the interface
state changes to DS Other. Otherwise, the interface state
is set to Waiting and the one-shot wait timer is started.
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Note
In the current version of VLSP, network
links are always considered to be multi-
access, regardless of the physical nature
of the link.
State(s): Waiting
Event: Backup Seen
New state: Depends on action routine
Action:
Select the designated switch and backup designated switch
for the attached link, as described in Section 9.3.1. As a
result of this selection, the new state of the interface
will be either DS Other, Backup or DS.
State(s): Waiting
Event: Wait Timer
New state: Depends on action routine
Action:
Select the designated switch and backup designated switch
for the attached link, as described in Section 9.3.1. As a
result of this selection, the new state of the interface
will be either DS Other, Backup or DS.
State(s): DS Other, Backup or DS
Event: Neighbor Change
New state: Depends on action routine
Action:
Reselect the designated switch and backup designated switch
for the attached link, as described in Section 9.3.1. As a
result of this selection, the new state of the interface
will be either DS Other, Backup or DS.
State(s): Any State
Event: Interface Down
New state: Down
Action:
All variables in the interface data structure are reset and
all timers are disabled. In addition, all neighbor
connections associated with the interface are destroyed by
generating the KillNbr event on all neighbors listed in the
interface data structure.
State(s): Any State
Event: Loop Ind
New state: Loopback
Action:
All variables in the interface data structure are reset and
all timers are disabled. In addition, all neighbor
connections associated with the interface are destroyed by
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generating the KillNbr event on all neighbors listed in the
interface data structure.
State(s): Loopback
Event: Unloop Ind
New state: Down
Action:
No action is necessary beyond changing the interface state
to Down because the interface was reset on entering the
Loopback state.
6. Neighbor Data Structure
Each switch conducts a conversation with its neighboring
switches and each conversation is described by a neighbor data
structure. A conversation is associated with a switch
interface, and is identified by the neighboring switch ID.
Note that if two switches have multiple attached links in
common, multiple conversations ensue, each described by a
unique neighbor data structure. Each separate conversation is
treated as a separate neighbor.
The neighbor data structure contains all information relevant
to any adjacency formed between the two neighbors. Remember,
however, that not all neighbors become adjacent. An adjacency
can be thought of as a highly developed conversation between
two switches.
State
The functional level of the neighbor conversation. See
Section 6.1 for a complete description of neighbor states.
Inactivity timer
A one-shot timer used to determine when to declare the
neighbor down if no Hello packet is received from this
neighbor. The length of the timer is SwitchDeadInterval
seconds, as contained in the neighbors Hello packet.
Master/slave flag
A flag indicating whether the local switch is to act as the
master or the slave in the database exchange process (see
Section 10.2). The master/slave relationship is negotiated
when the conversation changes to the ExStart state.
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Sequence number
A 4-octet number identifying individual Database Description
packets. When the neighbor state ExStart is entered and the
database exchange process is started, the sequence number is
set to a value not previously seen by the neighboring
switch. (One possible scheme is to use the switch's time of
day counter.) The sequence number is then incremented by
the master with each new Database Description packet sent.
See Section 10.2 for more information on the database
exchange process.
Neighbor ID
The switch ID of the neighboring switch, as contained in the
neighbors Hello packets.
Neighbor priority
The switch priority of the neighboring switch, as contained
in the neighbor's Hello packets. Switch priorities are used
when selecting the designated switch for the attached link.
Interface identifier
A 10-octet value that uniquely identifies the interface over
which this conversation is being held. This value consists
of the 6-octet base MAC address of the neighbor switch,
followed by the 4-octet local port number of the interface.
Neighbor's designated switch
The switch ID identifying the neighbors idea of the
designated switch, as contained in the neighbors Hello
packets. This value is used in the local selection of the
designated switch. It is not used on point-to-point links.
Neighbor's backup designated switch
The switch ID identifying the neighbors idea of the backup
designated switch, as contained in the neighbors Hello
packets. This value is used in the local selection of the
backup designated switch. It is not used on point-to-point
links.
Link state retransmission list
The list of link state advertisements that have been
forwarded over but not acknowledged on this adjacency. The
local switch retransmits these link state advertisements at
periodic intervals until they are acknowledged or until the
adjacency is destroyed. (For more information on
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retransmitting link state advertisements, see Section
11.2.5.)
Database summary list
The set of link state advertisement headers that summarize
the local link state database. When the conversation
changes to the Exchange state, this list is sent to the
neighbor via Database Description packets. (For more
information on the synchronization of databases, see Section
10.)
Link state request list
The list of link state advertisements that must be received
in order to synchronize with the neighbor switchs link
state database. This list is created as Database
Description packets are received, and is then sent to the
neighbor in Link State Request packets. (For more
information on the synchronization of databases, see Section
10.)
6.1 Neighbor States
This section describes the various states of a conversation
with a neighbor switch. The states are listed in order of
progressing functionality. For example, the inoperative state
is listed first, followed by a list of the intermediate states
through which the conversation passes before attaining the
final, fully functional state. The specification makes use of
this ordering by references such as "those
neighbors/adjacencies in state greater than X".
Figure 2 represents the neighbor state machine. The arrows on
the graph represent the events causing each state change.
These events are described in Section 6.2. The neighbor state
machine is described in detail in Section 6.3.
Down
This is the initial state of a neighbor conversation. In
this state, there has been no recent information received
from the neighbor.
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+----------+ KillNbr, LLDown, +-----------+
| Down | <--------------------- | any state |
+----------+ or Inactivity Timer +-----------+
|
Hello Rcvd |
|
V
+----------+ 1-Way +----------+
| Init | <-------- | >= 2-way |
+----------+ +----------+
|
|
2-Way Rcvd | +-------+ AdjOK?/no +------------+
+----------------> | 2-Way | <----------- | >= ExStart |
| (no adjacency) +-------+ +------------+
|
V
+---------+ Seq Number Mismatch +-------------+
| ExStart | <--------------------- | >= Exchange |
+---------+ or BadLSReq +-------------+
|
Negotiation |
Done |
V
+----------+
| Exchange |
+----------+
|
Exchange | +--------+
Done +----------------------> | Full |
| (request list empty) +--------+
| ^
V |
+---------+ Loading Done |
| Loading | ----------------------->
+---------+
Figure 2: Neighbor State Machine
Init
In this state, a Hello packet has been received from the
neighbor. However, bidirectional communication has not yet
been established with the neighbor -- that is, the local
switch has not yet appeared in the neighbor's Hello packets.
All neighbors in this state (or higher) are listed in the
Hello packets sent by the local switch over the associated
interface.
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2-Way
In this state, communication between the two switches is
bidirectional -- that is, the local switch has seen its own
switch ID listed in the neighbor switchs Hello packets.
This is the most advanced state short of beginning to
establish an adjacency. The designated switch and the
backup designated switch are selected from the set of
neighbors in state 2-Way or greater.
ExStart
This state indicates that the two switches have begun to
establish an adjacency by determining which switch is the
master, as well as the initial sequence number for Database
Descriptor packets. Neighbor conversations in this state or
greater are called adjacencies.
Exchange
In this state, the switches are exchanging Database
Description packets. (See Section 10.2 for a complete
description of this process.) All adjacencies in the
Exchange state or greater are used by the distribution
procedure (see Section 11.2), and are capable of
transmitting and receiving all types of VLSP routing
packets.
Loading
In this state, the local switch is sending Link State
Request packets to the neighbor asking for the more recent
advertisements that were discovered in the Exchange state.
Full
In this state, the two switches are fully adjacent. These
adjacencies will now appear in switch link and network link
advertisements generated for the link.
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6.2 Events Causing Neighbor State Changes
The state of a neighbor conversation changes due to neighbor
events. This section describes these events.
Neighbor events are shown as arrows in Figure 2, the graphic
representation of the neighbor state machine. For more
information on the neighbor state machine, see Section 6.3.
Hello Received
This event is generated when a Hello packet has been
received from a neighbor.
2-Way Received
This event is generated when the local switch sees its own
switch ID listed in the neighbors Hello packet, indicating
that bidirectional communication has been established
between the two switches.
Negotiation Done
This event is generated when the master/slave relationship
has been successfully negotiated and initial packet sequence
numbers have been exchanged. This event signals the start
of the database exchange process (see Section 10.2).
Exchange Done
This event is generated when the database exchange process
is complete and both switches have successfully transmitted
a full sequence of Database Description packets. (For more
information on the database exchange process, see Section
10.2.)
BadLSReq
This event is generated when a Link State Request has been
received for a link state advertisement that is not
contained in the database. This event indicates an error in
the synchronization process.
Loading Done
This event is generated when all Link State Updates have
been received for all out-of-date portions of the database.
(See Section 10.3.)
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AdjOK?
This event is generated when a decision must be made as to
whether an adjacency will be established or maintained with
the neighbor. This event will initiate some adjacencies and
destroy others.
Seq Number Mismatch
This event is generated when a Database Description packet
has been received with any of the following conditions:
- The packet contains an unexpected sequence number.
- The packet (unexpectedly) has the Init bit set.
- The packet has a different Options field than was
previously seen.
These conditions all indicate that an error has occurred
during the establishment of the adjacency.
1-Way
This event is generated when bidirectional communication
with the neighbor has been lost. That is, a Hello packet
has been received from the neighbor in which the local
switch is not listed.
KillNbr
This event is generated when further communication with
the neighbor is impossible.
Inactivity Timer
This event is generated when the inactivity timer has
expired, indicating that no Hello packets have been received
from the neighbor in switchDeadInterval seconds.
LLDown
This event is generated by the lower-level protocols and
indicates that the neighbor is now unreachable.
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6.3 Neighbor State Machine
This section presents a detailed description of the neighbor
state machine.
Neighbor states (see Section 6.1) change as the result of
various events (see Section 6.2). However, the effect of each
event can vary, depending on the current state of the
conversation with the neighbor. For this reason, the state
machine described in this section is organized according to the
current neighbor state and the occurring event. For each
state/event pair, the new neighbor state is listed, along with
a description of the required processing.
Note that when the neighbor state changes as a result of an
interface Neighbor Change event (see Section 5.2), it may be
necessary to rerun the designated switch selection algorithm.
In addition, if the interface associated with the neighbor
conversation is in the DS state (that is, the local switch is
the designated switch), changes in the neighbor state may cause
a new network link advertisement to be originated (see Section
11.1).
When the neighbor state machine must invoke the interface state
machine, it is invoked as a scheduled task. This simplifies
processing, by ensuring that neither state machine executes
recursively.
State(s): Down
Event: Hello Received
New state: Init
Action:
Start the inactivity timer for the neighbor. If the timer
expires before another Hello packet is received, the
neighbor switch is declared dead.
State(s): Init or greater
Event: Hello Received
New state: No state change
Action:
Reset the inactivity timer for the neighbor.
State(s): Init
Event: 2-Way Received
New state: Depends on action routine
Action:
Determine whether an adjacency will be formed with the
neighbor (see Section 9.4). If no adjacency is to be
formed, the neighbor state changes to 2-Way.
Otherwise, the neighbor state changes to ExStart. The
switch initializes the sequence number for this neighbor.
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It then declares itself master for the database exchange
process. (See Section 10.2.)
State(s): ExStart
Event: Negotiation Done
New state: Exchange
Action:
The Negotiation Done event signals the start of the database
exchange process. See Section 10.2 for a detailed
description of this process.
State(s): Exchange
Event: Exchange Done
New state: Depends on action routine
Action:
If the neighbor Link state request list is empty, the
neighbor state changes to Full. This is the adjacency's
final state.
Otherwise, the neighbor state changes to Loading. The
switch begins sending Link State Request packets to the
neighbor requesting the most recent link state
advertisements, as discovered during the database exchange
process. (See Section 10.2.) These advertisements are
listed in the link state request list associated with the
neighbor.
State(s): Loading
Event: Loading Done
New state: Full
Action:
No action is required beyond changing the neighbor state to
Full. This is the adjacency's final state.
State(s): 2-Way
Event: AdjOK?
New state: Depends on action routine
Action:
If no adjacency is to be formed with the neighboring switch
(see Section 9.4), the neighbor state remains at 2-Way.
Otherwise, the neighbor state changes to ExStart. The
switch initializes the sequence number for this neighbor.
It then declares itself master for the database exchange
process. (See Section 10.2.)
State(s): ExStart or greater
Event: AdjOK?
New state: Depends on action routine
Action:
If an adjacency should still be formed with the neighboring
switch (see Section 9.4), no state change and no further
action is necessary.
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Otherwise, the (possibly partially formed) adjacency is torn
down. The link state retransmission list, database summary
list and link state request list are cleared of link state
advertisements. The neighbor state changes to 2-Way.
State(s): Exchange or greater
Event: Seq Number Mismatch
New state: ExStart
Action:
The (possibly partially formed) adjacency is torn down. The
link state retransmission list, database summary list and
link state request list are cleared of link state
advertisements. The neighbor state then changes to ExStart
and another attempt is made to establish the adjacency.
State(s): Exchange or greater
Event: BadLSReq
New state: ExStart
Action:
The (possibly partially formed) adjacency is torn down. The
link state retransmission list, database summary list and
link state request list are cleared of link state
advertisements. The neighbor state then changes to ExStart
and another attempt is made to establish the adjacency.
State(s): Any state
Event: KillNbr
New state: Down
Action:
The neighbor conversation is terminated. The inactivity
timer is disabled, and the link state retransmission list,
database summary list and link state request list are
cleared of link state advertisements.
State(s): Any state
Event: LLDown
New state: Down
Action:
The neighbor conversation is terminated. The inactivity
timer is disabled, and the link state retransmission list,
database summary list and link state request list are
cleared of link state advertisements.
State(s): Any state
Event: Inactivity Timer
New state: Down
Action:
The neighbor conversation is terminated. The inactivity
timer is disabled, and the link state retransmission list,
database summary list and link state request list are
cleared of link state advertisements.
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State(s): 2-Way or greater
Event: 1-Way Received
New state: Init
Action:
The adjacency between the switches, if any, is torn down.
The link state retransmission list, database summary list
and link state request list are cleared of link state
advertisements.
State(s): 2-Way or greater
Event: 2-Way received
New state: No state change
Action:
No action required.
State(s): Init
Event: 1-Way received
New state: No state change
Action:
No action required.
7. Area Data Structure
The area data structure contains all the information needed to
run the basic routing algorithm. One of its components is the
link state database -- the collection of all switch link and
network link advertisements generated by the switches.
The area data structure contains the following items:
Area ID
A 4-octet value identifying the area. Since VLSP does not
support multiple areas, the value here is always zero.
Associated switch interfaces
A list of interface IDs of the local switch interfaces
connected to network links.
Link state database
The collection of all current link state advertisements for
the switch fabric. This collection consists of the
following:
Switch link advertisements
A list of the switch link advertisements for all switches
in the fabric. Switch link advertisements describe the
state of each switchs interfaces.
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Network link advertisements
A list of the network link advertisements for all multi-
access network links in the switch fabric. Network link
advertisements describe the set of switches currently
connected to each link.
Best path(s)
A set of end-to-end hop descriptions for all equal-cost best
paths from the local switch to every other switch in the
fabric. Each hop is specified by the interface ID of the
next link in the path. Best paths are derived from the
collected switch link and network link advertisements using
the Dijkstra algorithm. [Perlman]
7.1 Adding and Deleting Link State Advertisements
The link state database within the area data structure must
contain, at most, a single instance of each link state
advertisement. To keep the database current, a switch adds
link state advertisements to the database under the following
conditions:
- When a link state advertisement is received during the
distribution process
- When the switch itself generates a link state advertisement
(See Section 11.2.4 for information on installing link state
advertisements.)
Likewise, a switch deletes link state advertisements from the
database under the following conditions:
- When a link state advertisement has been superseded by a
newer instance during the flooding process
- When the switch generates a newer instance of one of its
self-originated advertisements
Note that when an advertisement is deleted from the link state
database, it must also be removed from the link state
retransmission list of all neighboring switches.
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7.2 Accessing Link State Advertisements
An implementation of the VLS protocol must provide access to
individual link state advertisements, based on the
advertisement's type, link state identifier, and advertising
switch.[1] This lookup function is invoked during the link
state distribution procedure and during calculation of the
routing table. In addition, a switch can use the function to
determine whether it has originated a particular link state
advertisement, and if so, with what sequence number.
8. Routing Table
The routing table contains all the information necessary to
forward a data packet toward its destination. There is a
single routing table in each switch. Each routing table entry
describes the collection of best paths to a particular
destination switch, using the local switch as the start of the
path.
Each entry in the routing table contains the following data
items:
Destination ID
The interface ID of the destination switch, as known by its
adjacent designated switch. That is, the value here
consists of the 6-octet base MAC address of the destination
switch, followed by the 4-octet port number of the
interface, local to the designated switch of the network
link.
Destination type
The type of the destination switch. The value here is
always Network.
Type of service
The type of service (TOS) of the paths. Note that since the
current version of VLSP does not support routing based on
non-zero TOS, the value here is always zero.
Area ID
The 4-octet identifier of the area. Since VLSP does not
support multiple areas, the value here is always zero.
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Cost metric
The link state cost of the path(s), calculated as the sum of
the costs of a path's constituent links. Note that even
when multiple paths to the destination are calculated, there
is only one path cost because, by definition, such multiple
paths are of equal cost.
Link state ID
The link state identifier of the network link advertisement
that references the destination switch. This value consists
of the interface ID of the network link, as known by the
designated switch of the link.
Advertising switch
The switch ID of the designated switch that originated the
network link advertisement specified by the link state ID
data item.
Next hop(s)
The interface ID(s) of the local outgoing interface(s) over
which to forward traffic to the destination switch. When
multiple paths of equal cost exist to the destination
switch, their initial hops are all stored here. End-to-end
path information for all equal-cost paths are stored in the
area data structure (see Section 7).
8.1 Routing Table Lookup
An implementation of the VLS protocol must provide access to
multiple equal-cost best paths, based on the base MAC addresses
of the source and destination switches. This lookup function
should return up to three equal-cost paths. Paths should be
returned as lists of end-to-end hop information, with each hop
specified as a interface ID of the next link in the path -- the
6-octet base MAC address of the next switch and the 4-octet
local port number of the link interface.
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9. Discovery Process
The first operational stage of the VLS protocol is the
discovery process. During this stage, each switch dynamically
detects its neighboring switches and establishes a relationship
with each of these neighbors. This process has the following
component steps:
- Neighboring switches are discovered through the exchange of
Hello packets over each functioning interface.
- Bidirectional communication is established with each
neighbor switch.
- A designated switch and backup designated switch are
selected for each multi-access network link.
- An adjacent relationship is established with selected
neighbors on each link.
The following subsections describe each of these steps in
detail.
9.1 Hello Packets
Each functioning switch in the fabric periodically sends a
Hello packet out each of its functioning switch interfaces.
Each Hello packet contains the following data used during the
discovery process:
- The switch ID and priority of the sending switch
- Values specifying the interval timers to be used for sending
Hello packets and deciding whether to declare a neighbor
switch Down
- The switch ID of the designated switch and the backup
designated switch for the interface link, as understood by
the sending switch
- A list of switch IDs of all neighboring switches seen so far
on the interface link
For a detailed description of the Hello packet format, see
Section 13.5.1.
When a switch receives a Hello packet, it first attempts to
identify the sending switch by matching its switch ID to one of
the known neighbors listed in the interface data structure. If
this is the first Hello packet received from the switch, the
switch ID is entered in the list of known neighbors and a new
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neighbor data structure is created with a neighbor status of
Down.
At this point, the remainder of the Hello packet is examined
and the appropriate interface and neighbor events are
generated. In all cases, a neighbor Hello Received event is
generated. Other events may also be generated, triggering
further steps in the discovery process or other actions, as
appropriate.
For a detailed description of the interface state machine, see
Section 5.3. For a detailed description of the neighbor state
machine, see Section 6.3.
9.2 Bidirectional Communication
When a switch sees its own switch ID listed in a Hello packet
received from one of its neighbors, bidirectional communication
has been established with that neighbor. A neighbor 2-Way
Received event is generated.
Once bidirectional communication has been established with a
neighbor, the local switch determines whether an adjacency will
be formed with the neighbor. However, before that decision can
be made, a designated switch and a backup designated switch
must be selected for the link, if the link is a multi-access
link. The next section contains a description of the
designated switch, the backup designated switch, and the
selection process.
9.3 Designated Switch
Every multi-access network link has a designated switch. The
designated switch performs the following functions for the
routing protocol:
- The designated switch originates a network link
advertisement on behalf of the link, listing the set of
switches (including the designated switch itself) currently
attached to the link. For a detailed description of network
link advertisements, see Section 14.3.
- The designated switch becomes adjacent to all other switches
on the link. Since the link state databases are
synchronized across adjacencies, the designated switch plays
a central part in the synchronization process. For a
description of the synchronization process, see Section 10.
Each multi-access network link also has a backup designated
switch. The primary function of the backup designated switch
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is to act as a standby for the designated switch. If the
current designated switch fails, the backup designated switch
becomes the designated switch.
To facilitate this transition, the backup designated switch
forms an adjacency with every other switch on the link. Thus,
when the backup designated switch must take over for the
designated switch, its link state database is already
synchronized with the databases of all other switches on the
link.
Note
Point-to-point network links have neither a
designated switch or a backup designated
switch. However, in the current version of
VLSP, network links are always treated as
multi-access, regardless of the physical
nature of the link. Therefore, all network
links have both a designated switch and a
backup designated switch.
9.3.1 Selecting the Designated Switch
When a link interface first becomes functional, the switch sets
a one-shot Wait timer (with a value of SwitchDeadInterval
seconds) for the interface. The purpose of this timer is to
ensure that all switches attached to the link have a chance to
establish bidirectional communication before the designated
switch and backup designated switch are selected for the link.
When the Wait timer is set, the interface enters the Waiting
state. During this state, the switch exchanges Hello packets
with its neighbors attempting to establish bidirectional
communication. The interface leaves the Waiting state under
one of the following conditions:
- The Wait timer expires.
- A Hello packet is received indicating that a designated
switch or a backup designated switch has already been
specified for the interface.
At this point, if the switch sees that a designated switch has
already been selected for the link, the switch accepts that
designated switch, regardless of its own switch priority and
MAC address. This situation typically means the switch has
come up late on a fully functioning link. Although this makes
it harder to predict the identity of the designated switch on a
particular link, it ensures that the designated switch does not
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change needlessly, necessitating a resynchronization of the
databases.
If no designated switch is currently specified for the link,
the switch begins the actual selection process. Note that this
selection algorithm operates only on a list of neighbor
switches that are eligible to become the designated switch. A
neighbor is eligible to be the designated switch if it has a
switch priority greater than zero and its neighbor state is 2-
Way or greater. The local switch includes itself on the list
of eligible switches as long as it has a switch priority
greater than zero.
The selection process includes the following steps:
1) The current values of the link's designated switch and
backup designated switch are saved for use in step 6.
2) The new backup designated switch is selected as follows:
a) Eliminate from consideration those switches that have
declared themselves to be the designated switch.
b) If one or more of the remaining switches have declared
themselves to be the backup designated switch, eliminate
from consideration all other switches.
c) From the remaining list of eligible switches, select the
switch having the highest switch priority as the backup
designated switch. If multiple switches have the same
(highest) priority, select the switch with the highest
switch ID as the backup designated switch.
3) The new designated switch is selected as follows:
a) If one or more of the switches have declared themselves to
be the designated switch, eliminate from consideration all
other switches.
b) From the remaining list of eligible switches, select the
switch having the highest switch priority as the
designated switch. If multiple switches have the same
(highest) priority, select the switch with the highest
switch ID as the designated switch.
4) If the local switch has been newly selected as either the
designated switch or the backup designated switch, or is now
no longer the designated switch or the backup designated
switch, repeat steps 2 and 3, above, and then proceed to
step 5.
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If the local switch is now the designated switch, it will
eliminate itself from consideration at step 2a when the
selection of the backup designated switch is repeated.
Likewise, if the local switch is now the backup designated
switch, it will eliminate itself from consideration at step
3a when the selection of the designated switch is repeated.
This ensures that no switch will select itself as both
backup designated switch and designated switch.[2]
5) Set the interface state to the appropriate value, as
follows:
- If the local switch is now the designated switch, set the
interface state to DS.
- If the local switch is now the backup designated switch,
set the interface state to Backup.
- Otherwise, set the interface state to DS Other.
6) If either the designated switch or backup designated switch
has now changed, the set of adjacencies associated with this
link must be modified. Some adjacencies may need to be
formed, while others may need to be broken. Generate the
neighbor AdjOK? event for all neighbors with a state of 2-
Way or higher to trigger a reexamination of adjacency
eligibility.
Caution
If VLSP is implemented with configurable parameters,
care must be exercised in specifying the switch
priorities. Note that if the local switch is not
itself eligible to become the designated switch
(i.e., it has a switch priority of 0), it is
possible that neither a backup designated switch
nor a designated switch will be selected by the
above procedure. Note also that if the local
switch is the only attached switch that is eligible
to become the designated switch, it will select
itself as designated switch and there will be no
backup designated switch for the link. For this
reason, it is advisable to specify a default switch
priority of 1 for all switches.
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9.4 Adjacencies
VLSP creates adjacencies between neighboring switches for the
purpose of exchanging routing information. Not every two
neighboring switches will become adjacent. On a multi-access
link, an adjacency is only formed between two switches if one
of them is either the designated switch or the backup
designated switch.
Note that an adjacency is bound to the network link that the
two switches have in common. Therefore, if two switches have
multiple links in common, they may have multiple adjacencies
between them.
The decision to form an adjacency occurs in two places in the
neighbor state machine:
- When bidirectional communication is initially established
with the neighbor
- When the designated switch or backup designated switch on
the attached link changes.
The rules for establishing an adjacency between two neighboring
switches are as follows:
- On a point-to-point link, the two neighboring switches
always establish an adjacency.
- On a multi-access link, an adjacency is established with the
neighboring switch under one of the following conditions:
- The local switch itself is the designated switch.
- The local switch itself is the backup designated switch.
- The neighboring switch is the designated switch.
- The neighboring switch is the backup designated switch.
If no adjacency is formed between two neighboring switches, the
state of the neighbor conversation remains set to 2-Way.
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10. Synchronizing the Databases
In an SPF-based routing algorithm, it is important for the link
state databases of all switches to stay synchronized. VLSP
simplifies this process by requiring only adjacent switches to
remain synchronized.
The synchronization process begins when the switches attempt to
bring up the adjacency. Each switch in the adjacency describes
its database by sending a sequence of Database Description
packets to its neighbor. Each Database Description packet
describes a set of link state advertisements belonging to the
database. When the neighbor sees a link state advertisement
that is more recent than its own database copy, it makes a note
to request this newer advertisement.
During this exchange of Database Description packets (known as
the database exchange process), the two switches form a
master/slave relationship. Database Description packets sent
by the master are known as polls, and each poll contains a
sequence number. Polls are acknowledged by the slave by
echoing the sequence number in the Database Description
response packet.
When all Database Description packets have been sent and
acknowledged, the database exchange process is completed. At
this point, each switch in the exchange has a list of link
state advertisements for which its neighbor has more recent
instances. These advertisements are requested using Link State
Request packets.
Once the database exchange process has completed and all Link
State Requests have been satisfied, the databases are deemed
synchronized and the neighbor states of the two switches are
set to Full, indicating that the adjacency is fully functional.
Fully functional adjacencies are advertised in the link state
advertisements of the two switches.[3]
10.1 Link State Advertisements
Link state advertisements form the core of the database from
which a switch builds its routing table and calculates the set
of best paths to the other switches in the fabric.
Each link state advertisement begins with a standard header.
This header contains three data items that uniquely identify
the link state advertisement:
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- The link state type. Possible values are as follows:
1 Switch link advertisement -- describes the collected
states of the switch's interfaces.
2 Network link advertisement -- describes the set of
switches attached to the network link.
- The link state ID, defined as follows:
- For a switch link advertisement -- the switch ID of the
originating switch
- For a network link advertisement -- the switch ID of the
designated switch for the link
- The switch ID of the advertising switch -- the switch that
generated the advertisement
The link state advertisement header also contains three data
items that are used to determine which instance of a particular
link state advertisement is the most current. (See Section
10.1.1 for a description of how to determine which instance of
a link state advertisement is the most current.)
- The link state sequence number
- The link state age, stored in seconds
- The link state checksum, a 16-bit unsigned value calculated
for the entire contents of the link state advertisement,
with the exception of the age field
The remainder of each link state advertisement contains data
specific to the type of the advertisement. See Section 14 for
a detailed description of the link state header, as well as the
format of a switch link or network link advertisement.
10.1.1 Determining Which Link State Advertisement Is Newer
At various times while synchronizing or updating the link state
database, a switch must determine which instance of a
particular link state advertisement is the most current. This
decision is made as follows:
- The advertisement having the greater sequence number is the
most current.
- If both instances have the same sequence number, then:
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- If the two instances have different checksum values, then
the instance having the larger checksum is considered the
most current.[4]
- If both instances have the same sequence number and the same
checksum value, then:
- If one (and only one) of the instances is of age MaxAge,
then the instance of age MaxAge is considered the most
current.[5]
- Else, if the ages of the two instances differ by more than
MaxAgeDiff, the instance having the smaller (younger) age
is considered the most current.[6]
- Else, the two instances are considered identical.
10.2 Database Exchange Process
There are two stages to the database exchange process:
- Negotiating the master/slave relationship
- Exchanging database summary information
In both these stages, the neighboring switches exchange
Database Description packets.
10.2.1 Database Description Packets
Database Description packets are used to describe a switchs
link state database during the database exchange process. Each
Database Description packet contains a list of headers of the
link state advertisements currently stored in the sending
switchs database. (See Section 14.1 for a description of a
link state advertisement header.)
In addition to the link state headers, each Database
Description packet contains the following data items:
- A flag (the M-bit) indicating whether or not more packets
are to follow. Depending on the size of the local database
and the maximum size of the packet, the list of headers in
any particular Database Description packet may be only a
partial list of the total database. When the M-bit is set,
the list of headers is only a partial list and more headers
are to follow in subsequent packets.
- A flag (the I-bit) indicating whether or not this is the
first Database Description packet sent for this execution of
the database exchange process.
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- A flag (the MS-bit) indicating whether the sending switch
thinks it is the master or the slave in the database
exchange process. If the flag is set, the switch thinks it
is the master.
- A 4-octet sequence number for the packet.
While the switches are negotiating the master/slave
relationship, they exchange "empty" Database Description
packets. That is, packets that contain no link summary
information. Instead, the flags and sequence number constitute
the information required for the negotiation process.
See Section 13.5.2 for a more detailed description of a
Database Description packet.
10.2.2 Negotiating the Master/Slave Relationship
Before two switches can begin the actual exchange of database
information, they must decide between themselves who will be
the master in the exchange process and who will be the slave.
They must also agree on the starting sequence number for the
Database Description packets.
Once a switch has decided to form an adjacency with a
neighboring switch, it sets the neighbor state to ExStart and
begins sending empty Database Description packets to its
neighbor. These packets contain the starting sequence number
the switch plans to use in the exchange process. Also, the I-
bit and M-bit flags are set, as well as the MS-bit. Thus, each
switch in the exchange begins by believing it will be the
master.
Empty Database Description packets are retransmitted every
RxmtInterval seconds until the neighbor responds.
When a switch receives an empty Database Description packet
from its neighbor, it determines which switch will be the
master by comparing the switch IDs. The switch with the
highest switch ID becomes the master of the exchange. Based on
this determination, the switch proceeds as follows:
- If the switch is to be the slave of the database exchange
process, it acknowledges that it is the slave by sending
another empty Database Description packet to the master.
This packet contains the masters sequence number and has
the MS-bit and the I-bit cleared.
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The switch then generates a neighbor event of Negotiation
Done to change its neighbor state to Exchange and waits for
the first non-empty Database Description packet from the
master.
- If the switch is to be the master of the database exchange,
it waits to receive an acknowledgment from its neighbor --
that is, an empty Database Description packet with the MS-
bit and I-bit cleared and containing the sequence number it
(the master) previously sent.
When it receives the acknowledgment, it generates a neighbor
event of Negotiation Done to change its neighbor state to
Exchange and begin the actual exchange of Database
Description packets.
Note that during the negotiation process, the receipt of an
inconsistent packet will result in a neighbor event of Seq
Number Mismatch, terminating the process. See Section 6.3 for
more information.
10.2.3 Exchanging Database Description Packets
Once the neighbor state changes to Exchange, the switches begin
the exchange of Database Description packets containing link
state summary data. The process proceeds as follows:
1) The master sends a packet containing a list of link state
headers. If the packet contains only a portion of the
unexchanged database -- that is, more Database Description
packets are to follow -- the packet has the M-bit set. The
MS-bit is set and the I-bit is clear.
If the slave does not acknowledge the packet within
RxmtInterval seconds, the master retransmits the packet.
2) When the slave receives a packet, it first checks the
sequence number to see if the packet is a duplicate. If so,
it simply acknowledges the packet by clearing the MS-bit and
returning the packet to the master. (Note that the slave
acknowledges all Database Description packets that it
receives, even those that are duplicates.)
Otherwise, the slave processes the packet by doing the
following:
- For each link state header listed in the packet, the slave
searches its own link state database to determine whether
it has an instance of the advertisement.
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- If the slave does not have an instance of the link state
advertisement, or if the instance it does have is older
than the instance listed in the packet, it creates an
entry in its link state request list in the neighbor data
structure. See Section 10.1.1 for a description of how to
determine which instance of a link state advertisement is
the newest.
- When the slave has examined all headers, it acknowledges
the packet by turning the MS-bit off and returning the
packet to the master.
3) When the master receives the first acknowledgment for a
particular Database Description packet, it processes the
acknowledgment as follows:
- For each link state header listed in the packet, the
master checks to see if the slave has indicated it has an
instance of the link state advertisement that is newer
than the instance the master has in its own database. If
so, the master creates an entry in its link state request
list in the neighbor data structure.
- The master then increments the sequence number and sends
another packet containing the next set of link state
summary information, if any.
Subsequent acknowledgments for the Database Description
packet (those with the same sequence number) are discarded.
When the master sends the last portion of its database
summary information, it clears the M-bit in the packet to
indicate that no more packets are to be sent.
4) When the slave receives a Database Description packet with
the M-bit clear, it processes the packet, as described above
in step 2. After it has completed processing and has
acknowledged the packet to the master, it generates an
Exchange Done neighbor event and its neighbor state changes
to Loading.
The database exchange process is now complete for the slave,
and it begins the process of requesting those link state
advertisements for which the master has more current
instances (see Section 10.3).
5) When the master receives an acknowledgment for the final
Database Description packet, it processes the acknowledgment
as described above in step 3. Then it generates an Exchange
Done neighbor event and its neighbor state changes to
Loading.
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The database exchange process is now complete for the
master, and it begins the process of requesting those link
state advertisements for which the slave has more current
instances (see Section 10.3).
Note that during this exchange, the receipt of an inconsistent
packet will result in a neighbor event of Seq Number Mismatch,
terminating the process. See Section 6.3 for more information.
10.3 Updating the Database
When either switch completes the database exchange process and
its neighbor state changes to Loading, it has a list of link
state advertisements for which the neighboring switch has a
more recent instance. This list is stored in the neighbor data
structure as the link state request list.
To complete the synchronization of its database with that of
its neighbor, the switch must obtain the most current instances
of those link state advertisements.
The switch requests these advertisements by sending its
neighbor a Link State Request packet containing the description
of one or more link state advertisement, as defined by the
advertisements type, link state ID, and advertising switch.
(For a detailed description of the Link State Request packet,
see Section 13.5.3.) The switch continues to retransmit this
packet every RxmtInterval seconds until it receives a reply
from the neighbor.
When the neighbor switch receives the Link State Request
packet, it responds with a Link State Update packet containing
its most current instance of each of the requested
advertisements. (Note that the neighboring switch can be in
any of the Exchange, Loading or Full neighbor states when it
responds to a Link State Request packet.)
If the neighbor cannot locate a particular link state
advertisement in its database, something has gone wrong with
the synchronization process. The switch generates a BadLSReq
neighbor event and the partially formed adjacency is torn down.
See Section 6.3 for more information.
Depending on the size of the link state request list, it may
take more than one Link State Request packet to obtain all the
necessary advertisements. Note, however, that there must at
most one Link State Request packet outstanding at any one time.
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10.4 An Example
Figure 3 shows an example of an adjacency being formed between
two switches -- S1 and S2 -- connected to a network link. S2
is the designated switch for the link and has a higher switch
ID than S1.
The neighbor state changes that each switch goes through are
listed on the sides of the figure.
At the top of Figure 3, S1's interface to the link becomes
operational, and S1 begins sending Hello packets over the
interface. At this point, S1 does not yet know the identity of
the designated switch or of any other neighboring switches.
S2 receives the Hello packet from S1 and changes its neighbor
state to Init. In its next Hello packet, S2 indicates that it
is itself the designated switch and that it has received a
Hello packet from S1. S1 receives the Hello packet and changes
its state to ExStart, starting the process of bringing up the
adjacency.
S1 begins by asserting itself as the master. When it sees that
S2 is indeed the master (because of S2's higher switch ID), S1
changes to slave and adopts S2's sequence number. Database
Description packets are then exchanged, with polls coming from
the master (S2) and acknowledgments from the slave (S1). This
sequence of Database Description packets ends when both the
poll and associated acknowledgment have the M-bit off.
In this example, it is assumed that S2 has a completely up-to-
date database and immediately changes to the Full state. S1
will change to the Full state after updating its database by
sending Link State Request packets and receiving Link State
Update packets in response.
Note that in this example, S1 has waited until all Database
Description packets have been received from S2 before sending
any Link State Request packets. However, this need not be the
case. S1 could interleave the sending of Link State Request
packets with the reception of Database Description packets.
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+--------+ +--------+
| Switch | | Switch |
+ S1 + + S2 +
+--------+ +--------+
Down Down
Hello (DS=0, seen=0)
------------------------------------->
Init
Hello (DS=S2, seen=...,S1)
<-------------------------------------
ExStart
DB Description (Seq=x, I, M, Master)
------------------------------------->
ExStart
DB Description (Seq=y, I, M, Master)
<-------------------------------------
Exchange
DB Description (Seq=y, M, Slave)
------------------------------------->
Exchange
DB Description (Seq=y+1, M, Master)
<-------------------------------------
DB Description (Seq=y+1, M, Slave)
------------------------------------->
.
.
.
DB Description (Seq=y+n, Master)
<-------------------------------------
DB Description (Seq=y+n, Slave)
------------------------------------->
Loading Full
Link State Request
<-------------------------------------
Link State Update
------------------------------------->
.
.
.
Link State Request
<-------------------------------------
Link State Update
------------------------------------->
Full
Figure 3: An Example of Bringing Up an Adjacency
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11. Maintaining the Databases
Each switch advertises its state (also known as its link state)
by originating switch link advertisements. In addition, the
designated switch on each network link advertises the state of
the link by originating network link advertisements.
As described in Section 10.1, link state advertisements are
uniquely identified by their type, link state ID, and
advertising switch.
Link state advertisements are distributed throughout the switch
fabric using a reliable flooding algorithm that ensures that
all switches in the fabric are notified of any link state
changes.
11.1 Originating Link State Advertisements
A new instance of each link state advertisement is originated
any time the state of the switch or link changes. When a new
instance of a link state advertisement is originated, its
sequence number is incremented, its age is set to zero, and its
checksum is calculated. The advertisement is then installed
into the local link state database and forwarded out all fully
operational interfaces (that is, those interfaces with a state
greater than Waiting) for distribution throughout the switch
fabric. See Section 11.2.4 for a description of the
installation of the advertisement into the link state database
and Section 11.2.3 for a description of how advertisements are
forwarded.
A switch originates a new instance of a link state
advertisement as a result of the following events:
1) The state of one of the switchs interfaces changes such
that the contents of the associated switch link
advertisement changes.
2) The designated switch on any of the switchs attached
network links changes. The switch originates a new switch
link advertisement. Also, if the switch itself is now the
designated switch, it originates a new network link
advertisement for the link.
3) One of the switchs neighbor states changes to or from Full.
If this changes the contents of the associated switch link
advertisement, a new instance is generated. Also, if the
switch is the designated switch for the attached network
link, it originates a new network link advertisement for the
link.
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Two instances of the same link state advertisement must not be
originated within the time period MinLSInterval. Note that
this may require that the generation of the second instance to
be delayed up to MinLSInterval seconds.
11.1.1 Switch Link Advertisements
A switch link advertisement describes the collected states of
all functioning links attached to the originating switch --
that is, all attached links with an interface state greater
than Down. A switch originates a switch link advertisement
when it first becomes functional. It then generates a new
instance of the advertisement each time one of its interfaces
changes state.
Each link in the advertisement is assigned a type, based on the
state of interface, as shown in Table 4.
Note
A stub link is a link that is unavailable
for network traffic.
Interface state Link type Description
Down (n/a) (n/a)
Loopback 3 Stub link
Waiting 3 Stub link
Point-to-Point 1 Point-to-point link
DS Other* 2 Multi-access link
Backup* 2 Multi-access link
DS** 2 Multi-access link
*If a full adjacency has been formed with the designated
switch. Otherwise, the link type is 3 until the
adjacency has been established.
**If a full adjacency has been formed with at least one
other switch on the link. Otherwise, the link type is 3
until an adjacency has been established.
Table 4: Link Types in a Switch Link Advertisement
Each link in the advertisement is also assigned a link
identifier based on its link type. In general, this value
identifies another switch that also originates advertisements
for the link, thereby providing a key for accessing other link
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state advertisements for the link. The relationship between
link type and ID is shown in Table 5.
Link type Description Link ID
1 Point-to-point link Switch ID of neighbor
switch
2 Multi-access link Switch ID of
designated switch
3 Stub link (n/a)
Table 5: Link IDs in a Switch Link Advertisement
In addition to a type and an identifier, the description of
each link specifies the interface ID of the associated network
link.
Finally, each link description includes the cost of sending a
packet over the link. This output cost is expressed in the
link state metric and must be greater than zero.
To illustrate the format of a switch link advertisement,
consider the switch fabric shown in Figure 4.
00-00-1d-22-23-c5
+-------+
| SW2 |
+-------+
|
| Point-to-Point
|
| 01
+-------+ Waiting +-------+
| SW3 |----------------| SW1 | 00-00-1d-1f-05-81
+-------+ 02 +-------+
00-00-1d-17-35-a4 | 03
|
| DS Other
|
+-------------------------+-------------------------+
DS | | |
| | |
+-------+ +-------+ +-------+
| SW4 | | SW5 | | SW6 |
+-------+ +-------+ +-------+
00-00-1d-4a-26-b3 00-00-1d-4a-27-1c 00-00-1d-7e-84-2e
Figure 4: Sample Switch Fabric
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In this example, switch SW1 has 5 neighboring switches (shown
as boxes) distributed over 3 network links (shown as lines).
The base MAC address of each switch is also shown adjacent to
each box. On switch SW1, ports 01 and 02 attach to point-to-
point network links, while port 03 attaches to a multi-access
network link with three attached switches. The interface state
of each port is shown next to the line representing the
corresponding link.
The switch link advertisement generated by switch SW1 would
contain the following data items:
; switch link advertisement for switch SW1
LS age = 0 ; always true on origination
Options = (T-bit|E-bit) ; options
LS type = 1 ; this is a switch link advertisement
; SW1s switch ID
Link State ID = 00-00-1d-1f-05-81-00-00-00-00
Advertising switch = 00-00-1d-1f-05-81-00-00-00-00
# links = 3
; link on interface port 1
Link ID = 00-00-1d-22-23-c5-00-00-00-00 ; switch ID
Link Data = 00-00-1d-1f-05-81-00-00-00-01 ; interface ID
Type = 1 ; pt-to-pt link
# other metrics = 0 ; TOS 0 only
TOS 0 metric = 1
; link on interface port 2
Link ID = 00-00-1d-17-35-a4-00-00-00-00 ; switch ID
Link Data = 00-00-1d-1f-05-81-00-00-00-02 ; interface ID
Type = 3 ; stub link
# other metrics = 0 ; TOS 0 only
TOS 0 metric = 1
; link on interface port 3
Link ID = 00-00-1d-4a-26-b3-00-00-00-00 ; switch ID of DS
Link Data = 00-00-1d-1f-05-81-00-00-00-03 ; interface ID
Type = 1 ; multi-ax link
# other metrics = 0 ; TOS 0 only
TOS 0 metric = 2
(See Section 14.2 for a detailed description of the format of a
switch link advertisement.)
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11.1.2 Network Link Advertisements
Network link advertisements are used to describe the switches
attached to each multi-access network link.
Note
Network link advertisements are not generated for
point-to-point links. However, in the current
version of VLSP, network links are always treated
as multi-access, regardless of the physical nature
of the link. Therefore, network link advertisements
are generated for all network links, regardless of
physical type.
A network link advertisement is originated by the designated
switch for the associated link once the switch has established
a full adjacency with at least one other switch on the link.
Each advertisement lists the switch IDs of those switches that
are fully adjacent to the designated switch. The designated
switch includes itself in this list.
To illustrate the format of a network link advertisement,
consider again the switch fabric shown in Figure 4. In this
example, network link advertisements will be generated only
switch SW4, the designated switch of the multi-access network
link between switches SW1 and SW4.
The network link advertisement generated by switch SW4 would
contain the following data items:
; network link advertisement for switch SW4
LS age = 0 ; always true on origination
Options = (T-bit|E-bit) ; options
LS type = 2 ; this is a switch link advertisement
; SW4s switch ID
Link State ID = 00-00-1d-4a-26-b3-00-00-00-00
Advertising switch = 00-00-1d-4a-26-b3-00-00-00-00
Attached switch = 00-00-1d-4a-26-b3-00-00-00-00
Attached switch = 00-00-1d-1f-05-81-00-00-00-00
Attached switch = 00-00-1d-4a-27-1c-00-00-00-00
Attached switch = 00-00-1d-7e-84-2e-00-00-00-00
(See Section 14.3 for a detailed description of the format of a
network link advertisement.)
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11.2 Distributing Link State Advertisements
Link state advertisements are distributed throughout the switch
fabric encapsulated within Link State Update packets. A single
Link State Update packet may contain several distinct
advertisements.
To make the distribution process reliable, each advertisement
must be explicitly acknowledged in a Link State Acknowledgment
packet. Note, however, that multiple acknowledgments can be
grouped together into a single Link State Acknowledgment
packet. A sending switch retransmits unacknowledged Link State
Update packets at regular intervals until they are
acknowledged.
The remainder of this section is structured as follows:
- Section 11.2.1 presents an overview of the distribution
process.
- Section 11.2.2 describes how an incoming Link State Update
packet is processed.
- Section 11.2.3 describes how a Link State Packet is
forwarded -- both by the originating switch and an
intermediate receiving switch.
- Section 11.2.4 describes how advertisements are installed
into the local database.
- Section 11.2.5 describes the retransmission of
unacknowledged advertisements.
- Section 11.2.6 describes how advertisements are acknowledged.
11.2.1 Overview
The philosophy behind the distribution of link state
advertisements is based on the concept of adjacencies -- that
is, each switch is only required to remain synchronized with
its adjacent neighbors.
When a switch originates a new instance of a link state
advertisement, it formats the advertisement into a Link State
Update packet and floods the packet out each fully operational
interface -- that is, each interface with a state greater than
Waiting. However, only those neighbors that are adjacent to
the sending switch need to process the packet.
The sending switch indicates which of its neighbor switches
should process the advertisement by specifying a particular
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multicast destination in the network layer address information
(see Section 13.2). The sending switch sets the value of the
network layer destination switch ID field according to the
state of the interface over which the packet is sent:
- If the interface state is Point-to-Point, DS, or Backup, the
switch is adjacent to all other switches on the link and all
neighboring switches must process the packet. Therefore,
the destination field is set to the multicast switch ID
AllSPFSwitches.
- If the interface state is DS Other, the switch is only
adjacent to the designated switch and the backup designated
switch and only those two neighboring switches must process
the packet. Therefore, the destination field is set to the
multicast switch ID AllDSwitches.
A similar logic is used when a switch receives a Link State
Update packet containing a new instance of a link state
advertisement. After processing and acknowledging the packet,
the receiving switch forwards the Link State Update packet as
follows:
- On the interface over which the original Link State Update
packet was received:
- If the receiving switch is the designated switch for the
attached network link, the packet is forwarded to all
other switches on the link. (The destination field is set
to AllSPFSwitches.) The originating switch will recognize
that it was the advertisement originator and discard the
packet.
- If the receiving switch is not the designated switch for
the attached network link, the packet is not sent back out
the interface over which it was received.
- On all other interfaces:
- If the receiving switch is the designated switch for the
attached network link, the packet is forwarded to all
switches on the link. (The destination field is set to
AllSPFSwitches.)
- If the receiving switch is neither the designated switch
or the backup designated switch for the attached network
link, the packet is forwarded only to the designated
switch and the backup designated switch. (The destination
field is set to AllDSwitches.)
Each Link State Update packet is forwarded and processed in
this fashion until all switches in the fabric have received
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notification of the new instance of the link state
advertisement.
11.2.2 Processing an Incoming Link State Update Packet
When the a Link State Update packet is received, it is first
subjected to a number of consistency checks. In particular,
the Link State Update packet is associated with a specific
neighbor. If the state of that neighbor is less than Exchange,
the entire Link State Update packet is discarded.
Each link state advertisement contained in the packet is
processed as follows:
1) Validate the advertisement's link state checksum and type.
If the checksum is invalid or the type is unknown, discard
the advertisement without acknowledging it.
2) If the advertisement's age is equal to MaxAge and there is
currently no instance of the advertisement in the local link
state database, then do the following:
a) Acknowledge the advertisement by sending a Link State
Acknowledgment packet to the sending neighbor (see Section
11.2.6).
b) Purge all outstanding requests for equal or previous
instances of the advertisement from the sending neighbor's
Link State Request list.
c) If the neighbor is Exchange or Loading, install the
advertisement in the link state database (see Section
11.2.4). Otherwise, discard the advertisement.
3) If the advertisements age is equal to MaxAge and there is
an instance of the advertisement in the local link state
database, then do the following:
a) If the advertisement is listed in the link state
retransmission list of any neighbor, remove the
advertisement from the retransmission list(s) and delete
the database copy of the advertisement.
b) Discard the received (MaxAge) advertisement without
acknowledging it.
4) If the advertisement's age is less than MaxAge, attempt to
locate an instance of the advertisement in the local link
state database. If there is no database copy of this
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advertisement, or the received advertisement is more recent
than the database copy (see Section 10.1.1), do the
following:
a) If there is already a database copy, and if the database
copy was installed less than MinLSInterval seconds ago,
discard the new advertisement without acknowledging it.
b) Otherwise, forward the new advertisement out some subset
of the local interfaces (see Section 11.2.3). Note
whether the advertisement was sent back out the receiving
interface for later use by the acknowledgment process.
c) Remove the current database copy from the Link state
retransmission lists of all neighbors.
d) Install the new advertisement in the link state database,
replacing the current database copy. (Note that this may
cause the routing table calculation to be scheduled. See
Section 12.) Timestamp the new advertisement with the
time that it was received to prevent installation of
another instance within MinLSInterval seconds.
e) Acknowledge the advertisement, if necessary, by sending a
Link State Acknowledgment packet back out the receiving
interface. (See Section 11.2.6.)
f) If the link state advertisement was initially advertised
by the local switch itself, advance the advertisement
sequence number and issue a new instance of the
advertisement. (Receipt of a newer instance of an
advertisement means that the local copy of the
advertisement is left over from before the last time the
switch was restarted.)
5) If the received advertisement is the same instance as the
database copy (as determined by the algorithm described in
Section 10.1.1), do the following:
a) If the advertisement is listed in the neighbors link
state retransmission list, the local switch is expecting
an acknowledgment for this advertisement. Treat the
received advertisement as an implied acknowledgment, and
remove the advertisement from the link state
retransmission list. Note this implied acknowledgment for
later use by the acknowledgment process (Section 11.2.6).
b) Acknowledge the advertisement, if necessary, by sending a
Link State Acknowledgment packet back out the receiving
interface. (See Section 11.2.6.)
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6) If the database copy of the advertisement is more recent
than the instance just received, do the following:
a) Determine whether the instance is listed in the neighbor
link state request list. If so, an error has occurred in
the database exchange process. Restart the database
exchange process by generating a neighbor BadLSReq event
for the sending neighbor and terminate processing of the
Link State Update packet.
b) Otherwise, generate an unusual event to network management
and discard the advertisement.
11.2.3 Forwarding Link State Advertisements
When a new instance of an advertisement is originated or after
an incoming advertisement has been processed, the switch must
decide over which interfaces and to which neighbors the
advertisement will be forwarded. In some instances, the switch
may decide not to forward the advertisement over a particular
interface because it is able to determine that the neighbors on
that attached link have or will receive the advertisement from
another switch on the link.
The decision of whether to forward an advertisement over each
of the switchs interfaces is made as follows:
1) Each neighboring switch attached to the interface is
examined to determine whether it should receive and process
the new advertisement. For each neighbor, the following
steps are executed:
a) If the neighbor state is less than Exchange, the neighbor
need not receive or process the new advertisement.
b) If the neighbor state is Exchange or Loading, examine the
link state request list associated with the neighbor. If
an instance of the new advertisement is on the list, the
neighboring switch already has an instance of the
advertisement. Compare the new advertisement to the
neighbor's copy:
- If the new advertisement is less recent, the neighbor
need not receive or process the new advertisement.
- If the two copies are the same instance, delete the
advertisement from the link state request list. The
neighbor need not receive or process the new
advertisement.[7]
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- Otherwise, the new advertisement is more recent.
Delete the advertisement from the link state request
list. The neighbor may need to receive and process the
new advertisement.
c) If the new advertisement was received from this neighbor,
the neighbor need not receive or process the
advertisement.
d) Add the new advertisement to the link state retransmission
list for the neighbor.
2) The switch must now decide whether to forward the new
advertisement out the interface.
a) If the link state advertisement was not added to any of
the link state retransmission lists for neighbors attached
to the interface, there is no need to forward the
advertisement out the interface.
b) If the new advertisement was received on this interface,
and it was received from either the designated switch or
the backup designated switch, there is no need to forward
the advertisement out the interface. Chances are all
neighbors on the attached network link have also received
the advertisement already.
c) If the new advertisement was received on this interface
and the state of the interface is Point-to-Point, there is
no need to forward the advertisement since the received
advertisement was originated by the neighbor switch.
d) If the new advertisement was received on this interface,
and the interface state is Backup -- that is, the switch
itself is the backup designated switch -- there is no need
to forward the advertisement out the interface. The
designated switch will distribute advertisements on the
attached network link.
e) Otherwise, the advertisement must be forwarded out the
interface.
To forward a link state advertisement, the switch first
increments the advertisements age by InfTransDelay seconds
to account for the transmission time over the link. The
switch then copies the advertisement into a Link State
Update packet
Forwarded advertisements are sent to all adjacent switches
associated with the interface. If the interface state is
Point-to-Point, DS, or Backup, the destination switch ID
field of the network layer address information is set to the
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multicast switch ID AllSPFSwitches. If the interface state
is DS Other, the destination switch ID field is set to the
multicast switch ID AllDSwitches.
11.2.4 Installing Link State Advertisements in the Database
When a new link state advertisement is installed into the link
state database, as the result of either originating or
receiving a new instance of an advertisement, the switch must
determine whether the routing table and best paths need to be
recalculated. To make this determination, do the following:
1) Compare the contents of the new instance with the contents
of the old instance (assuming the older instance is
available). Note that this comparison does not include any
data from the link state header. Differences in fields
within the header (such as the sequence number and checksum,
which are guaranteed to be different in different instances
of an advertisement) are of no consequence when deciding
whether or not to recalculate the routing table.
2) If there are no differences in the contents of the two
advertisement instances, there is no need to recalculate the
routing table.
3) Otherwise, the entire routing table must be recalculated,
starting with the best path calculations
Note also that the older instance of the advertisement must be
removed from the link state database when the new advertisement
is installed. The older instance must also be removed from the
link state retransmission lists of all neighbors.
11.2.5 Retransmitting Link State Advertisements
When a switch sends a link state advertisement to an adjacent
neighbor, it records the advertisement in the neighbors link
state retransmission list. To ensure the reliability of the
distribution process, the switch continues to periodically
retransmit the advertisements specified in the list until they
are acknowledged.
The interval timer used to trigger retransmission of the
advertisements is set to RxmtInterval seconds, as found in the
interface data structure. Note that if this value is too low,
needless retransmissions will ensue. If the value is too high,
the speed with which the databases synchronize across adjacencies
may be affected if there are lost packets.
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When the interval timer expires, entries in the retransmission
list are formatted into one or more Link State Update packets.
(Remember that multiple advertisements can fit into a single
Link State Update packet.) The age field of each advertisement
is incremented by InfTransDelay, as found in the interface data
structure, before the advertisement is copied into the outgoing
packet.
Link State Update packets containing retransmitted
advertisements are always sent directly to the adjacent switch.
That is, the destination field of the network layer addressing
information is set to the switch ID of the neighboring switch.
If the adjacent switch goes down, retransmissions will continue
until the switch failure is detected and the adjacency is torn
down by the VLSP Hello protocol. When the adjacency is torn
down, the link state retransmission list is cleared.
11.2.6 Acknowledging Link State Advertisements
Each link state advertisement received by a switch must be
acknowledged. In most cases, this is done by sending a Link
State Acknowledgment packet. However, acknowledgments can also
be done implicitly by sending Link State Update packets (see
step 4a of Section 11.2.2).
Multiple acknowledgments can be grouped together into a single
Link State Acknowledgment packet.
Sending an acknowledgment
Link State Acknowledgment packets are sent back out the
interface over which the advertisement was received. The
packet can be sent immediately to the sending neighbor, or it
can be delayed and sent when an interval timer expires.
- Sending delayed acknowledgments facilitates the formatting
of multiple acknowledgments into a single packet. This
enables a single packet to send acknowledgments to several
neighbors at once by using a multicast switch ID in the
destination field of the network layer addressing
information (see below). Delaying acknowledgments also
randomizes the acknowledgment packets sent by the multiple
switches attached to a multi-access network link.
Note that the interval used to time delayed acknowledgments
must be short (less than RxmtInterval) or needless
retransmissions will ensue.
Delayed acknowledgments are sent to all adjacent switches
associated with the interface. If the interface state is
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Point-to-Point, DS, or Backup, the destination field of the
network layer addressing information is set to the multicast
switch ID AllSPFSwitches. If the interface state is DS Other,
the destination field is set to the multicast switch ID
AllDSwitches.
- Immediate acknowledgments are sent directly to a specific
neighbor in response to the receipt of duplicate link state
advertisements. These acknowledgments are sent immediately
when the duplicate is received.
The method used to send a Link State Acknowledgment packet --
either delayed or immediate -- depends on the circumstances
surrounding the receipt of the advertisement, as shown in
Table 6. Note that switches with an interface state of Backup
send acknowledgments differently than other switches because
they play a slightly different role in the distribution
process (see Section 11.2.3).
Acknowledgment type by state
Circumstance Backup All others
Advertisement was None None
forwarded back out
receiving interface
Advertisement more Delayed if advert Delayed
recent than database received from DS,
copy, but was not else do nothing
forwarded back out
receiving interface
Advertisement was a Delayed if advert None
duplicate treated received from DS,
as an implied acknow- else do nothing
ledgment (step 4a of
Section 11.2.2)
Advertisement was a Immediate Immediate
duplicate not treated
as an implied acknow-
ledgment
Advertisement age Immediate Immediate
equal to MaxAge and
no current instance
found in database
Table 6: Sending Link State Acknowledgments
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Receiving an acknowledgment
When the a Link State Acknowledgment packet is received, it is
first subjected to a number of consistency checks. In
particular, the packet is associated with a specific neighbor.
If the state of that neighbor is less than Exchange, the entire
Link State Acknowledgment packet is discarded.
Each acknowledgment contained in the packet is processed as
follows:
- If the advertisement being acknowledged has an instance in
the link state retransmission list for the sending neighbor,
do the following:
- If the acknowledgment is for the same instance as that
specified in the list (as determined by the procedure
described in Section 10.1.1), remove the instance from the
retransmission list.
- Otherwise, log the acknowledgment as questionable.
11.3 Aging the Link State Database
Each link state advertisement has an age field, containing the
advertisements age, expressed in seconds. When the
advertisement is copied into a Link State Update packet for
forwarding out a particular interface, the age is incremented
by InfTransDelay seconds to account for the transmission time
over the link. An advertisement's age is never incremented
past the value MaxAge. Advertisements with an age of MaxAge are
not used to build the routing table or calculate best paths.
If a link state advertisements age reaches MaxAge, the switch
flushes the advertisement from the switch fabric by doing the
following:
- Originate a new instance of the advertisement with the age
field set to MaxAge. The distribution process will
eventually result in the advertisement being removed from
the retransmission lists of all switches in the fabric.
- Once the advertisement is no longer contained in the link
state retransmission list of any neighbor and no neighbor is
in a state of Exchange or Loading, remove the advertisement
from the local link state database.
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11.3.1 Premature Aging of Advertisements
A link state advertisement can be prematurely flushed from the
switch fabric by forcing its age to MaxAge and redistributing
the advertisement.
A switch that was previously the designated switch for a multi-
access network link but has lost that status due to a failover
to the backup designated switch prematurely ages the network
link advertisements it originated for the link.
Premature aging also occurs when an advertisement's sequence
number must wrap -- that is, when the current advertisement
instance has a sequence number of 0x7fffffff. In this
circumstance, the advertisement is prematurely aged so that the
next instance of the advertisement can be originated with a
sequence number of 0x80000001 and be recognized as the most
recent instance.
A switch may only prematurely age those link state
advertisements for which it is the advertising switch.
12. Calculating the Routing Table
Once an adjacency has been formed and the two switches have
synchronized their databases, each switch in the adjacency
builds its routing table and calculates the best path(s) to all
other switches in the fabric, using itself as the root of each
path. In this context, "best" path means that path with the
lowest total cost metric across all hops. If there are
multiple paths with the same (lowest) total cost metric, they
are all calculated. Best paths are stored in the area data
structure.
Paths are calculated using the well-known Dijkstra algorithm.
For a detailed description of this algorithm, the reader is
referred to [Perlman], or any of a number of standard textbooks
dealing with network routing.
Note that whenever there is a change in an adjacency
relationship, or any change that alters the topology of the
switch fabric, the routing table must be rebuilt and the best
paths recalculated.
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13. Protocol Packets
This section describes VLS protocol packets and link state
advertisements.
Note
All VLSP packets are encapsulated in a standard ISMP
packet, as described in Section 3. In the current
section, the term "packet" refers to the payload of
the ISMP packet -- that is, the ISMP message body.
It is understood that the packet format descriptions
that follow are preceded by the ISMP frame header and
ISMP packet header, as described in Sections 3.1 and
3.2.
There are five distinct VLSP packet types, as listed in Table 7.
Type Packet Name Function Section
1 Hello Discover/maintain
neighbors 13.5.1
2 Database Description Summarize database
contents 13.5.2
3 Link State Request Database download 13.5.3
4 Link State Update Database update 13.5.4
5 Link State Ack Flooding acknow-
ledgment 13.5.5
Table 7: VLSP Packet Types
Since it is important that the link state databases remain
synchronized throughout the switch fabric, processing of both
incoming and outgoing routing protocol packets should take
priority over ordinary data packets. Section 13.1 discusses
packet processing.
All VLSP packets begin with network layer addressing
information, described in Section 13.2, followed by a standard
header, described in Section 13.3.
With the exception of Hello packets, all VLSP packets deal with
lists of link state advertisements. The format of a link state
advertisement is described in Section 14.
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13.1 Packet Processing
Note that with the exception of Hello packets, VLSP packets are
sent only between adjacent neighbors. Therefore, all packets
travel a single hop.
VLSP does not support fragmentation and reassembly of packets.
Therefore, packets containing lists of link state
advertisements or advertisement headers must be formatted such
that they contain only as many advertisements or headers as
will fit within the size constraints of a standard ethernet
frame.
When a protocol packet is received by a switch, it must first
pass the following criteria before being accepted for further
processing:
- The checksum and protocol version number must be correct.
- The destination switch ID (as found in the network layer
address information) must be the switch ID of the receiving
switch, or one of the multicast switch IDs AllSPFSwitches or
AllDSwitches.
If the destination switch ID is the multicast switch ID
AllDSwitches, the state of the receiving interface must be
Point-to-Point, DS, or Backup.
- The source switch ID (as found in the network layer address
information) must not be that of the receiving switch.
(That is, locally originated packets should be discarded.)
At this point, if the packet is a Hello packet, it is accepted
for further processing.
Since all other packet types are only sent between adjacent
neighbors, the packet must have been sent by one of the
switch's active neighbors. If the source switch ID matches the
switch ID of one of the receiving switchs active neighbors (as
stored in the interface data structure associated with the
inport interface), the packet is accepted for further
processing. Otherwise, the packet is discarded.
13.2 Network Layer Address Information
As mentioned in Section 4.2.1, portions of the VLS protocol (as
derived from OSPF) are dependent on certain network layer
addresses -- in particular, the AllSPFSwitches and AllDSwitches
multicast addresses that drive the distribution of link state
advertisements throughout the switch fabric. In order to
facilitate the implementation of the protocol at the physical
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MAC layer, network layer address information is encapsulated in
the VSLP packets. This information immediately follows the
ISMP frame and packet header and immediately precedes the VLSP
packet header, as shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: frame header / ISMP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Unused (16 octets) :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 | |
+ Destination switch ID +
20 | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
24 | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
28 | |
+ Source switch ID +
32 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
36 | |
: VLSP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Destination switch ID
This 10-octet field contains the switch ID of the packet
destination. The value here is set as follows:
- Hello packets are addressed to the multicast switch ID
AllSPFSwitches.
- The designated switch and the backup designated switch
address initial Link State Update packets and Link State
Acknowledgment packets to the multicast switch ID
AllSPFSwitches.
- All other switches address initial Link State Update
packets and Link State Acknowledgment packets to the
multicast switch ID AllDSwitches.
- Retransmissions of Link State Update packets are always
addressed directly to the nonresponding switch.
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- Database Description packets and Link State Request are
always addressed directly to the other switch
participating in the database exchange process.
Source switch ID
This 10-octet field contains the switch ID of the sending
switch.
13.3 VLSP Packet Header
Every VLSP packet starts with a common 30-octet header. This
header, along with the data found in the network layer address
information, contains all the data necessary to determine
whether the packet should be accepted for further processing.
(See Section 13.1.)
The format of the VLSP header is shown below. Note that the
header starts at offset 36 of the ISMP message body, following
the network layer address information.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: frame header / ISMP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer address information :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
36 | Version # | Type | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
40 | |
+ Source switch ID +
44 | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
48 | | Area ID . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
52 | Area ID . . . | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
56 | Autype | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Authentication +
60 | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
64 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Version #
This 1-octet field contains the version number of the VLS
protocol to which this packet adheres. This document
describes VLSP Version 1.
Type
This 1-octet field contains the packet type. Possible
values are as follows:
1 Hello
2 Database Description
3 Link State Request
4 Link State Update
5 Link State Acknowledgment
Packet length
This 2-octet field contains the length of the protocol
packet, in bytes, calculated from the start of the VLSP
header, at offset 20 of the ISMP message body. If the
packet length is not an integral number of 16-bit words, the
packet is padded with an octet of zero (see the description
of the checksum field, below).
Switch ID
This 10-octet field contains the switch ID of the sending
switch.
Area ID
This 4-octet field contains the area identifier. Since VLSP
does not support multiple areas, the value here is always
zero.
Checksum
This 2-octet field contains the packet checksum value. The
checksum is calculated as the 16-bit one's complement of the
one's complement sum of all the 16-bit words in the packet,
beginning with the VLSP header, excluding the authentication
field. If the packet length is not an integral number of
16-bit words, the packet is padded with an octet of zero
before calculating the checksum.
AuType
This 2-octet field identifies the authentication scheme to
be used for the packet. Since authentication is not
supported by this version of VLSP, this field contains zero.
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Authentication
This 8-octet field is reserved for use by the authentication
scheme. Since authentication is not supported by this
version of VLSP, this field contains zeroes.
13.4 Options Field
Hello packets and Database Description packets, as well as link
state advertisements, contain a 1-octet options field. Using
this field, a switch can communicate its optional capabilities
to other VLSP switches. The receiving switch can then choose
whether or not to support those optional capabilities. Thus,
switches of differing capabilities potentially can be mixed
within a single VLSP routing domain.
Two optional capabilities are currently defined in the options
field: routing based on Type of Service (TOS) and support for
external routing beyond the local switch fabric. These two
capabilities are specified in the options field as shown below.
+-+-+-+-+-+-+-+-+
|0|0|0|0|0|0|E|T|
+-+-+-+-+-+-+-+-+
The options field
T-bit
The T-bit specifies the switchs Type of Service (TOS)
capability. If the T-bit is set, the switch supports
routing based on nonzero types of service.
E-bit
The E-bit specifies the switchs external routing
capability. If the E-bit is set, the switch supports
external routing.
Note
The current version of VLSP supports
neither of these capabilities. Therefore,
both the T-bit and the E-bit are clear and
the options field contains a value of zero.
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13.5 Packet Formats
This section contains detailed descriptions of the five VLS
protocol packets.
13.5.1 Hello Packets
Hello packets are sent periodically over all switch interfaces
in order to discover and maintain neighbor relationships.
Since all switches connected to a common network link must
agree on certain interface parameters, these parameters are
included in each Hello packet. A switch receiving a Hello
packet that contains parameters inconsistent with its own view
of the interface will not establish a neighbor relationship
with the sending switch.
The format of a Hello packet is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer addressing / VLSP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
66 | (unused - must be 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
70 | HelloInt | Options | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
74 | DeadInt |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
78 | |
+ Designated switch ID +
82 | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
86 | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
90 | |
+ Backup designated switch ID +
94 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
98 | |
+ +
: Neighbor list :
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Network layer addressing / VLSP header
This 66-octet field contains the network layer addressing
information and the standard VLS protocol packet header.
The packet header type field contains a value of 1.
HelloInt
This 2-octet field contains the interval, in seconds, at
which this switch sends Hello packets.
Options
This 1-octet field contains the optional capabilities
supported by the switch, as described in Section 13.4.
Priority
This 1-octet field contains the switch priority used in
selecting the designated switch and backup designated switch
(see Section 9.3.1). If the value here is zero, the switch
is ineligible to become the designated switch or the backup
designated switch.
DeadInt
This 4-octet field contains the length of time, in seconds,
that neighboring switches will wait before declaring the
interface down once they stop receiving Hello packets over
the interface. The value here is equal to the value of
SwitchDeadInterval, as found in the interface data
structure.
Designated switch
This 10-octet field contains the switch ID of the designated
switch for this network link, as currently understood by the
sending switch. This value is set to zero if the designated
switch selection process has not yet begun.
Backup designated switch
This 10-octet field contains the switch ID of the backup
designated switch for this network link, as currently
understood by the sending switch. This value is set to zero
if the backup designated switch selection process has not
yet begun.
Neighbor list
This variable-length field contains a list of switch IDs of
each switch from which the sending switch has received a
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valid Hello packet within the last SwitchDeadInterval
seconds.
13.5.2 Database Description Packets
Database Description packets are exchanged while an adjacency
is being formed between two neighboring switches and are used
to describe the contents of the topological database. For a
complete description of the database exchange process, see
Section 10.2.
The format of a Database Description packet is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer addressing / VLSP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
66 | (unused - must be 0) | Options | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
70 | Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
74 | |
+ +
: Link state advertisement headers :
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network layer addressing / VLSP header
This 66-octet field contains the network layer addressing
information and the standard VLS protocol packet header.
The packet header type field contains a value of 2.
Options
This 1-octet field contains the optional capabilities
supported by the switch, as described in Section Section
13.4.
Flags
This 1-octet field contains a set of bit flags that are used
to coordinate the database exchange process. The format of
this octet is as follows:
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+-+-+-+-+-+-+-+-+
|0|0|0|0|0|I|M|MS
+-+-+-+-+-+-+-+-+
I-bit (Init)
The I-bit is used to signal the start of the exchange. It
is set while the two switches negotiate the master/slave
relationship and the starting sequence number.
M-bit (More)
The M-bit is set to indicate that more Database
Description packets to follow.
MS-bit (Master/Slave)
The MS-bit is used to indicate which switch is the master
of the exchange. If the bit is set, the sending switch is
the master during the database exchange process. If the
bit is clear, the switch is the slave.
Sequence number
This 4-octet field is used to sequence the Database
Description packets during the database exchange process.
The two switches involved in the exchange process agree on
the initial value of the sequence number during the
master/slave negotiation. The number is then incremented
for each Database Description packet in the exchange.
To acknowledge each Database Description packet sent by the
master, the slave sends a Database Description packet that
echoes the sequence number of the packet being acknowledged.
Link state advertisement headers
This variable-length field contains a list of link state
headers that describe a portion of the masters topological
database. Each header uniquely identifies a link state
advertisement and its current instance. (See Section 14.1
for a detailed description of a link state advertisement
header.) The number of headers included in the list is
calculated implicitly from the length of the packet, as
stored in the VLSP packet header (see Section 13.3).
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13.5.3 Link State Request Packets
Link State Request packets are used to request those pieces of
the neighbor's database that the sending switch has discovered
(during the database exchange process) are more up-to-date than
instances in its own database. Link State Request packets are
sent as the last step in bringing up an adjacency. (See
Section 10.3.)
The format of a Link State Request packet is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer addressing / VLSP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
66 | Link state type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
70 | |
+ Link state ID +
74 | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
78 | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
82 | |
+ Advertising switch ID +
86 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
90 | |
: . . . :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network layer addressing / VLSP header
This 66-octet field contains the network layer addressing
information and the standard VLS protocol packet header.
The packet header type field contains a value of 3.
Link state type
This 4-octet field contains the link state type of the
requested link state advertisement, as stored in the
advertisement header.
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Link state ID
This 10-octet field contains the link state ID of the
requested link state advertisement, as stored in the
advertisement header.
Advertising switch
This 10-octet field contains the switch ID of advertising
switch for the requested link state advertisement, as stored
in the advertisement header.
Note that the last three fields uniquely identify the
advertisement, but not its instance. The receiving switch will
respond with its most recent instance of the specified
advertisement.
Multiple link state advertisements can be requested in a single
Link State Request packet by repeating the link state type, ID,
and advertising switch for each requested advertisement. The
number of advertisements requested is calculated implicitly
from the length of the packet, as stored in the VLSP packet
header.
13.5.4 Link State Update Packets
Link State Update packets are used to respond to a Link State
Request packet or to advertise a new instance of one or more
link state advertisements. Link State Update packets are
acknowledged with Link State Acknowledgment packets. For more
information on the use of Link State Update packets, see
Section 10 and Section 11.
The format of a Link State Update packet is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer addressing / VLSP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
66 | # advertisements |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
70 | |
+ +
: Link state advertisements :
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Network layer addressing / VLSP header
This 66-octet field contains the network layer addressing
information and the standard VLS protocol packet header.
The packet header type field contains a value of 4.
# advertisements
This 4-octet field contains the number of link state
advertisements included in the packet.
Link state advertisements
This variable-length field contains a list of link state
advertisements. For a detailed description of the different
types of link state advertisements, see Section 14.
13.5.5 Link State Acknowledgment Packets
Link State Acknowledgment Packets are used to explicitly
acknowledge one or more Link State Update packets, thereby
making the distribution of link state advertisements reliable.
(See Section 11.2.6.)
The format of a Link State Acknowledgment packet is shown
below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer addressing / VLSP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
66 | |
+ +
: Link state advertisement headers :
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network layer addressing / VLSP header
This 66-octet field contains the network layer addressing
information and the standard VLS protocol packet header.
The packet header type field contains a value of 5.
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Link state advertisement headers
This variable-length field contains a list of link state
headers that are being acknowledged by this packet. Each
header uniquely identifies a link state advertisement and
its current instance. (See Section 14.1 for a detailed
description of a link state advertisement header.) The
number of headers included in the list is calculated
implicitly from the length of the packet, as stored in the
VLSP packet header (see Section 13.3).
14. Link State Advertisement Formats
Link state advertisements are used to describe various pieces
of the routing topology within the switch fabric. Each switch
in the fabric maintains a complete set of all link state
advertisements generated throughout the fabric. (Section 11.1
describes the circumstances under which a link state
advertisement is originated. Section 11.2 describes how
advertisements are distributed throughout the switch fabric.)
This collection of advertisements, known as the link state (or
topological) database, is used to build the switchs routing
table and calculate a set of best paths to all other switches
in the fabric.
There are two types of link state advertisement, as listed in
Table 8.
Type Name Function Section
1 Switch link Lists all network links
advertisement attached to a switch 14.2
2 Network link Lists all adjacencies on
advertisement a network link 14.3
Table 8: Link State Advertisement Types
Each link state advertisement begins with a standard header,
described in Section 14.1.
14.1 Link State Advertisement Headers
All link state advertisements begin with a common 32-octet
header. This header contains information that uniquely
identifies the advertisement -- its type, link state ID, and
the switch ID of its advertising switch. Also, since multiple
instances of a link state advertisement can exist concurrently
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in the switch fabric, the header contains information that
permits a switch to determine which instance is the most recent
-- the age, sequence number and checksum.
The format of the link state advertisement header is shown
below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | Age | Options | LS Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
04 | |
+ Link state ID +
08 | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
16 | |
+ Advertising switch ID +
20 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
24 | Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
28 | Checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Age
This 2-octet field contains the time, in seconds, since this
instance of the link state advertisement was originated.
Options
This 1-octet field contains the optional capabilities
supported by the advertising switch, as described in Section
13.4.
LS type
This 1-octet field contains the type of the link state
advertisement. Possible values are:
1 Switch link advertisement
2 Network link advertisement
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Link state ID
This 10-octet field identifies the switch that originates
advertisements for the link. The content of this field
depends on the advertisement's type.
- For a switch link advertisement, this field contains the
switch ID of the originating switch
- For a network link advertisement, this field contains the
switch ID of the designated switch for the link
Note
In VLSP, the link state ID of an advertisement is
always the same as the advertising switch. This level
of redundancy results from the fact that OSPF uses
additional types of link state advertisements for
which the originating switch is not the advertising
switch.
Advertising switch
This 10-octet field contains the switch ID of the switch
that originated the link state advertisement.
Sequence number
This 4-octet field is used to sequence the instances of a
particular link state advertisement. The number is
incremented for each new instance.
Checksum
This 2-octet field contains the checksum of the complete
contents of the link state advertisement, excluding the age
field. The checksum used is commonly referred to as the
Fletcher checksum and is documented in [RFC905]. Note that
since this checksum is calculated for each separate
advertisement, a protocol packet containing lists of
advertisements or advertisement headers will contain
multiple checksum values.
Length
This 2-octet field contains the total length, in octets, of
the link state advertisement, including the header.
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14.2 Switch Link Advertisements
A switch link advertisement is used to describe all functioning
network links of a switch, including the cost of using each
link.
Each functioning switch in the fabric originates one, and only
one, switch link advertisement -- all of the switch's links
must be described in a single advertisement. A switch
originates its first switch link advertisement (containing no
links) when it first becomes functional. It then originates a
new instance of the advertisement each time any of its neighbor
states changes such that the contents of the advertisement
changes.
The format of a switch link advertisement is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Link state header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
32 | (unused - must be 0) | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
36 | |
+ Link ID +
40 | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
44 | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
48 | |
+ Link data +
52 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
56 | Link type | # TOS | TOS 0 metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
60 | |
: . . . :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link state header
This 32-octet field contains the standard link state
advertisement header. The type field contains a 1, and the
link state ID field contains the switch ID of the
advertising switch.
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# links
This 2-octet field contains the number of links described by
this advertisement. This value must be equal to the total
number of functioning network links attached to the switch.
Link ID
This 10-octet field identifies the other switch that
originates link state advertisements for the link, providing
a key for accessing other link state advertisements for the
link. The value here is based on the link type, as follow:
- For point-to-point links, this field contains the switch
ID of the neighbor switch connected to the other end of
the link.
- For multi-access links, this field contains the switch ID
of the designated switch for the link.
- For stub links, this field does not apply.
Link data
This 10-octet field contains additional data necessary to
build the routing table and calculate the set of best paths.
Typically, this field contains the interface ID of the link.
Link type
This 1-octet field contains the type of link being
described. Possible values are as follows:
1 Point-to-point link
2 Multi-access link
3 Stub link (unavailable for network traffic)
# TOS
This 1-octet field contains the number of nonzero type of
service metrics specified for the link. Since the current
version of VLSP does not support routing based on nonzero
types of service, this field contains a value of zero.
TOS 0 metric
This 2-octet field contains the cost of using this link for
the zero TOS. This value is expressed in the link state
metric and must be greater than zero.
Note that the last five fields are repeated for all functioning
network links attached to the advertising switch. If the
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interface state of attached link changes, the switch must
originate a new instance of the switch link advertisement.
14.3 Network Link Advertisements
A network link advertisement is originated by the designated
switch of each multi-access network link. The advertisement
describes all switches attached to the link that are currently
fully adjacent to the designated switch, including the
designated switch itself.
Network link advertisements are not generated for point-to-
point network links.
The format of a network link advertisement is show below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Link state header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
32 | (unused) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
36 | |
+ +
: Switch list :
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link state header
This 32-octet field contains the standard link state
advertisement header. The type field contains a 2, and the
link state ID field contains the switch ID of the designated
switch.
Switch list
The switch IDs of all switches attached to the network link
that are currently fully adjacent to the designated switch.
The designated switch includes itself in this list.
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15. Protocol Parameters
This section contains a compendium of the parameters used in
the VLS protocol.
15.1 Architectural Constants
Several VLS protocol parameters have fixed architectural
values. The name of each architectural constant follows,
together with its value and a short description of its
function.
AllSPFSwitches
The multicast switch ID to which Hello packets and certain
other protocol packets are addressed, as specified in the
destination switch ID field of the network layer address
information (see Section 13.2). The value of AllSPFSwitches
is E0-00-00-05-00-00-00-00.
AllDSwitches
The multicast switch ID to which Link State Update packets
and Link State Acknowledgment packets are addressed, as
specified in the destination switch ID field of the network
layer address information (see Section 13.2), when they are
destined for the designated switch or the backup designated
switch of a network link. The value of AllDSwitches is
E0-00-00-06-00-00-00-00.
LSRefreshTime
The interval at which the routing table is rebuilt and the
set of best paths recalculated if no other state changes
have forced a recalculation. The value of LSRefreshTime is
set to 1800 seconds (30 minutes).
MinLSInterval
The minimum time between distinct originations of any
particular link state advertisement. The value of
MinLSInterval is set to 5 seconds.
MaxAge
The maximum age that a link state advertisement can attain.
When an advertisement's age reaches MaxAge, it is
redistributed throughout the switch fabric. When the
originating switch receives an acknowledgment for the
advertisement, indicating that the advertisement has been
removed from all neighbor Link state retransmission lists,
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the advertisement is removed from the originating switchs
database. Advertisements having age MaxAge are not used in
the routing table calculation. The value of MaxAge must be
greater than LSRefreshTime. The value of MaxAge is set to
3600 seconds (1 hour).
MaxAgeDiff
The maximum time disparity in ages that can occur for a
single link state instance as it is distributed throughout
the switch fabric. Most of this time is accounted for by
the time the advertisement sits on switch output queues (and
therefore not aging) during the distribution process. The
value of MaxAgeDiff is set to 900 seconds (15 minutes).
LSInfinity
The link state metric value indicating that the destination
is unreachable. It is defined to be a binary value of all
ones.
15.2 Configurable Parameters
Many of the switch interface parameters used by VLSP may be
made configurable if the implementer so desires. These
parameters are listed below. Sample default values are given
for some of the parameters.
Note that some of these parameters specify properties of the
individual interfaces and their attached network links. These
parameters must be consistent across all the switches attached
to that link.
Interface output cost(s)
The cost of sending a packet over the interface, expressed
in the link state metric. This is advertised as the link
cost for this interface in the switch's switch link
advertisement. The interface output cost must always be
greater than zero.
RxmtInterval
The number of seconds between link state advertisement
retransmissions for adjacencies established on this
interface. This value is also used when retransmitting
Database Description packets and Link State Request packets.
This value must be greater than the expected round-trip
delay between any two switches on the attached link.
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However, the value should be conservative or needless
retransmissions will result. A typical value for a local
area network would be 5 seconds.
InfTransDelay
The estimated number of seconds it takes to transmit a Link
State Update packet over this interface. Link state
advertisements contained in the Link State Update packet
must have their age incremented by this amount before
transmission. This value must take into account the
transmission and propagation delays for the interface and
must be greater than zero. A typical value for a local area
network would be 1 second.
Switch priority
An 8-bit unsigned integer. When two switches attached to
the same network link contend for selection as the
designated switch, the switch with the highest priority
takes precedence. If both switches have the same priority,
the switch with the highest base MAC address becomes the
designated switch. A switch whose switch priority is set to
zero is ineligible to become the designated switch on the
attached link.
HelloInterval
The length of time, in seconds, between the Hello packets
that the switch sends over the interface. This value is
advertised in the switch's Hello packets. It must be the
same for all switches attached to a common network link.
The smaller this value is set, the faster topological
changes will be detected. However, a smaller interval will
also generate more routing traffic. A typical value for a
local area network would be 10 seconds.
SwitchDeadInterval
The length of time, in seconds, that neighboring switches
will wait before declaring the interface down once they stop
receiving Hello packets over the interface. This value is
advertised in the switch's Hello packets. It must be the
same for all switches attached to a common network link and
should be some multiple of the HelloInterval parameter. A
typical value would be 4 times HelloInterval.
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Footnotes
[1]During calculation of the routing table, a network link
advertisement must be located based solely on its link state
ID. Note, however, that the lookup in this case is still well
defined, since no two network advertisements can have the same
link state ID.
[2]It is instructive to see what happens when the designated
switch for a network link fails. Call the designated switch
for the link S1 and the backup designated switch S2. If
switch S1 fails (or its interface to the link goes down), the
other switches on the link will detect S1's absence within
switchDeadInterval seconds. All switches may not detect this
condition at precisely the same time. The switches that
detect S1's absence before S2 does will temporarily select S2
as both designated switch and backup designated switch. When
S2 detects that S1 is down, it will move itself to designated
switch and a new backup designated switch is selected.
[3]Note that it is possible for a switch to resynchronize any
of its fully established adjacencies by setting the neighbor
state back to ExStart. This causes the switch on the other
end of the adjacency to process a SeqNumberMismatch event and
also revert to the ExStart state.
[4]When two advertisements have different checksum values, they
are assumed to be separate instances. This can occur when a
switch restarts and loses track of its previous sequence
number. In this case, since the two advertisements have the
same sequence number, it is not possible to determine which
advertisement is actually newer. If the wrong advertisement
is accepted as newer, the originating switch will originate
another instance.
[5]An instance of an advertisement is originated with an age of
MaxAge only when it is to be flushed from the database. This
is done either when the advertisement has naturally aged to
MaxAge, or (more typically) when the sequence number must
wrap. Therefore, a received instance with an age of MaxAge
must be processed as the most recent in order to flush it
properly from the database.
[6]MaxAgeDiff is an architectural constant that defines the
maximum disparity in ages, in seconds, that can occur for a
single link state instance as it is distributed throughout the
switch fabric. If two advertisements differ by more than this
amount, they are assumed to be different instances of the same
advertisement. This can occur when a switch restarts and
loses track of its previous sequence number.
L. Kane, et. al. [Page 91]
I/D VLS Protocol Specification May 1997
[7]This is how the link state request list is emptied, causing
the neighbor state to change to Full.
References
[Perlman] Perlman, Radia. Interconnections: Bridges and
Routers. Addison-Wesley Publishing Company. 1992.
[RFC905] McKenzie, A., ISO Transport Protocol specification
ISO DP 8073. April 1984.
[RFC1583] Moy, J. OSPF Version 2. March 1994.
[RFC1700] Reynolds, S.J., Postel, J. Assigned Numbers.
October 1994.
[RFCxxxx] Dobbins, K., et. al. ARLD Protocol Specification
[RFCxxxx] Dobbins, K., et. al. LSMP Protocol Specification
[RFCxxxx] Dobbins, K., et. al. SBCD Protocol Specification
[RFCxxxx] Dobbins, K., et. al. SFCT Protocol Specification
[RFCxxxx] Dobbins, K., et. al. SNDM Protocol Specification
Security Considerations
Security issues are not discussed in this document.
Authors Addresses
Cabletron Systems, Inc., is located at:
Post Office Box 5005
Rochester, NH 03866-5005
(603) 332-9400
Laura Kane Email: lkane@ctron.com
Kurt Dobbins Email: dobbins@ctron.com
Rich Soczewinski Email: soczew@ctron.com
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