Network Working Group                                     B Decraene
  Internet Draft                                            JL Le Roux
  Document: draft-decraene-mpls-ldp-interarea-02.txt    France Telecom
  Expiration Date: December 2006
                                                               I Minei
                                                Juniper Networks, Inc.

                                                             June 2006

                   LDP extensions for Inter-Area LSP

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   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",
   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 IS-IS 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 (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

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

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

   Note that this applies equally to IS-IS and OSPF. An ABR refers here
   either to an OSPF ABR or to an IS-IS 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:                                    

   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:            

   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:

   In this situation, one LSP is established between ingress PE4 and
   every egress PE of area C while maintaining IP prefixes aggregation
   on the ASBRs.

4.2.  Use of static routes

   Consider the following example where a LER is dual connected to two

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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

   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

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

   Ina Minei
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA 94089

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