Network Working Group                                         B Decraene
Internet Draft                                                JL Le Roux
Document: draft-decraene-mpls-ldp-interarea-00.txt        France Telecom
Expiration Date: January 2006                                  July 2005

                   LDP extensions for Inter-Area LSP

Status of this Memo

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   To facilitate the establishment of Label Switched Paths (LSP) that
   would span multiple IGP areas in a given autonomous system, 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",
   document are to be interpreted as described in RFC-2119 [1].

2. Introduction

   Link state IGP such as OSPF [RFC 2328] and IS-IS [RFC-1195] allow the
   partition of an autonomous system into areas or levels to increase
   routing scalability within a domain.

   However, LDP mandates ([LDP] section "Label Mapping Messages
   Procedures") that the IP address of the FEC Element *exactly* match
   an entry in the IP RIB. 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

   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.

   The new label mapping procedure described in this document allows IP
   prefixes aggregation on the Area Border Routers and therefore eases
   the use of LDP in a multi-area/level context and restores the
   advantages introduced by IGP hierarchy.

3. Label Mapping Procedure

3.1. Control plane behavior

   This document defines a new label mapping procedure for LDP. It MUST
   be possible to activate/deactivate this procedure by configuration.

   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 the 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

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   Note also that this label mapping procedure only applies to an
   ordered label distribution control mode.

3.2. Populating Forwarding Tables

   The new label mapping procedure MUST only populates the Next Hop
   Label Forwarding Entry (NHLFE) and the Incoming Label Map (ILM)
   defined in [MPLS] and sometimes referred to as Label Forwarding
   Information Base (LFIB).

   This allows the establishment of multi-area LDP LSPs, without
   requiring any leaking of /32 loopback addresses across area

   The new label mapping procedure MUST neither populate the IP
   Forwarding Information Base (FIB) nor the FEC-to-NHLFE Map (FTN)
   defined in [MPLS]. FTN MUST only be populated by procedures described
   in [LDP] where the FEC exactly match an entry in the routing table.

   Any protocol using LSP SHOULD be aware of these inter-area LSPs and
   SHOULD treat them like any other LSP.  In particular (MP-)BGP SHOULD
   consider these LSP to route prefixes advertised by a BGP next-hop
   whose address correspond to the LSP FEC.

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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 |
      |          |                           |             |
      |PE4      ABR2                        ABR1       PE2 |
      |          |        P3                 |             |
      |          |                           |         PE3 |
      +----------+                           +-------------+
                 |                           |

     Figure 1: An IGP domain with two areas attached to the Backbone

   Note that this applies equally to ISIS and OSPF. An ABR refers here
   either to an OSPF ABR or to an ISIS L1/L2 node.

   All routers are MPLS enabled and MPLS connectivity is required
   between all PE routers.

   In the "egress" area "C", the records available are:
   IGP RIB                          LDP FEC elements:                                    

   The area border router ABR1 advertises in the backbone area:
     - the aggregated IP prefix in the IGP
     - all the individual IP FEC elements (/32) in LDP

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   In "backbone" area "B", the records available are:
   IGP RIB                          LDP FEC elements:            

   The area border router ABR2 advertises in the area "A":
     - a default IP route (implicit or explicit) 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:              

   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

                    +--------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 check the connectivity.

   The current [LDP] specification would require the configuration and
   the maintenance of one IP route per egress LER and per outgoing

   The behavior described in this document would only require one IP
   route per outgoing interface.

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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

   Introducing IGP Areas have a mixed impact regarding convergence time.
   On one hand, it may increase convergence time as the ABR needs to
   process IGP topology messages and generates new ones. This is likely
   to require more time on the ABRs than simply flooding IGP messages.
   On the other hand, it reduces the network graph size in the IGP
   database of each router thus decrease computing time.

   Performing IP aggregation on the ABR:
     - requires careful IP addresses allocations to allow aggregation;
     - can lead to sub-optimal routing;
     - improves converge time as less entries need to be updated in the

   In case of an egress LER failure in an area, the new procedure
   described in this document will increase the convergence time. As IP
   aggregation is performed, the IGP will not advertise the failure in
   remote areas. Therefore, the ingress LER will have to wait until the
   reception of the LDP withdraw message. As LDP messages are processed
   on each node before being propagated, information propagation is
   likely to be slower than with IGP messages which are flooded.

   All in all, the convergence time is expected to be improved for link
   and P (LSR) node failures because FIBs have fewer entries to update.
   In case of a LER failure, the impact on convergence time is likely to
   be implementation dependant (IGP versus LDP processing) and network
   dependant (diameter size versus number of IP prefixes removed).

6. Security Considerations

   The new 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

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     [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 Ina Minei, 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

   Jean-Louis Le Roux
   France Telecom
   2, avenue Pierre-Marzin
   22307 Lannion Cedex

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11. Intellectual Property Considerations

   The IETF takes no position regarding the validity or scope of any
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   tain to the implementation or use of the technology described in this
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   related documentation can be found in BCP-11. Copies of claims of
   rights made available for publication and any assurances of licenses
   to be made available, or the result of an attempt made to obtain a
   general license or permission for the use of such proprietary rights
   by implementors or users of this specification can be obtained from
   the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
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   this standard. Please address the information to the IETF Executive

   Disclaimer of Validity

   This document and the information contained herein are provided on an

   Full Copyright Statement

   Copyright (C) The Internet Society (2005). This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

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