One document matched: draft-decraene-mpls-ldp-interarea-01.txt
Differences from draft-decraene-mpls-ldp-interarea-00.txt
Network Working Group B Decraene
Internet Draft JL Le Roux
Document: draft-decraene-mpls-ldp-interarea-01.txt France Telecom
Expiration Date: April 2006
I Minei
Juniper Networks, Inc.
Proposed Status: Informational
October 2005
LDP extensions for Inter-Area LSP
Status of this Memo
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Abstract
To facilitate the establishment of Label Switched Paths (LSP) that
would span multiple IGP areas in a given Autonomous System (AS), this
document proposes a new optional label mapping procedure for the
Label Distribution Protocol (LDP).
This procedure allows the use of a label if the Forwarding
Equivalence Class (FEC) Element matches an entry in the routing table
(RIB). Matching is defined by an IP longest match search and does not
mandate an exact match.
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1. Conventions used in this document
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 [1].
IGP Area: OSPF Area or IS-IS level
ABR: OSPF Area Border Router or ISIS L1/L2 router
2. Introduction
Link state IGP such as OSPF [RFC 2328] and IS-IS [RFC-1195] allows
the partition of an autonomous system into areas or levels so as to
increase routing scalability within a routing domain.
However, LDP requires that the IP address of the FEC Element should
*exactly* match an entry in the IP RIB: according to [LDP] section
3.5.7.1 (Label Mapping Messages Procedures) "An LSR receiving a Label
Mapping message from a downstream LSR for a Prefix or Host Address
FEC Element should not use the label for forwarding unless its
routing table contains an entry that exactly matches the FEC Element.
Therefore, the establishment of MPLS LSPs between LERs across
areas/levels requires the redistribution of the exact (/32 for IPv4)
loopback addresses of all the LERs across all areas.
As a consequence, the potential benefits that a multi-area domain may
yield are diminished since the number of IP entries in the LSDB, RIB
and FIB maintained by every LSR of the domain (whatever the
area/level it belongs to) cannot be minimized.
Note however that IP prefixes and IGP events may still be reduced
since IP addresses of links are usually not redistributed outside of
their area.
In that context, this document extend the Label Mapping Procedure
defined in LDP so as to support the setup of inter-area LSPs while
maintaining IP prefixes aggregation on the ABRs. This basically
consists of extending the Label Mapping procedure so as to allow for
"Longest Match Based" Label Mapping.
3. Label Mapping Procedure
This document defines a new label mapping procedure for LDP. It MUST
be possible to activate/deactivate this procedure by configuration
and it SHOULD be deactivated by default. It MAY be possible to
activate it on a per prefix basis.
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With this new label mapping procedure, a LSR receiving a Label
Mapping message from a neighbor LSR for a Prefix Address FEC Element
SHOULD use the label for MPLS forwarding if its routing table
contains an entry that matches the FEC Element and the advertising
LSR is a next hop to reach the FEC. If so, it SHOULD advertise the
FEC Element and a label to its LDP peers.
By "matching FEC Element", one should understand an IP longest match.
Note that with this new Label Mapping Procedure, each LSP established
by LDP still strictly follows the shortest path(s) defined by the
IGP.
FECs selected by this "Longest Match" label mapping procedure
will be distributed in an ordered way. However this procedure is
applicable to both independent and ordered distribution control mode.
4. Application examples
4.1. Inter-area LSPs
Consider the following example of an autonomous system with one
backbone area and two edge areas:
Area "B"
Level 2 / Backbone area
+---------------------------+
Area "A" | | Area "C"
| |
Level 1 | | Level 1 / area
| P1 |
+----------+ +-------------+
| | P2 | PE1 | 10.0.0.1/32
| | | |
|PE4 ABR2 ABR1 PE2 | 10.0.0.2/32
| | P3 | |
| | | PE3 | 10.0.0.3/32
+----------+ +-------------+
| |
+---------------------------+
Figure 1: An IGP domain with two areas attached to the Backbone
Area.
Note that this applies equally to IS-IS and OSPF. An ABR refers here
either to an OSPF ABR or to an ISIS L1/L2 node.
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All routers are MPLS enabled and MPLS connectivity (LSP) is required
between all PE routers.
In the "egress" area "C", the records available are:
IGP RIB LDP FEC elements:
10.0.0.1/32 10.0.0.1/32
10.0.0.2/32 10.0.0.2/32
10.0.0.3/32 10.0.0.3/32
The area border router ABR1 advertises in the backbone area:
- the aggregated IP prefix 10.0.0/24 in the IGP
- all the individual IP FEC elements (/32) in LDP
In "backbone" area "B", the records available are:
IGP RIB LDP FEC elements:
10.0.0.0/24 10.0.0.1/32
10.0.0.2/32
10.0.0.3/32
The area border router ABR2 advertises in the area "A":
- an aggregated IP prefix 10.0/16 in the IGP
- all the individual IP FEC elements (/32) in LDP
In the "ingress" area "A", the records available are:
IGP RIB LDP FEC elements:
10.0/16 10.0.0.1/32
10.0.0.2/32
10.0.0.3/32
In this situation, one LSP is established between ingress PE4 and
every egress PE of area C.
4.2. Use of static routes
Consider the following example where a LER is dual connected to two
LSRs:
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+--------LSR1---- .
| |
LER |
| |
+--------LSR2---- .
Figure 2: LER dual connected to two LSRs.
In some situations, especially on the edge of the network, it is
valid to use static IP routes between the LER and the two LSRs. If
necessary, the BFD protocol can be used to quickly detect loss of
connectivity.
The current [LDP] specification would require on the ingress LER the
configuration and the maintenance of one IP route per egress LER and
per outgoing interface.
The new longest match Label Mapping Procedure described in this
document would only require one IP route per outgoing interface.
5. Caveats for deployment
5.1. Deployment consideration
LSRs compliant with this document are backward compatible with LSRs
that comply with [LDP].
For the successful establishment of end to end MPLS LSPs whose FEC
are aggregated in the RIB, this new behavior must be implemented on
all LSR in all areas where IP aggregation is used.
If all IP prefixes are leaked in the backbone area and only stub
areas use IP aggregation, LSRs in the backbone area don't need to be
compliant with this document.
5.2. Impact on routing convergence time
In case of an egress LER failure in an area, performing IP route
aggregation on ABRs will change the routing convergence behavior. The
IGP will not propagate the notification of the egress LER failure
outside of the egress area and failure notification will rely on LDP
signaling through the end to end propagation of the LDP withdraw
message. This failure notification may be faster or longer depending
on the implementations, the IGP timers used and the network topology
(network diameter).
For link and P (LSR) node failures, the failure notification is
unchanged and the convergence time is expected to be improved because
RIB and FIBs have fewer entries to update.
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6. Security Considerations
The "Longest Match" Label Mapping procedure described in this
document does not introduce any change as far as the Security
Consideration section of [LDP] is concerned.
7. Normative References
[LDP] L. Andersson, P. Doolan, N. Feldman, A. Fredette, B.
Thomas, "LDP Specification", RFC 3036, January 2001
[MPLS] E. Rosen, A. Viswanathan, R. Callon, " Multiprotocol
Label Switching Architecture", RFC 3031, January 2001
[MP-BGP] Bates, T., Rekhter, Y, Chandra, R. and D. Katz,
"Multiprotocol Extensions for BGP-4", RFC 2858, June 2000.
[BGP L3 VPN] E. Rosen, Y. Rekhter, "BGP/MPLS VPNs", RFC 2547,
March 1999
[3107] Y. Rekhter, E. Rosen, "Carrying Label Information in
BGP-4", RFC 3107, May 2001
[OSPF] J. Moy, "OSPF Version 2", RFC 1583, March 1994
[IS-IS] R. Callon, "Use of OSI IS-IS for Routing in TCP/IP and
Dual Environments", RFC 1195, December 1990
8. Informative Reference
[BGP L2 VPN] K. Kompella, Y. Rekhter "Virtual Private LAN
Service", draft-ietf-l2vpn-vpls-bgp-05.txt April 8, 2005, work
in progress.
9. Acknowledgments
Authors would like to thank Yakov Rekhter, Stefano Previdi, Benoit
Fondeviole, Gilles Bourdon and Christian Jacquenet for the useful
discussions on this subject, their review and comments.
10. Author's Addresses
Bruno Decraene
France Telecom
38-40 rue du General Leclerc
92794 Issy Moulineaux cedex 9
France
bruno.decraene@francetelecom.com
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Jean-Louis Le Roux
France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex
France
jeanlouis.leroux@francetelecom.com
Ina Minei
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
ina@juniper.net
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