One document matched: draft-serbest-l2vpn-vpls-mcast-00.txt
INTERNET DRAFT Y. Serbest
Internet Engineering Task Force SBC
Document: Marc Lasserre
draft-serbest-l2vpn-vpls-mcast-00.txt Rob Nath
October 2004 Riverstone
Category: Informational Vach Kompella
Expires: April 2005 Ray Qiu
Sunil Khandekar
Alcatel
Supporting IP Multicast over VPLS
Status of this memo
By submitting this Internet-Draft, we represent that any applicable
patent or other IPR claims of which we are aware have been
disclosed, or will be disclosed, and any of which we are aware have
been or will be disclosed, and any of which we become aware will be
disclosed in accordance with RFC 3668.
This document is an Internet-Draft and is in full conformance with
Sections 5 and 6 of RFC 3667 and Section 5 of RFC 3668.
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Abstract
In Virtual Private LAN Service (VPLS), the PE devices provide a
logical interconnect such that CE devices belonging to a specific
VPLS instance appear to be connected by a single LAN. A VPLS
solution performs replication for multicast traffic at the ingress
PE devices. When replicated at the ingress PE, multicast traffic
wastes bandwidth when 1. Multicast traffic is sent to sites with no
members, and 2. Pseudo wires to different sites go through a shared
path. This document is addressing the former by IGMP and PIM
snooping.
Conventions used in this document
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Internet Draft draft-serbest-l2vpn-vpls-mcast-00.txt October, 2004
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119.
Table of Contents
1 Introduction......................................................3
2 Overview of VPLS..................................................3
3 Multicast Traffic over VPLS.......................................4
4 Constraining of IP Multicast in a VPLS............................5
4.1General Rules for IGMP/PIM Snooping in VPLS......................6
4.2IGMP Snooping for VPLS...........................................6
4.2.1 IGMP Join.................................................7
4.2.2 IGMP Leave...............................................11
4.2.3 Failure Scenarios........................................11
4.3PIM Snooping for VPLS...........................................12
4.3.1 PIM-DM...................................................13
4.3.2 PIM-SM...................................................17
4.3.3 PIM-SSM..................................................21
4.3.4 Bidirectional-PIM (BIDIR-PIM)............................23
5 Security Considerations..........................................27
6 References.......................................................27
6.1Normative References............................................28
6.2Informative References..........................................28
7 Authors' Addresses...............................................28
8 Intellectual Property Statement..................................29
9 Full copyright statement.........................................29
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1 Introduction
In Virtual Private LAN Service (VPLS), the Provider Edge (PE)
devices provide a logical interconnect such that Customer Edge (CE)
devices belonging to a specific VPLS instance appear to be connected
by a single LAN. Forwarding information base for particular VPLS
instance is populated dynamically by source MAC address learning.
This is a straightforward solution to support unicast traffic, with
reasonable flooding for unicast unknown traffic. Since a VPLS
provides LAN emulation for IEEE bridges as wells as for routers, the
unicast and multicast traffic need to follow the same path for
layer-2 protocols to work properly. As such, multicast traffic is
treated as broadcast traffic and is flooded to every site in the
VPLS instance.
VPLS solutions (i.e., [VPLS-LDP] and [VPLS-BGP]) perform replication
for multicast traffic at the ingress PE devices. When replicated at
the ingress PE, multicast traffic wastes bandwidth when: 1.
Multicast traffic is sent to sites with no members, 2. Pseudo wires
to different sites go through a shared path, and 3. Multicast
traffic is forwarded along a shortest path tree as opposed to the
minimum cost spanning tree. This document is addressing the first
problem by IGMP and PIM snooping. Using VPLS in conjunction with
IGMP and/or PIM snooping has the following advantages:
- It improves VPLS to support IP multicast efficiently (not
necessarily optimum, as there can still be bandwidth waste),
- It prevents sending multicast traffic to sites with no members,
- It keeps P routers in the core stateless,
- The Service Provider (SP) does not need to perform the tasks to
provide multicast service (e.g., running PIM, managing P-group
addresses, managing multicast tunnels etcą)
- The SP does not need to maintain PIM adjacencies with the
customers.
In this document, we describe the procedures for Internet Group
Management Protocol (IGMP) and Protocol Independent Multicast (PIM)
snooping over VPLS for efficient distribution of IP multicast
traffic.
2 Overview of VPLS
In case of VPLS, the PE devices provide a logical interconnect such
that CE devices belonging to a specific VPLS appear to be connected
by a single LAN. End-to-end VPLS consists of a bridge module and a
LAN emulation module ([L2VPN-FR]).
In a VPLS, a customer site receives Layer-2 service from the SP.
The PE is attached via an access connection to one or more CEs. The
PE performs forwarding of user data packets based on information in
the Layer-2 header, that is, MAC destination address. The CE sees a
bridge.
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The details of VPLS reference model, which we summarize here, can be
found in [L2VPN_FR]. In VPLS, the PE can be viewed as containing a
Virtual Switching Instance (VSI) for each L2VPN that it serves. A
CE device attaches, possibly through an access network, to a bridge
module of a PE. Within the PE, the bridge module attaches, through
an Emulated LAN Interface to an Emulated LAN. For each VPLS, there
is an Emulated LAN instance. The Emulated LAN consists of VPLS
Forwarder module (one per PE per VPLS service instance) connected by
pseudo wires (PW), where the PWs may be traveling through Packet
Switched Network (PSN) tunnels over a routed backbone. VSI is a
logical entity that contains a VPLS forwarder module and part of the
bridge module relevant to the VPLS service instance [L2VPN-FR].
Hence, the VSI terminates PWs for interconnection with other VSIs
and also terminates attachment circuits (ACs) for accommodating CEs.
A VSI includes the forwarding information base for a L2VPN [L2VPN-
FR] which is the set of information regarding how to forward Layer-2
frames received over the AC from the CE to VSIs in other PEs
supporting the same L2VPN service (and/or to other ACs), and
contains information regarding how to forward Layer-2 frames
received from PWs to ACs. Forwarding information bases can be
populated dynamically (such as by source MAC address learning) or
statically (e.g., by configuration). Each PE device is responsible
for proper forwarding of the customer traffic to the appropriate
destination(s) based on the forwarding information base of the
corresponding VSI.
3 Multicast Traffic over VPLS
In VPLS, if a PE receives a frame from an Attachment Circuit (AC)
with no matching entry in the forwarding information base for that
particular VPLS instance, it floods the frame to all other PEs
(which are part of this VPLS instance) and to directly connected ACs
(other than the one that the frame is received from). The flooding
of a frame occurs when:
- The destination MAC address has not been learned,
- The destination MAC address is a broadcast address,
- The destination MAC address is a multicast address.
Malicious attacks (e.g., receiving unknown frames constantly) aside,
the first situation is handled by VPLS solutions as long as
destination MAC address can be learned. After that point on, the
frames will not be flooded. A PE is REQUIRED to have safeguards,
such as unknown unicast limiting and MAC table limiting, against
malicious unknown unicast attacks.
There is no way around flooding broadcast frames. To prevent
runaway broadcast traffic from adversely affecting the VPLS service
and the SP network, a PE is REQUIRED to have tools to rate limit the
broadcast traffic as well.
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Similar to broadcast frames, multicast frames are flooded as well,
as a PE can not know where multicast members reside. Rate limiting
multicast traffic, while possible, should be should be done
carefully since several network control protocols relies on
multicast. For one thing, layer-2 and layer-3 protocols utilize
multicast for their operation. For instance, Bridge Protocol Data
Units (BPDUs) use an IEEE assigned all bridges multicast MAC
address, and OSPF is multicast to all OSPF routers multicast MAC
address. If the rate-limiting of multicast traffic is not done
properly, the customer network will experience instability and poor
performance. For the other, it is not straightforward to determine
the right rate limiting parameters for multicast.
A VPLS solution MUST NOT affect the operation of customer layer-2
protocols (e.g., BPDUs). Additionally, a VPLS solution MUST NOT
affect the operation of layer-3 protocols.
In the following section, we describe procedures to constrain the
flooding of IP multicast traffic in a VPLS.
4 Constraining of IP Multicast in a VPLS
The objective of improving the efficiency of VPLS for multicast
traffic that we are trying to optimize here has the following
constraints:
- The service is VPLS, i.e., a layer-2 VPN,
- In VPLS, ingress replication is required,
- There is no layer-3 adjacency (e.g., PIM) between a CE and a
PE.
Under these circumstances, the most obvious approach is
implementation of IGMP and PIM snooping in VPLS. Other multicast
routing protocols such as DVMRP and MOSPF are outside the scope of
this document.
Another approach to constrain multicast traffic in a VPLS is to
utilize point-multipoint LSPs (e.g., [PMP-RSVP-TE]). In such case,
one has to establish a point-multipoint LSP from a source PE (i.e.,
the PE to which the source router is connected to) to all other PEs
participating in the VPLS instance. In this case, if nothing is
done, all PEs will receive multicast traffic even if they donĘt have
any members hanging off of them. One can apply IGMP/PIM snooping,
but this time IGMP/PIM snooping should be done in P routers as well.
One can propose a dynamic way of establishing point-multipoint LSPs,
for instance by mapping IGMP/PIM messages to RSVP-TE signaling. One
should consider the effect of such approach on the signaling load
and on the delay between the time the join request received and the
traffic is received (this is important for IPTV application for
instance). This approach is outside the scope of this document.
Although, in some extremely controlled cases, such as a ring
topology of PE routers with no P routers or a tree topology, the
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efficiency of the replication of IP multicast can be improved. For
instance, spoke PWs of a hierarchical VPLS can be daisy-chained
together and some replication rules can be devised. These cases are
not expected to be common and will not be considered in this
document.
In the following sections, we provide some guidelines for the
implementation of IGMP and PIM snooping in VPLS.
4.1 General Rules for IGMP/PIM Snooping in VPLS
The following rules for the correct operation of IGMP/PIM snooping
MUST be followed.
Rule 1: IGMP and PIM messages forwarded by PEs MUST follow the
split-horizon rule for mesh PWs as defined in [VPLS-LDP].
Rule 2: IGMP/PIM snooping states in a PE MUST be per VPLS instance.
Rule 3: If a PE does not have any entry in a IGMP/PIM snooping state
for multicast group (*,G) or (S,G), the multicast traffic to that
group in the VPLS instance MUST be flooded.
Rule 4: A PE MUST support PIM mode selection per VPLS instance via
CLI and/or EMS. Another option could be to deduce the PIM mode from
RP address for a specific multicast group. For instance, a RP
address can be learned during the Designated Forwarder (DF) election
stage for Bidirectional-PIM.
4.2 IGMP Snooping for VPLS
IGMP is a mechanism to inform the routers on a subnet of a hostĘs
request to become a member of a particular multicast group. IGMP is
a stateful protocol. The router (i.e., the querier) regularly
verifies that the hosts want to continue to participate in the
multicast groups by sending periodic queries, transmitted to all
hosts multicast group (IP:224.0.0.1, MAC:01-00-5E-00-00-01) on the
subnet. If the hosts are still interested in that particular
multicast group, they respond with membership report message,
transmitted to the multicast group of which they are members. In
IGMPv1 [RFC1112], the hosts simply stop responding to IGMP queries
with membership reports, when they want to leave a multicast group.
IGMPv2 [RFC2236] adds a leave message that a host will use when it
needs to leave a particular multicast group. IGMPv3 [RFC3376]
extends the report/leave mechanism beyond multicast group to permit
joins and leaves to be issued for specific source/group (S,G) pairs.
In IGMP snooping, a PE snoops on the IGMP protocol exchange between
hosts and routers, and based on that restricts the flooding of IP
multicast traffic. In the following, we explore the mechanisms
involved in implementing IGMP snooping for VPLS. Please refer to
Figure 1 as an example of VPLS with IGMP snooping. In the figure,
Router 1 is the Querier. If multiple routers exist on a single
subnet (basically that is what a VPLS instance is), they can
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mutually elect a designated router (DR) that will manage all of the
IGMP messages for that subnet.
VPLS Instance
+------+ AC1 +------+ +------+ AC4 +------+
| Host |-----| PE |-------------| PE |-----|Router|
| 1 | | 1 |\ PW1to3 /| 3 | | 1 |
+------+ +------+ \ / +------+ +------+
| \ / |
| \ / |
| \ /PW2to3 |
| \ / |
PW1to2| \ |PW3to4
| / \ |
| / \PW1to4 |
| / \ |
| / \ |
+------+ +------+ / \ +------+ +------+
| Host | | PE |/ PW2to4 \| PE | |Router|
| 2 |-----| 2 |-------------| 4 |-----| 2 |
+------+ AC2 +------+ +------+ AC5 +------+
|
|AC3
+------+
| Host |
| 3 |
+------+
Figure 1 Reference Diagram for IGMP Snooping for VPLS
4.2.1 IGMP Join
The IGMP snooping mechanism for joining a multicast group (for all
IGMP versions) works as follows:
- The Querier sends a membership query to all hosts
(IP:224.0.0.1, MAC:01-00-5E-00-00-01). The membership query
can be either general query or group specific query.
- PE 3 replicates the query message and forwards it to all PEs
participating in the VPLS instance (i.e., PE 1, PE 2, PE 4).
- PE 3 notes that there is already a directly connected Querier.
Basically, it keeps a state of [(*,G);Querier: AC4], if it is a
group specific query. It keeps a state of [(*,*);Querier:AC4],
if it is a general query.
- All PEs then forward the query to ACs which are part of the
VPLS instance.
- At this point all PEs learn the place of the Querier. For
instance, for PE 1 it is behind PW1to3, for PE 2 behind PW2to3,
for PE 3 behind AC4, for PE 4 behind PW3to4.
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- Suppose that all hosts (Host 1, Host 2, and Host 3) want to
participate in the multicast group.
- Host 2 first (for the sake of the example) sends a membership
report to the multicast group (e.g., IP: 239.1.1.1, MAC: 01-00-
5E-01-01-01), of which Host 2 wants to be a member.
- PE 2 replicates the membership report message and forwards it
to all PEs participating in the VPLS instance (i.e., PE 1, PE
3, PE 4).
- PE 2 notes that there is a directly connected host, which is
willing to participate in the multicast group and updates its
state to [(*,G);Querier:PW2to3;Hosts:AC2].
Guideline 1: A PE MUST NOT forward a membership report message to
ACs participating in the VPLS instance, unless it received query
message from that AC. This is necessary to avoid report
suppression for other members in order for the PEs to construct
correct states and to not have any orphan receiver hosts.
- PE 2 does not forward the membership report of Host 2 to Host
3.
- Per the guideline above, PE 1 does not forward the membership
report of Host 2 to Host 1.
- Per the guideline above, PE 3 does forward the membership
report of Host 2 to Router 1 (the Querier).
- PE 3 notes that there is a host in the VPLS instance, which is
willing to participate in the multicast group and updates its
state to [(*,G);Querier:AC4;Hosts:PW2to3] regardless of the
type of the query.
- LetĘs assume that Host 1 subsequently sends a membership report
to the same multicast group.
- PE 1 replicates the membership report message and forwards it
to all PEs participating in the VPLS instance (i.e., PE 2, PE
3, PE 4).
- PE 1 notes that there is a directly connected host, which is
willing to participate in the multicast group. Basically, it
keeps a state of [(*,G);Querier:PW1to3;Hosts:AC1,PW1to2].
- Per Guideline 1, PE 2 does not forward the membership report of
Host 1 to Host 2 and Host 3.
- PE 3 receives the membership report message of Host 1 and
checks its states. Per Guideline 1, it sends the report to
Router 1. It also updates its state to
[(*,G);Querier:AC4;Hosts:PW2to3,PW1to3].
- Now, Host 3 sends a membership report to the same multicast
group.
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- PE 2 updates its state to
[(*,G);Querier:PW2to3;Hosts:AC2,AC3,PW1to2]. It then floods the
report message to all PEs participating in the VPLS instance.
Per Guideline 1, only PE 3 forwards the membership report of
Host 3 to Router 1.
- At this point, all PEs have necessary states to ensure that no
multicast traffic will be sent to sites with no members.
The previous steps work the same way for all three (IGMPv1, IGMPv2,
and IGMPv3), when the query is general or source specific.
The group and source specific query for IGMPv3 is considered
separately below.
The IGMP snooping mechanism for joining a multicast group (for
IGMPv3) works as follows:
- The Querier sends a membership query to all hosts
(IP:224.0.0.1, MAC:01-00-5E-00-00-01). The membership query is
group and source specific query with a list of sources (e.g.,
S1, S2, ą, Sn).
- PE 3 replicates the query message and forwards it to all PEs
participating in the VPLS instance (i.e., PE 1, PE 2, PE 4).
- PE 3 notes that there is already a directly connected Querier.
Basically, it keeps a state of {[(S1,G);Querier: AC4],
[(S2,G);Querier: AC4], ą, [(Sn,G);Querier: AC4]}.
- All PEs then forward the query to ACs which are part of the
VPLS instance.
- At this point, all PEs learn the place of the Querier. For
instance, for PE 1 it is behind PW1to3, for PE 2 behind PW2to3,
for PE 3 behind AC4, for PE 4 behind PW3to4.
- Suppose that all hosts (Host 1, Host 2, and Host 3) want to
participate in the multicast group. Host 1 and Host 2 want to
subscribe to (Sn,G), and Host 3 wants to subscribe to (S3,G).
- Host 2 first (for the sake of the example) sends a membership
report message for (Sn,G) to IP address of 224.0.0.22 (MAC:01-
00-5E-00-00-16).
- PE 2 replicates the membership report message and forwards it
to all PEs participating in the VPLS instance (i.e., PE 1, PE
3, PE 4).
- PE 2 notes that there is a directly connected host, which is
willing to participate in the multicast group and updates its
state to {[(Sn,G);Querier:PW2to3;Hosts:AC2]}.
- Per Guideline 1, PE 2 does not forward the membership report of
Host 2 to Host 3.
- Per Guideline 1, PE 1 does not forward the membership report of
Host 2 to Host 1.
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- Per Guideline 1, PE 3 does forward the membership report of
Host 2 to Router 1 (the Querier).
- PE 3 notes that there is a host in the VPLS instance, which is
willing to participate in the multicast group. Basically, it
updates its state to {[(S1,G);Querier: AC4], [(S2,G);Querier:
AC4], ą, [(Sn,G);Querier: AC4;Hosts: PW2to3]}.
- LetĘs say Host 1 now sends a membership report to the same
multicast group.
- PE 1 replicates the membership report message and forwards it
to all PEs participating in the VPLS instance (i.e., PE 2, PE
3, PE 4).
- PE 1 notes that there is a directly connected host, which is
willing to participate in the multicast group. Basically, it
keeps a state of {[(Sn,G);Querier:PW1to3;Hosts:AC1,PW1to2]}.
- Per Guideline 1, PE 2 does not forward the membership report of
Host 1 to Host 2 and Host 3.
- PE 3 receives the membership report message of Host 1 and
checks its states. It updates its state to {[(S1,G);Querier:
AC4], [(S2,G);Querier: AC4], ą, [(Sn,G);Querier: AC4;Hosts:
PW2to3,PW1to3]}. It then forwards the membership report of Host
1 to Router 1 per Guideline 1.
- Finally, Host 3 sends a membership report to the same multicast
group (S3,G).
- PE 2 replicates the membership report message and forwards it
to all PEs participating in the VPLS instance (i.e., PE 1, PE
3, PE 4).
- Per Guideline 1, PE 2 does not forward the membership report of
Host 3 to Host 2.
- Per Guideline 1, PE 1 does not forward the membership report of
Host 3 to Host 1.
- Per Guideline 1, PE 3 does forward the membership report of
Host 3 to Router 1 (the Querier).
- PE 2 notes that there is a directly connected host, which is
willing to participate in the multicast group and updates its
state to {[(S3,G);Querier:PW2to3;Hosts:AC3] ,
[(Sn,G);Querier:PW2to3;Hosts:AC2,PW1to2]}.
- PE 3 receives the membership report message of Host 3 and
checks its states. It updates its state to {[(S1,G);Querier:
AC4], [(S2,G);Querier: AC4], [(S3,G);Querier:
AC4;Hosts:PW2to3], ą, [(Sn,G);Querier: AC4;Hosts:
PW2to3,PW1to3]}. It then forwards the membership report to the
Querier (Router 1).
At this point, all PEs have necessary states to not send multicast
traffic to sites with no members.
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Based on above description of IGMPv3 based snooping for VPLS, one
may conclude that the PEs MUST have the capability to store (S,G)
state and MUST forward/replicate traffic accordingly. This is,
however, not MANDATORY. A PE MAY only keep (*,G) based states
rather than on a per (S,G) basis with the understanding that this
will result in a less efficient IP multicast forwarding within each
VPLS instance.
Guideline 2: If a PE receives unsolicited report message and if it
does not possess a state for that particular multicast group, it
MUST flood that unsolicited membership report message to all PEs
participating in the VPLS instance, as well as to the Querier if it
is locally attached.
4.2.2 IGMP Leave
The IGMP snooping mechanism for leaving a multicast group works as
follows:
- In the case of IGMPv2/IGMPv3, when a PE receives a leave
(*,G)/(S,G) message from a host via its AC, first it removes
the AC from its state.
Guideline 3: A PE MUST NOT forward a leave (*,G)/(S,G) message to
ACs participating in the VPLS instance, If the PE still has
locally connected hosts or hosts connected over a H-VPLS spoke in
its state.
Guideline 4: A PE MUST forward a leave (*,G)/(S,G) message to all
PEs participating in the VPLS instance. A PE MAY forward the
leave (*,G)/(S,G) message to the Querier ONLY, if there are no
member hosts in its state.
Guideline 5: If a PE does not receive a membership report from an
AC for the three consecutive queries, the PE MUST remove the AC
from its state.
4.2.3 Failure Scenarios
Up to now, we did not consider any failures, which we will focus in
this section.
- In case the Querier fails (e.g., AC to the querier fails),
another router in the VPLS instance will be selected as the DR.
The new DR will be sending queries. In such circumstances, the
IGMP snooping states in the PEs will be updated/overwritten by
the same procedure explained above.
- In case a host fails (e.g., AC to the host fails), a PE removes
the host from its IGMP snooping state for that particular
multicast group. Guidelines 3, 4 and 5 still apply here.
- In case a PW (which is in IGMP snooping state) fails, the PEs
will remove the PW from their IGMP snooping state. For
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instance, if PW1to3 fails, then PE 1 will remove PW1to3 from
its state as the Querier connection, and PE 3 will remove
PW1to3 from its state as one of the host connections.
Guidelines 3, 4 and 5 still apply here. After PW is restored,
the IGMP snooping states in the PEs will be updated/overwritten
by the same procedure explained above. One can implement a
dead timer before making any changes to IGMP snooping states
upon PW failure. In that case, IGMP snooping states will be
altered if the PW can not be restored before the dead timer
expires.
4.3 PIM Snooping for VPLS
IGMP snooping procedures described above provide efficient delivery
of IP multicast traffic in a given VPLS service when end stations
are connected to the VPLS. However, when VPLS is offered as a WAN
service it is likely that the CE devices are routers and would run
PIM between them. To provide efficient IP multicasting in such
cases, it is necessary that the PE routers offering the VPLS service
do PIM snooping. This section describes the procedures for PIM
snooping.
PIM is a multicast routing protocol, which runs exclusively between
routers. PIM shares many of the common characteristics of a routing
protocol, such as discovery messages (e.g., neighbor discovery using
Hello messages), topology information (e.g., multicast tree), and
error detection and notification (e.g., dead timer and designated
router election). On the other hand, PIM does not participate in
any kind of exchange of databases, as it uses the unicast routing
table to provide reverse path information for building multicast
trees. There are a few variants of PIM. In PIM-DM ([PIM-DM]),
multicast data is pushed towards the members similar to broadcast
mechanism. PIM-DM constructs a separate delivery tree for each
multicast group. As opposed to PIM-DM, other PIM versions (PIM-SM
[RFC2362], PIM-SSM [PIM-SSM], and BIDIR-PIM [BIDIR-PIM]) invokes a
pull methodology instead of push technique.
PIM routers periodically exchange Hello messages to discover and
maintain stateful sessions with neighbors. After neighbors are
discovered, PIM routers can signal their intentions to join/prune
specific multicast groups. This is accomplished by having
downstream routers send an explicit join message (for the sake of
generalization, consider Graft messages for PIM-DM as join messages)
to the upstream routers. The join/prune message can be group
specific (*,G) or group and source specific (S,G).
In PIM snooping, a PE snoops on the PIM message exchange between
routers, and builds its multicast states. Based on the multicast
states, it forwards IP multicast traffic accordingly to avoid
unnecessary flooding.
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In the following, the mechanisms involved for implementing PIMv2
([RFC2362]) snooping in VPLS are specified. PIMv1 is out of the
scope of this document. Please refer to Figure 2 as an example of
VPLS with PIM snooping.
VPLS Instance
+------+ AC1 +------+ +------+ AC4 +------+
|Router|-----| PE |-------------| PE |-----|Router|
| 1 | | 1 |\ PW1to3 /| 3 | | 4 |
+------+ +------+ \ / +------+ +------+
| \ / |
| \ / |
| \ /PW2to3 |
| \ / |
PW1to2| \ |PW3to4
| / \ |
| / \PW1to4 |
| / \ |
| / \ |
+------+ +------+ / \ +------+ +------+
|Router| | PE |/ PW2to4 \| PE | |Router|
| 2 |-----| 2 |-------------| 4 |-----| 5 |
+------+ AC2 +------+ +------+ AC5 +------+
|
|AC3
+------+
|Router|
| 3 |
+------+
Figure 2 Reference Diagram for PIM Snooping for VPLS
In the following sub-sections, snooping mechanisms for each variety
of PIM are specified.
4.3.1 PIM-DM
The characteristics of PIM-DM is flood and prune behavior. Shortest
path trees are built as a multicast source starts transmitting.
In Figure 2, the multicast source is behind Router 4, and all
routers have at least one receiver except Router 3 and Router 5.
The PIM-DM snooping mechanism for neighbor discovery works as
follows:
- To establish PIM neighbor adjacencies, PIM multicast routers
(all routers in this example) send PIM Hello messages to the
ALL PIM Routers group address (IPv4: 224.0.0.13, MAC: 01-00-5E-
00-00-0D) on every PIM enabled interfaces. The IPv6 ALL PIM
Routers group is "ff02::d". In addition, PIM Hello messages
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are used to elect Designated Router for a multi-access network.
In PIM-DM, the DR acts as the Querier if IGMPv1 is used.
Otherwise, DR has no function in PIM-DM.
Guideline 6: PIM Hello messages MUST be flooded in the VPLS
instance. A PE MUST populate its "PIM Neighbors" list according
to the snooping results. This is a general PIM snooping guideline
and applies to all variants of PIM snooping.
Guideline 7: For PIM-DM only. The "Flood to" list is populated
with the ACs/PWs in the "PIM Neighbors" list. Changes to the "PIM
Neighbors" list MUST be replicated to the "Flood to" list.
- Every router starts sending PIM Hello messages. Per Guideline
6, every PE replicates Hello messages and forwards them to all
PEs participating in the VPLS instance.
- Based on PIM Hello exchanges PE routers populate PIM snooping
states as follows. PE 1: {[(,); Source:; Flood to: AC1,
PW1to2, PW1to3, PW1to4], [PIM Neighbors: (Router 1,AC1),
(Router 2,Router 3,PW1to2), (Router 4,PW1to3), (Router
5,PW1to4)] }, PE 2: {[(,); Source:; Flood to: AC2, AC3, PW1to2,
PW2to3, PW2to4], [PIM Neighbors: (Router 1,PW1to2), (Router
2,AC2), (Router 3,AC3), (Router 4,PW2to3), (Router 5,PW2to4)]},
PE 3: {[(,); Source:; Flood to: AC4, PW1to3, PW2to3, PW3to4],
[PIM Neighbors: (Router 1,PW1to3), (Router 2,Router 3,PW2to3),
(Router 4,AC4), (Router 5,PW3to4)]}, PE 4: {[(,); Source:;
Flood to: AC5, PW1to4, PW2to4, PW3to4], [PIM Neighbors: (Router
1,PW1to4), (Router 2,Router 3,PW2to4), (Router 4,PW3to4),
(Router 5,AC5)]}. The original "Flood to" list is populated
with ACs/PWs in the PIM neighbor list per Guideline 7..
- PIM Hello messages contain a Holdtime value, which tells the
receiver when to expire the neighbor adjacency (which is three
times the Hello period).
Guideline 8: If a PE does not receive a Hello message from a
router within its Holdtime, the PE MUST remove that router from
the PIM snooping state. If a PE receives a Hello message from a
router with Holdtime value set to zero, the PE MUST remove that
router from the PIM snooping state immediately. PEs MUST track
the Hello Holdtime value per PIM neighbor.
The PIM-DM snooping mechanism for multicast forwarding works as
follows:
- When the source starts sending traffic to multicast group
(S,G), PE 3 updates its state to PE 3: {[(S,G) ; Source:
(Router 4,AC4); Flood to: PW1to3, PW2to3, PW3to4], [PIM
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Neighbors: (Router 1,PW1to3), (Router 2,Router 3,PW2to3),
(Router 4,AC4), (Router 5,PW3to4)]}. AC4 is removed from the
"Flood to" list for (S,G), since it is where the multicast
traffic comes from.
Guideline 9: Multicast traffic MUST be replicated per PW and AC
basis, i.e., even if there are more than one PIM neighbor behind a
PW/AC, only one replication MUST be sent to that PW/AC.
- PE 3 replicates the multicast traffic and sends it to the other
PE routers in its "Flood to" list.
- Consequently, all PEs update their states as follows. PE 1:
{[(S,G); Source: (Router 4,PW1to3); Flood to: AC1], [PIM
Neighbors: (Router 1,AC1), (Router 2,Router 3,PW1to2), (Router
4,PW1to3), (Router 5,PW1to4)]}, PE 2: {[(S,G); Source: (Router
4,PW2to3); Flood to: AC2, AC3], [PIM Neighbors: (Router
1,PW1to2), (Router 2,AC2), (Router 3,AC3), (Router 4,PW2to3),
(Router 5,PW2to4)]}, PE 4: {[(S,G); Source: (Router 4,PW3to4);
Flood to: AC5], [PIM Neighbors: (Router 1,PW1to4), (Router
2,Router 3,PW2to4), (Router 4,PW3to4), (Router 5,AC5)]}.
At this point all the routers (Router 1, Router 2,Router 3, Router
5) receive the multicast traffic.
- However, Router 3 and Router 5 do not have any members for that
multicast group, so they send prune messages to leave the
multicast group to the ALL PIM Routers group. PE 2 updates its
state to PE 2: {[(S,G); Source: (Router 4,PW2to3); Flood to:
AC2], [PIM Neighbors: (Router 1,PW1to2), (Router 2,AC2),
(Router 3,AC3), (Router 4,PW2to3), (Router 5,PW2to4)]}. PE 4
also remove Router 3 and Router 5 from its state as well.
Guideline 10: PIM join and prune messages MUST be flooded in the
VPLS instance.
- PE 2 and PE 4 then flood the prune message and forward it to
all PEs participating in the VPLS instance per Guideline 10.
PE 4 updates its state to PE 4: {[(S,G); Source: (Router
4,AC4); Flood to:], [PIM Neighbors: (Router 1,PW1to4), (Router
2,Router 3,PW2to4), (Router 4,PW3to4), (Router 5,AC5)]}.
- PIM-DM prune messages contain a Holdtime value, which specifies
how many seconds the prune state should last.
Guideline 11: For PIM-DM only. A PE MUST keep the prune state for
a PW/AC according to the Holdtime in the prune message, unless a
corresponding Graft message is received.
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- Upon receiving the prune messages, each PE updates its state
accordingly. PE 1: {[(S,G); Source: (Router 4,PW1to3); Flood
to: AC1], [PIM Neighbors: (Router 1,AC1), (Router 2,Router 3,
PW1to2), (Router 4,PW1to3), (Router 5,PW1to4)]}, PE 2: {[(S,G);
Source: (Router 4,PW2to3); Flood to: AC2], [PIM Neighbors:
(Router 1,PW1to2), (Router 2,AC2), (Router 3,AC3), (Router
4,PW2to3), (Router 5,PW2to4)]}, PE 3: {[(S,G); Source: (Router
4,AC4); Flood to: PW1to3, PW2to3], [PIM Neighbors: (Router
1,PW1to3), (Router 2,Router 3,PW2to3), (Router 4,AC4), (Router
5, PW3to4)]}, PE 4: {[(S,G); Source: (Router 4,PW3to4); Flood
to:], [PIM Neighbors: (Router 1,PW1to4), (Router 2,Router
3,PW2to4), (Router 4,PW3to4), (Router 5,AC5)]}.
Guideline 12: To avoid overriding joins, a PE SHOULD suppress the
PIM prune messages to directly connected routers (i.e., ACs), as
long as there is a PW/AC in its corresponding "Flood to" list.
- In this case, PE 1, PE 2, and PE 3 do not forward the prune
messages to their directly connected routers.
The multicast traffic is now flowing only to points in the network
where receivers are present.
Guideline 13: For PIM-DM only. A PE MUST remove the AC/PW from
its corresponding prune state when it receives a graft message
from the AC/PW. That is, the corresponding AC/PW MUST be added to
the "Flood to" list.
Guideline 14: For PIM-DM only. PIM-DM graft messages MUST be
forwarded based on the destination MAC address. If the
destination MAC address is 01-00-5E-00-00-0D, then the graft
message MUST be flooded in the VPLS instance.
- For the sake of example, suppose now Router 3 has a receiver
the multicast group (S,G). Assuming Router 3 sends a graft
message in IP unicast to Router 4 to restart the flow of
multicast traffic. PE 2 updates its state to PE 2: {[(S,G);
Source: (Router 4,PW2to3); Flood to: AC2, AC3], [PIM Neighbors:
(Router 1,PW1to2), (Router 2,AC2), (Router 3,AC3), (Router
4,PW2to3), (Router 5,PW2to4)]}. PE 2 then forwards the graft
message to PE 3 according to Guideline 14.
- Upon receiving the graft message, PE 3 updates its state
accordingly to PE 3: {[(S,G); Source: (Router 4,AC4); Flood to:
PW1to3, PW2to3], [PIM Neighbors: (Router 1,PW1to3), (Router
2,Router 3,PW2to3), (Router 4,AC4), (Router 5,PW3to4)]}.
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Guideline 15: PIM Assert messages MUST be flooded in the VPLS
instance.
Guideline 16: If an AC/PW goes down, a PE MUST remove it from its
PIM snooping state.
Failures can be easily handled in PIM-DM snooping, as it uses push
technique. If an AC or a PW goes down, PEs in the VPLS instance
will remove it from their snooping state (if the AC/PW is not
already pruned). After the AC/PW comes back up, it will be
automatically added to the snooping state by PE routers, as all
PWs/ACs MUST be in the snooping state, unless they are pruned later
on.
4.3.2 PIM-SM
The key characteristics of PIM-SM is explicit join behavior. In
this model, the multicast traffic is only sent to locations that
specifically request it. The root node of a tree is the Rendezvous
Point (RP) in case of a shared tree or the first hop router that is
directly connected to the multicast source in the case of a shortest
path tree.
In Figure 2, the RP is behind Router 4, and all routers have at
least one member except Router 3 and Router 5.
The PIM-SM snooping mechanism for neighbor discovery works the same
way as t procedure defined in PIM-DM section, with the exception of
PIM-DM only guidelines.
- Based on PIM Hello exchanges PE routers populate PIM snooping
states as follows. PE 1: {[(,); Flood to:], [PIM Neighbors:
(Router 1,AC1), (Router 2,Router 3,PW1to2), (Router 4,PW1to3),
(Router 5,PW1to4)]}, PE 2: {[(,); Flood to:], [PIM Neighbors:
(Router 1,PW1to2), (Router 2,AC2), (Router 3,AC3), (Router
4,PW2to3), (Router 5,PW2to4)]}, PE 3: {[(,); Flood to:], [PIM
Neighbors: (Router 1,PW1to3), (Router 2,Router 3,PW2to3),
(Router 4,AC4), (Router 5,PW3to4)]}, PE 4: {[(,); Flood to:],
[PIM Neighbors: (Router 1,PW1to4), (Router 2,Router 3,PW2to4),
(Router 4,PW3to4), (Router 5,AC5)]}.
For PIM-SM to work properly, all routers within the domain must use
the same mappings of group addresses to RP addresses. Currently,
there are three methods for RP discovery: 1. Static RP
configuration, 2, Auto-RP, and 3. PIMv2 Bootstrap Router mechanism.
Guideline 17: Cisco RP-Discovery (IP:224.0.1.40, MAC:01-00-5E-00-
01-28), Cisco-RP-Announce (IP:224.0.1.39, MAC:01-00-5E-00-01-27),
all bootstrap router (BSR) (IP:224.0.0.13, MAC:01-00-5E-00-00-0D)
messages MUST be flooded in the VPLS instance.
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The PIM-SM snooping mechanism for joining a multicast group (*,G)
works as follows:
- Assume Router 1 wants to join the multicast group (*,G) sends a
join message for the multicsat group (*,G). PE 1 replicates
the join message and forwards it to all PE routers in the VPLS
instance.
Guideline 18: A PE MUST add a PW/AC to its (*,G) "Flood to" list,
if it receives a (*,G) join message from the PW/AC.
- PE 1 updates their states as follows: PE 1: {[(*,G); Flood to:
AC1], [PIM Neighbors: (Router 1,AC1), (Router 2,Router
3,PW1to2), (Router 4,PW1to3), (Router 5,PW1to4)]}.
A periodic refresh mechanism is used in PIM-SM to maintain the
proper state. PIM-SM join messages contain a Holdtime value, which
specifies for how many seconds the join state should be kept.
Guideline 19: If a PE does not receive a refresh join message from
a PW/AC within its Holdtime, the PE MUST remove the PW/AC from its
"Flood to" list.
- PE 1 floods the join message to all PEs in the VPLS instance
per Guideline 10.
- All PEs update their states accordingly as follows: PE 1:
{[(*,G); Flood to: AC1], [PIM Neighbors: (Router 1,AC1),
(Router 2,Router 3,PW1to2), (Router 4,PW1to3), (Router
5,PW1to4)]}, PE 2: {[(*,G); Flood to: PW1to2], [PIM Neighbors:
(Router 1,PW1to2), (Router 2,AC2), (Router 3,AC3), (Router
4,PW2to3), (Router 5,PW2to4)]}, PE 3: {[(*,G); Flood to:
PW1to3], [PIM Neighbors: (Router 1,PW1to3), (Router 2,Router
3,PW2to3), (Router 4,AC4), (Router 5,PW3to4)]}, PE 4: {[(*,G);
Flood to: PW1to4], [PIM Neighbors: (Router 1,PW1to4), (Router
2,Router 3,PW2to4), (Router 4,PW3to4), (Router 5,AC5)]}.
- After Router 2 joins the same multicast group, the states
become as follows: PE 1: {[(*,G); Flood to: AC1,PW1to2], [PIM
Neighbors: (Router 1,AC1), (Router 2,Router 3,PW1to2), (Router
4,PW1to3), (Router 5,PW1to4)]}, PE 2: {[(*,G); Flood to: AC2,
PW1to2], [PIM Neighbors: (Router 1,PW1to2), (Router 2,AC2),
(Router 3,AC3), (Router 4,PW2to3), (Router 5,PW2to4)]}, PE 3:
{[(*,G); Flood to: PW1to3, PW2to3], [PIM Neighbors: (Router
1,PW1to3), (Router 2,Router 3,PW2to3), (Router 4,AC4), (Router
5,PW3to4)]}, PE 4: {[(*,G); Flood to: PW1to4, PW2to4], [PIM
Neighbors: (Router 1,PW1to4), (Router 2,Router 3,PW2to4),
(Router 4,PW3to4), (Router 5,AC5)]}.
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- For the sake of example, Router 3 joins the multicast group.
PE 2 floods the join message to the VPLS instance (including
the Router 2 via AC2). In turn, PE routers forward the join
message to their directly connected routers. The states of the
PEs become as follows: PE 1: {[(*,G); Flood to: AC1,PW1to2],
[PIM Neighbors: (Router 1,AC1), (Router 2,Router 3,PW1to2),
(Router 4,PW1to3), (Router 5,PW1to4)]}, PE 2: {[(*,G); Flood
to: AC2, AC3, PW1to2], [PIM Neighbors: (Router 1,PW1to2),
(Router 2,AC2), (Router 3,AC3), (Router 4,PW2to3), (Router
5,PW2to4)]}, PE 3: {[(*,G); Flood to: PW1to3, PW2to3], [PIM
Neighbors: (Router 1,PW1to3), (Router 2,Router 3,PW2to3),
(Router 4,AC4), (Router 5,PW3to4)]}, PE 4: {[(*,G); Flood to:
PW1to4,PW2to4],[ PIM Neighbors: (Router 1,PW1to4), (Router
2,Router 3,PW2to4), (Router 4,PW3to4), (Router 5,AC5)]}.
- Next Router 5 joins the group, and the states are updated
accordingly.
At this point, all PEs have necessary states to not send multicast
traffic to sites with no members.
The PIM-SM snooping mechanism for leaving a multicast group works as
follows:
- Assume Router 5 sends a prune message.
Guideline 20: A PE MUST remove a PW/AC from its (*,G) "Flood to"
list if it receives a (*,G) prune message from the PW/AC. A
prune-delay timer SHOULD be implemented to support prune override.
- PE 4 floods the (*,G) prune to the VPLS instance. PE routers
participating in the VPLS instance also forward the (*,G) prune
to the ACs, which are connected to the VPLS instance. The
states are updated as follows: PE 1: {[(*,G); Flood to:
AC1,PW1to2], [PIM Neighbors: (Router 1,AC1), (Router 2,Router
3,PW1to2), (Router 4,PW1to3), (Router 5,PW1to4)]}, PE 2:
{[(*,G); Flood to: AC2, AC3, PW1to2], [PIM Neighbors: (Router
1,PW1to2), (Router 2,AC2), (Router 3,AC3), (Router 4,PW2to3),
(Router 5,PW2to4)]}, PE 3: {[(*,G); Flood to: PW1to3, PW2to3],
[PIM Neighbors: (Router 1,PW1to3), (Router 2,Router 3,PW2to3),
(Router 4,AC4), (Router 5,PW3to4)]}, PE 4: {[(*,G); Flood to:
AC5, PW1to4], [PIM Neighbors: (Router 1,PW1to4), (Router
2,Router 3,PW2to4), (Router 4,PW3to4), (Router 5,AC5)]}.
The PIM-SM snooping mechanism for source and group specific join
works as follows:
Guideline 21: A PE MUST add a PW/AC to its (S,G) "Flood to" list
if it receives a (S,G) join message from the PW/AC.
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Guideline 22: A PE MUST remove a PW/AC from its (S,G) "Flood to"
list if it receives a (S,G) prune message from the PW/AC. A
prune-delay timer SHOULD be implemented to support prune override.
Guideline 23: A PE MUST prefer (S,G) state to (*,G), if both S and
G match.
Guideline 24: When a (S,G) is state is first created, the initial
"Flood to" list MUST be populated by copying the "Flood to" list
from its parent (*,G) state.
Guideline 25: In case (*,G) state changes in a PE, all changes to
(*,G) state MUST (additions and deletions in "Flood to" list) be
replicated to (S,G) state.
- Now, we assume Router 5 sends a source and group specific join
(S,G). PE 4 floods the (S,G) join to the VPLS instance. PE
routers participating in the VPLS instance also forward the
(S,G) join to the ACs, which are connected to the VPLS
instance. The states are updated as follows: PE 1: {[(*,G);
Flood to: AC1,PW1to2],[(S,G); Flood to: AC1,PW1to2,PW1to4],
[PIM Neighbors: (Router 1,AC1), (Router 2,Router 3,PW1to2),
(Router 4,PW1to3), (Router 5,PW1to4)]}, PE 2: {[(*,G); Flood
to: AC2, AC3, PW1to2], [(S,G); Flood to:
AC2,AC3,PW1to2,PW2to4], [PIM Neighbors: (Router 1,PW1to2),
(Router 2,AC2), (Router 3,AC3), (Router 4,PW2to3), (Router
5,PW2to4)]}, PE 3: {[(*,G); Flood to: PW1to3, PW2to3], [(S,G);
Flood to: PW1to3,PW2to3,PW3to4], [PIM Neighbors: (Router
1,PW1to3), (Router 2,Router 3,PW2to3), (Router 4,AC4), (Router
5,PW3to4)]}, PE 4: {[(*,G); Flood to: AC5, PW1to4], [(S,G);
Flood to: AC5,PW1to4], [PIM Neighbors: (Router 1,PW1to4),
(Router 2,Router 3,PW2to4), (Router 4,PW3to4), (Router
5,AC5)]}.
- Afterwards, we assume Router 5 sends a (S,G)RP-bit prune, also
known as (S,G,RPT) prune. PE routers flood this prune message
and do not take any action.
As in the case with IGMPv3 snooping, we assume that the PEs have the
capability to store (S,G) states for PIM-SM snooping and
forward/replicate traffic accordingly. This is not mandatory. An
implementation, can fall back to (*,G) states, if its hardware can
not support it. In such case, the efficiency of multicast
forwarding will be less.
Failures can be easily handled in PIM-SM snooping, as it employs
state-refresh technique. PEs in the VPLS instance will remove any
entry for non-refreshing routers from their states.
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There are some special cases to consider for PIM-SM snooping. First
one is the RP-on-a-stick. The RP-on-a-stick scenario may occur when
the Shortest Path Tree and the Shared Tree shares a common Ethernet
segment, as all routers will be connected over a multicast access
network (i.e., VPLS). Such a scenario will be handled by PIM-SM
rules (particularly, the incoming interface can not also appear in
the outgoing interface list) very nicely. Second scenario is the
turnaround router. The turnaround router scenario occurs when
shortest path tree and shared tree share a common path. The router
at which these tree merge is the turnaround router. PIM-SM handles
this case by proxy (S,G) join implementation by the turnaround
router.
In PIM-SM snooping, prune messages are flooded by PE routers. In
such implementation, PE routers may receive overriding join
messages, which will not affect anything. A PE can do prune
suppression to optimize prune messages. Future versions will
include such a mechanism.
4.3.3 PIM-SSM
The key characteristics of PIM-SSM is explicit join behavior, but it
eliminates the shared tree and the rendezvous point in PIM-SM. In
this model, a shortest path tree for each (S,G) is built with the
first hop router (that is directly connected to the multicast
source) being the root node. PIM-SSM is ideal for one-to-many
multicast services.
In Figure 2, S1 is behind Router 1, and S4 is behind Router 4.
Routers 2 and 4 want to join (S1,G), while Router 5 wants to join
(S4,G).
The PIM-SSM snooping mechanism for neighbor discovery works the same
way as t procedure defined in PIM-DM section, with the exception of
PIM-DM only guidelines.
- Based on PIM Hello exchanges PE routers populate PIM snooping
states as follows. PE 1: {[(,); Flood to:], [PIM Neighbors:
(Router 1,AC1), (Router 2,Router 3,PW1to2), (Router 4,PW1to3),
(Router 5,PW1to4)]}, PE 2: {[(,); Flood to:], [PIM Neighbors:
(Router 1,PW1to2), (Router 2,AC2), (Router 3,AC3), (Router
4,PW2to3), (Router 5,PW2to4)]}, PE 3: {[(,); Flood to:], [PIM
Neighbors: (Router 1,PW1to3), (Router 2,Router 3,PW2to3),
(Router 4,AC4), (Router 5,PW3to4)]}, PE 4: {[(,); Flood to:],
[PIM Neighbors: (Router 1,PW1to4), (Router 2,Router 3,PW2to4),
(Router 4,PW3to4), (Router 5,AC5)]}.
PIM-SSM snooping is actually simpler than PIM-SM and only the
following guidelines (some of which are repetitions from PIM-SM
section) apply.
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Guideline 26: A PE MUST add a PW/AC to its (S,G) "Flood to" list
if it receives a (S,G) join message from the PW/AC.
Guideline 27: PIM join and prune messages MUST be flooded in the
VPLS instance.
Guideline 28: If A PE does not receive a refresh join message from
a PW/AC within its Holdtime, the PE MUST remove the PW/AC from its
"Flood to" list.
Guideline 29: A PE MUST remove a PW/AC from its (S,G) "Flood to"
list if it receives a (S,G) prune message from the PW/AC. A
prune-delay timer SHOULD be implemented to support prune override.
The PIM-SSM snooping mechanism for joining a multicast group works
as follows:
- Assume Router 2 requests to join the multicast group (S1,G).
- PE 2 updates its state, and then flood the join message in the
VPLS instance.
- All PEs update their states as follows: PE 1: {[(S1,G); Flood
to: PW1to2], [PIM Neighbors: (Router 1,AC1), (Router 2,Router
3,PW1to2), (Router 4,PW1to3), (Router 5,PW1to4)]}, PE 2:
{[(S1,G); Flood to: AC2], [PIM Neighbors: (Router 1,PW1to2),
(Router 2,AC2), (Router 3,AC3), (Router 4,PW2to3), (Router
5,PW2to4)]}, PE 3: {[(S1,G); Flood to: PW2to3], [PIM Neighbors:
(Router 1,PW1to3), (Router 2,Router 3,PW2to3), (Router 4,AC4),
(Router 5,PW3to4)]}, PE 4: {[(S1,G); Flood to: PW2to4], [PIM
Neighbors: (Router 1,PW1to4), (Router 2,Router 3,PW2to4),
(Router 4,PW3to4), (Router 5,AC5)]}.
- Next, assume Router 4 sends a join (S1,G) message. Following
the same procedures, all PEs update their states as follows:
PE 1: {[(S1,G); Flood to: PW1to2, PW1to3], [PIM Neighbors:
(Router 1,AC1), (Router 2,Router 3,PW1to2), (Router 4,PW1to3),
(Router 5,PW1to4)]}, PE 2: {[(S1,G); Flood to: AC2, PW2to3],
[PIM Neighbors: (Router 1,PW1to2), (Router 2,AC2), (Router
3,AC3), (Router 4,PW2to3), (Router 5,PW2to4)]}, PE 3: {[(S1,G);
Flood to: PW2to3, AC4], [PIM Neighbors: (Router 1,PW1to3),
(Router 2,Router 3,PW2to3), (Router 4,AC4), (Router
5,PW3to4)]}, PE 4: {[(S1,G); Flood to: PW2to4, PW3to4], [PIM
Neighbors: (Router 1,PW1to4), (Router 2,Router 3,PW2to4),
(Router 4,PW3to4), (Router 5,AC5)]}.
- Then, assume Router 5 requests to join the multicast group
(S4,G). After the same procedures are applied, all PEs update
their states as follows: PE 1: {[(S1,G); Flood to: PW1to2,
PW1to3], [(S4,G); Flood to: PW1to4], [PIM Neighbors: (Router
1,AC1), (Router 2,Router 3,PW1to2), (Router 4,PW1to3), (Router
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5,PW1to4)]}, PE 2: {[(S1,G); Flood to: AC2, PW2to3], [(S4,G);
Flood to: PW2to4], [PIM Neighbors: (Router 1,PW1to2), (Router
2,AC2), (Router 3,AC3), (Router 4,PW2to3), (Router 5,PW2to4)]},
PE 3: {[(S1,G); Flood to: PW2to3, AC4], [(S4,G); Flood to:
PW3to4], [PIM Neighbors: (Router 1,PW1to3), (Router 2,Router
3,PW2to3), (Router 4,AC4), (Router 5,PW3to4)]}, PE 4: {[(S1,G);
Flood to: PW2to4, PW3to4], [(S4,G); Flood to: AC5], [PIM
Neighbors: (Router 1,PW1to4), (Router 2,Router 3,PW2to4),
(Router 4,PW3to4), (Router 5,AC5)]}.
At this point, all PEs have necessary states to not send multicast
traffic to sites with no members.
The PIM-SSM snooping mechanism for leaving a multicast group works
as follows:
- Assume Router 2 sends a (S1,G) prune message to leave the
multicast group. The prune message gets flooded in the VPLS
instance. All PEs update their states as follows: PE 1:
{[(S1,G); Flood to: PW1to3], [(S4,G); Flood to: PW1to4], [PIM
Neighbors: (Router 1,AC1), (Router 2,Router 3,PW1to2), (Router
4,PW1to3), (Router 5,PW1to4)]}, PE 2: {[(S1,G); Flood to:
PW2to3], [(S4,G); Flood to: PW2to4], [PIM Neighbors: (Router
1,PW1to2), (Router 2,AC2), (Router 3,AC3), (Router 4,PW2to3),
(Router 5,PW2to4)]}, PE 3: {[(S1,G); Flood to: AC4], [(S4,G);
Flood to: PW3to4], [PIM Neighbors: (Router 1,PW1to3), (Router
2,Router 3,PW2to3), (Router 4,AC4), (Router 5,PW3to4)]}, PE 4:
{[(S1,G); Flood to: PW3to4], [(S4,G); Flood to: AC5], [PIM
Neighbors: (Router 1,PW1to4), (Router 2,Router 3,PW2to4),
(Router 4,PW3to4), (Router 5,AC5)]}.
We assume that the PEs have the capability to store (S,G) states for
PIM-SSM snooping and constrain multicast flooding scope accordingly.
An implementation, can fall back to (*,G) states, if its hardware
can not support it. In such case, the efficiency of multicast
forwarding will be less.
Similar to PIM-SSM snooping, failures can be easily handled in PIM-
SSM snooping, as it employs state-refresh technique. PEs in the
VPLS instance will remove entry for non-refreshing routers from
their states.
In PIM-SSM snooping, prune messages are flooded by PE routers.
However, a PE can do prune suppression to optimize prune messages.
Future versions might include such a mechanism.
4.3.4 Bidirectional-PIM (BIDIR-PIM)
BIDIR-PIM is a variation of PIM-SM. The main differences between
PIM-SM and Bidirectional-PIM are as follows:
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- There are no source-based trees, and source-specific multicast
is not supported (i.e., no (S,G) states) in BIDIR-PIM.
- Multicast traffic can flow up the shared tree in BIDIR-PIM.
- To avoid forwarding loops, one router on each link is elected
as the Designated Forwarder (DF) for each RP in BIDIR-PIM.
The main advantage of BIDIR-PIM is that it scales well for many-to-
many applications. However, the lack of source-based trees means
that multicast traffic is forced to remain on the shared tree.
In Figure 2, the RP for (*,G4) is behind Router 4, and the RP for
(*,G1) is behind Router 1. Router 2 and Router 4 want to join
(*,G1), whereas Router 5 wants to join (*,G4). On the VPLS
instance, Router 4 is the DF for the RP of (*,G4), and Router 1 is
the DF of the RP for (*,G1).
The PIM-SSM snooping mechanism for neighbor discovery works the same
way as t procedure defined in PIM-DM section, with the exception of
PIM-DM only guidelines.
- Based on PIM Hello exchanges PE routers populate PIM snooping
Based on PIM Hello exchanges PE routers populate PIM snooping
states as follows. PE 1: {[(,); Upstream to:; Downstream to:],
[PIM Neighbors: (Router 1,AC1), (Router 2,Router 3,PW1to2),
(Router 4,PW1to3), (Router 5,PW1to4)], [(,), DF:]}, PE 2:
{[(,); Upstream to:; Downstream to:], [PIM Neighbors: (Router
1,PW1to2), (Router 2,AC2), (Router 3,AC3), (Router 4,PW2to3),
(Router 5,PW2to4)], [(,), DF:]}, PE 3: {[(,); Upstream to:;
Downstream to:], [PIM Neighbors: (Router 1,PW1to3), (Router
2,Router 3,PW2to3), (Router 4,AC4), (Router 5,PW3to4)], [(,),
DF:]}, PE 4: {[(,); Upstream to:; Downstream to:], [PIM
Neighbors: (Router 1,PW1to4), (Router 2,Router 3,PW2to4),
(Router 4,PW3to4), (Router 5,AC5)], [(,), DF:]}.
For BIDIR-PIM to work properly, all routers within the domain must
know the address of the RP. There are three methods to do that: 1.
Static RP configuration, 2, Auto-RP, and 3. PIMv2 Bootstrap.
Guideline 17 applies here as well.
During RP discovery time, PIM routers elect DF per subnet for each
RP. The algorithm to elect the DF is as follows: all PIM neighbors
in a subnet advertise their unicast route to elect the RP and the
router with the best route is elected.
Guideline 30: All PEs MUST snoop the DF elections messages and
determine the DF for each [(*,G),RP)] pair. The "Upstream" list
MUST be updated with PW/AC that leads to the DF. When the DF
changes, the "Upstream" list MUST be updated accordingly.
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- Based on DF election messages, PE routers populate PIM snooping
states as follows: PE 1: {[(*,G1); Upstream to: AC1; Downstream
to:], [(*,G4); Upstream to: PW1to3; Downstream to:], [PIM
Neighbors: (Router 1,AC1), (Router 2,Router 3,PW1to2), (Router
4,PW1to3), (Router 5,PW1to4)], [(*,G1), DF: AC1], [(*,G4), DF:
PW1to4]}, PE 2: {[(*,G1); Upstream to: PW1to2; Downstream to:],
[(*,G4); Upstream to: PW2to3; Downstream to:], [PIM Neighbors:
(Router 1,PW1to2), (Router 2,AC2), (Router 3,AC3), (Router
4,PW2to3), (Router 5,PW2to4)], [(*,G1), DF:PW1to2], [(*,G4),
DF:PW2to3]}, PE 3: {[(*,G1); Upstream to: PW1to3; Downstream
to:], [(*,G4); Upstream to: AC4; Downstream to:], [PIM
Neighbors: (Router 1,PW1to3), (Router 2,Router 3,PW2to3),
(Router 4,AC4), (Router 5,PW3to4)], [(*,G1), DF: PW1to3],
[(*,G4), DF: AC4]}, PE 4: {[(*,G1); Upstream to: PW1to4;
Downstream to:], [(*,G4); Upstream to: PW3to4; Downstream to:],
[PIM Neighbors: (Router 1,PW1to4), (Router 2,Router 3,PW2to4),
(Router 4,PW3to4), (Router 5,AC5)], [(*,G1), DF: PW1to4],
[(*,G4), DF: PW3to4]}.
The BIDIR-PIM snooping for Join and Prune messages is similar to
PIM-SM and the following guidelines (some of which are repetitions
from PIM-SM section) apply.
Guideline 31: A PE MUST add a PW/AC to its (*,G) "Downstream" list
if it receives a (*,G) join message from the PW/AC.
Guideline 32: PIM join and prune messages MUST be flooded in the
VPLS instance.
Guideline 33: If A PE does not receive a refresh join message from
a PW/AC within its Holdtime, the PE MUST remove the PW/AC from its
"Downstream" list.
Guideline 34: A PE MUST remove a PW/AC from its (*,G) "Downstream"
list if it receives a (*,G) prune message from the PW/AC. A
prune-delay timer SHOULD be implemented to support prune override.
Guideline 35: A PE MUST replicate multicast traffic for (*,G) to
the members in its (*,G) "Upstream" and "Downstream" lists.
The BIDIR-PIM snooping mechanism for joining a multicast group works
as follows:
- Assume Router 2 wants to join the multicast group (*,G1). PE 2
floods the join message in the VPLS instance. All PEs update
their states as follows: PE 1: {[(*,G1); Upstream to: AC1;
Downstream to: PW1to2], [(*,G4); Upstream to: PW1to3;
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Downstream to:], [PIM Neighbors: (Router 1,AC1), (Router
2,Router 3,PW1to2), (Router 4,PW1to3), (Router 5,PW1to4)],
[(*,G1), DF: AC1], [(*,G4), DF: PW1to4]}, PE 2: {[(*,G1);
Upstream to: PW1to2; Downstream to: AC2], [(*,G4); Upstream to:
PW2to3; Downstream to:], [PIM Neighbors: (Router 1,PW1to2),
(Router 2,AC2), (Router 3,AC3), (Router 4,PW2to3), (Router
5,PW2to4)], [(*,G1), DF:PW1to2], [(*,G4), DF:PW2to3]}, PE 3:
{[(*,G1); Upstream to: PW1to3; Downstream to: PW2to3], [(*,G4);
Upstream to: AC4; Downstream to:], [PIM Neighbors: (Router
1,PW1to3), (Router 2,Router 3,PW2to3), (Router 4,AC4), (Router
5,PW3to4)], [(*,G1), DF: PW1to3], [(*,G4), DF: AC4]}, PE 4:
{[(*,G1); Upstream to: PW1to4; Downstream to: PW2to4], [(*,G4);
Upstream to: PW3to4; Downstream to:], [PIM Neighbors: (Router
1,PW1to4), (Router 2,Router 3,PW2to4), (Router 4,PW3to4),
(Router 5,AC5)], [(*,G1), DF: PW1to4], [(*,G4), DF: PW3to4]}.
- Next, assume Router 4 wants to join the multicast group (*,G1).
All PEs update their states as follows: PE 1: {[(*,G1);
Upstream to: AC1; Downstream to: PW1to2, PW1to3], [(*,G4);
Upstream to: PW1to3; Downstream to:], [PIM Neighbors: (Router
1,AC1), (Router 2,Router 3,PW1to2), (Router 4,PW1to3), (Router
5,PW1to4)], [(*,G1), DF: AC1], [(*,G4), DF: PW1to4]}, PE 2:
{[(*,G1); Upstream to: PW1to2; Downstream to: AC2, PW2to3],
[(*,G4); Upstream to: PW2to3; Downstream to:], [PIM Neighbors:
(Router 1,PW1to2), (Router 2,AC2), (Router 3,AC3), (Router
4,PW2to3), (Router 5,PW2to4)], [(*,G1), DF:PW1to2], [(*,G4),
DF:PW2to3]}, PE 3: {[(*,G1); Upstream to: PW1to3; Downstream
to: PW2to3, AC4], [(*,G4); Upstream to: AC4; Downstream to:],
[PIM Neighbors: (Router 1,PW1to3), (Router 2,Router 3,PW2to3),
(Router 4,AC4), (Router 5,PW3to4)], [(*,G1), DF: PW1to3],
[(*,G4), DF: AC4]}, PE 4: {[(*,G1); Upstream to: PW1to4;
Downstream to: PW2to4, PW3to4], [(*,G4); Upstream to: PW3to4;
Downstream to:], [PIM Neighbors: (Router 1,PW1to4), (Router
2,Router 3,PW2to4), (Router 4,PW3to4), (Router 5,AC5)],
[(*,G1), DF: PW1to4], [(*,G4), DF: PW3to4]}.
- Then, assume Router 5 wants to join the multicast group (*,G4).
Following the same procedures, all PEs update their states as
follows: PE 1: {[(*,G1); Upstream to: AC1; Downstream to:
PW1to2], [(*,G4); Upstream to: PW1to3; Downstream to:PW1to4],
[PIM Neighbors: (Router 1,AC1), (Router 2,Router 3,PW1to2),
(Router 4,PW1to3), (Router 5,PW1to4)], [(*,G1), DF: AC1],
[(*,G4), DF: PW1to4]}, PE 2: {[(*,G1); Upstream to: PW1to2;
Downstream to: AC2], [(*,G4); Upstream to: PW2to3; Downstream
to: PW2to4], [PIM Neighbors: (Router 1,PW1to2), (Router 2,AC2),
(Router 3,AC3), (Router 4,PW2to3), (Router 5,PW2to4)], [(*,G1),
DF:PW1to2], [(*,G4), DF:PW2to3]}, PE 3: {[(*,G1); Upstream to:
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PW1to3; Downstream to: PW2to3], [(*,G4); Upstream to: AC4;
Downstream to:PW3to4], [PIM Neighbors: (Router 1,PW1to3),
(Router 2,Router 3,PW2to3), (Router 4,AC4), (Router 5,PW3to4)],
[(*,G1), DF: PW1to3], [(*,G4), DF: AC4]}, PE 4: {[(*,G1);
Upstream to: PW1to4; Downstream to: PW2to4], [(*,G4); Upstream
to: PW3to4; Downstream to: AC5], [PIM Neighbors: (Router
1,PW1to4), (Router 2,Router 3,PW2to4), (Router 4,PW3to4),
(Router 5,AC5)], [(*,G1), DF: PW1to4], [(*,G4), DF: PW3to4]}.
At this point, all PEs have necessary states to not send multicast
traffic to sites with no members.
One example of the BIDIR-PIM snooping mechanism for leaving a
multicast group works as follows:
- Assume Router 2 wants to leave the multicast group (*,G1) and
sends a (*,G1) prune message. The prune message gets flooded
in the VPLS instance. All PEs update their states as follows:
PE 1: {[(*,G1); Upstream to: AC1; Downstream to: PW1to3],
[(*,G4); Upstream to: PW1to3; Downstream to:PW1to4], [PIM
Neighbors: (Router 1,AC1), (Router 2,Router 3,PW1to2), (Router
4,PW1to3), (Router 5,PW1to4)], [(*,G1), DF: AC1], [(*,G4), DF:
PW1to4]}, PE 2: {[(*,G1); Upstream to: PW1to2; Downstream to:
PW2to3], [(*,G4); Upstream to: PW2to3; Downstream to: PW2to4],
[PIM Neighbors: (Router 1,PW1to2), (Router 2,AC2), (Router
3,AC3), (Router 4,PW2to3), (Router 5,PW2to4)], [(*,G1),
DF:PW1to2], [(*,G4), DF:PW2to3]}, PE 3: {[(*,G1); Upstream to:
PW1to3; Downstream to: AC4], [(*,G4); Upstream to: AC4;
Downstream to:PW3to4], [PIM Neighbors: (Router 1,PW1to3),
(Router 2,Router 3,PW2to3), (Router 4,AC4), (Router 5,PW3to4)],
[(*,G1), DF: PW1to3], [(*,G4), DF: AC4]}, PE 4: {[(*,G1);
Upstream to: PW1to4; Downstream to: PW3to4], [(*,G4); Upstream
to: PW3to4; Downstream to: AC5], [PIM Neighbors: (Router
1,PW1to4), (Router 2,Router 3,PW2to4), (Router 4,PW3to4),
(Router 5,AC5)], [(*,G1), DF: PW1to4], [(*,G4), DF: PW3to4]}.
Once again, failures can be easily handled in BIDIR-PIM snooping, as
it employs state-refresh technique. PEs in the VPLS instance will
remove entry for non-refreshing routers from their states.
Optionally, prune suppression can be done on a PE to optimize prune
message handling. Future versions might include such a mechanism.
5 Security Considerations
Security considerations provided in VPLS solution documents (i.e.,
[VPLS-LDP] and [VPLS-BGP) apply to this document as well.
6 References
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6.1 Normative References
6.2 Informative References
[VPLS-LDP] Lasserre, M, et al. "Virtual Private LAN Services
over MPLS", work in progress
[VPLSD-BGP] Kompella, K, et al. "Virtual Private LAN Service",
work in progress
[L2VPN-FR] Andersson, L, et al. "L2VPN Framework", work in
progress
[PMP-RSVP-TE] Aggarwal, R, et al. "Extensions to RSVP-TE for
Point to Multipoint TE LSPs", work in progress
[RFC1112] Deering, S., "Host Extensions for IP Multicasting",
RFC 1112, August 1989.
[RFC2236] Fenner, W., "Internet Group Management Protocol,
Version 2", RFC 2236, November 1997.
[RFC3376] Cain, B., et al. "Internet Group Management
Protocol, Version 3", RFC 3376, October 2002.
[PIM-DM] Deering, S., et al. "Protocol Independent Multicast
Version 2 ū Dense Mode Specification", draft-ietf-
pim-v2-dm-03.txt, June 1999.
[RFC2362] Estrin, D, et al. "Protocol Independent Multicast-
Sparse Mode (PIM-SM): Protocol Specification", RFC
2362, June 1998.
[PIM-SSM] Holbrook, H., et al. "Source-Specific Multicast for
IP", work in progress
[BIDIR-PIM] Handley, M., et al. "Bi-directional Protocol
Independent Multicast (BIDIR-PIM)", work in
progress
7 Authors' Addresses
Yetik Serbest
SBC Labs
9505 Arboretum Blvd.
Austin, TX 78759
Yetik_serbest@labs.sbc.com
Marc Lasserre
Riverstone Networks
Marc@riverstonenet.com
Rob Nath
Riverstone Networks
5200 Great America Parkway
Santa Clara, CA 95054
Rnath@riverstonenet.com
Vach Kompella
Alcatel North America
701 East Middlefield Rd.
Mountain View, CA 94043
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Internet Draft draft-serbest-l2vpn-vpls-mcast-00.txt October, 2004
Ray Qiu
Alcatel North America
701 East Middlefield Rd.
Mountain View, CA 94043
Ray_Qiu@alcatel.com
Sunil Khandekar
Alcatel North America
701 East Middlefield Rd.
Mountain View, CA 94043
Sunil_khandekar@alcatel.com
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Copyright (C) The Internet Society (2004). This document is subject
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