Network Working Group                                     B Decraene
  Internet Draft                                            JL Le Roux
  Document: draft-decraene-mpls-ldp-interarea-04.txt    France Telecom
  Expiration Date: September 2007
                                                               I Minei
                                                Juniper Networks, Inc.

                                                            March 2007

                    LDP extension 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.

2.   Terminology

   IGP Area: OSPF Area or IS-IS level

   ABR: OSPF Area Border Router or IS-IS L1/L2 router

   LSP: Label Switched Path

   Intra-area LSP: LSP that does not traverse any IGP area boundary.

   Inter-area LSP: LSP that traverses at least one IGP area boundary.

3.   Introduction

   Link state IGPs such as OSPF [OSPFv2] and IS-IS [IS-IS] allow 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

   Therefore, MPLS LSPs between LERs in different areas/levels are not
   setup unless the exact (/32 for IPv4) loopback addresses of all the
   LERs are redistributed across all areas.

   The problem statement is discussed in section 3. Then, in section 4
   we extend the Label Mapping Procedure defined in [LDP] so as to
   support the setup of contiguous inter-area LSPs while maintaining IP
   prefix aggregation on the ABRs. This basically consists of allowing
   for "Longest Match Based" Label Mapping.

4.   Problem statement

   Provider based MPLS VPN networks are expanding with the success of
   Layer 3 VPN ([L3-VPN]) and the new deployments of layer 2 VPNs
   ([VPLS-BGP], [VPLS-LDP]). Service Provider MPLS backbones are
   significantly growing both in terms of density with the addition of
   PEs to connect new customers and in terms of footprint as traditional

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   layer two aggregation networks are being replaced by IP/MPLS
   networks. As a consequence many providers need to introduce IGP
   areas. Inter-area LSPs, that is LSPs that traverse at least two IGP
   areas are required to ensure MPLS connectivity between PEs located in
   distinct IGP areas.

   To set up the required MPLS LSPs between PEs in different IGP areas,
   services providers have currently three solutions: LDP with IGP route
   leaking, BGP [MPLS-BGP] over LDP with MPLS hierarchy, or also inter-
   area RSVP-TE [ID-RSVP-TE].

   IGP route leaking consists in redistributing all /32 PE loopback
   addresses across area boundaries. As a result, LDP finds in the RIB
   an exact match for its FEC and sets up the LSP.
   As a consequence, the potential benefits that a multi-area domain may
   yield are significantly diminished since a lot of addresses have to
   be redistributed by ABRs, and the number of IP entries in the LSDB
   and RIB maintained by every LSR of the domain (whatever the
   area/level it belongs to) cannot be minimized.

   Service providers may also set up these inter-area LSPs by using MPLS
   hierarchy with BGP [MPLS-BGP] as a label distribution protocol
   between areas. The BGP next hop would typically be the ABRs and the
   BGP-created LSPs would be nested within intra-area LSPs setup by LDP
   between PEs and ABRs and between ABRs.
   This solution is not adequate for Service Providers which don't want
   to run BGP on their P routers as it requires BGP on all ABRs. In
   addition, this scheme has an impact on the availability, as the
   recovery upon ABR failure relies on BGP convergence. Also MPLS
   hierarchy does not allow locally protecting the LSP against ABR
   failures (LDP Fast Reroute), and hence ensuring sub-50ms recovery
   upon ABR failure. The resulting convergence time may not be
   acceptable for stringent SLAs required for voice or mission critical
   applications. Finally, this solution requires a significant migration
   effort for Service Providers which started with LDP and IGP route
   leaking to quickly set-up the fist inter-area LSPs.

   Service providers may also setup these inter-area LSPs by using
   inter-area RSVP-TE [ID-RSVP-TE]. This is a relevant solution when
   RSVP-TE is already used for setting up intra-area LSPs, and inter-
   area traffic engineering features are required. In return this is not
   a desired solution when LDP is already used for setting up intra-area
   LSPs, and inter-area traffic engineering features are not required.

   To avoid the above drawbacks, there is a need for an LDP based
   solution which allows setting up contiguous inter-area LSPs while
   avoiding leaking of /32 PE loopback addresses across area boundaries,
   and hence keeping all the benefits of IGP hierarchy.

   In that context, this document defines a new LDP Label Mapping
   Procedure so as to support the setup of contiguous inter-area LSPs

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   while maintaining IP prefix aggregation on the ABRs. This procedure
   is similar to the one defined in [LDP] but performs a longest match
   when searching the FEC element in the RIB.

5.   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.

   With this new longest match 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.
   That is, either the LDP FEC element exactly matches an entry in the
   IP RIB or the FEC element is a subset of an IP RIB entry. There is no
   match for other cases such as the FEC element is a superset of a RIB

   Note that with this longest match 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.

   As per RFC 3036, LDP has already some interactions with the RIB. In
   particular, it needs to be aware of the following events:
     - prefix UP when a new IP prefix appears in the RIB
     - prefix DOWN when an existing prefix disappears
     - next-hop change when an existing prefix have new next hop
        following a routing change.

   With the longest match procedure, multiple FECs may be concerned by a
   single RIB prefix change. The LSR must check all the FECs which are a
   subset of this RIB prefix. So some LDP reactions following a RIB
   event are changed:
     - When a new prefix appears in the RIB, the LSR MUST check if this
        prefix is a better match for some existing FECs. E.g. the FEC
        elements and used the IP RIB entry
        192.0.0/16 and a new more specific IP RIB entry 192.0.2/24
        appears. This may result in changing the LSR used as next hop
        and hence the NHLFE for this FEC.
     - When a prefix disappears in the RIB, the LSR MUST check all FEC
        elements which are using this RIB prefix as best match. For each

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        FEC, if another RIB prefix is found as best match, LDP MUST use
        it. This may result in changing the LSR used as next hop and
        hence the NHLFE for this FEC. Otherwise, the LSR MUST remove the
        FEC binding and send a label withdraw message.
     - When the next-hop of a RIB prefix change, the LSR must change
        the NHLFE of all the FEC elements using this prefix.

6.   Application examples

6.1.     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.

   All routers are MPLS enabled and MPLS connectivity (ie an LSP) is
   required between all PE routers.

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

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   The area border router ABR1 advertises in the backbone area:
     - the aggregated IP prefix 192.0.2/24 in the IGP
     - all the individual IP FEC elements (/32) in LDP

   In the "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 192.0/16 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 the ingress PE4 and
   every egress PE of area C while maintaining IP prefix aggregation on
   the ASBRs.

6.2.     Use of static routes

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

                    |         |
                   LER        |
                    |         |

                 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 LDP specification defined in [LDP] would require on the ingress
   LER the configuration and the maintenance of one IP route per egress
   LER and per outgoing interface.

   The longest match Label Mapping Procedure described in this document
   only requires one IP route per outgoing interface.

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7.   Caveats for deployment

7.1.     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 specification must be implemented on
   all LSRs in all areas where IP aggregation is used.

   If all IP prefixes are leaked in the IGP backbone area and only stub
   areas use IP aggregation, LSRs in the backbone area don't need to be
   compliant with this document.

7.2.     Impact on routing convergence time

   In case of an egress LER failure, 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 slower depending on the
   implementations, the IGP timers used and the network topology
   (network diameter).

   For failure of links and other nodes (Ps, ABRs), the failure
   notification and the convergence is unchanged. The convergence time
   may be improved because the RIB has fewer entries to update.

8.   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.

9.   References

9.1.     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

9.2.     Informative References

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     [MP-BGP]   Bates, T., Chandra, R., Katz, D. and Rekhter, Y,
          "Multiprotocol Extensions for BGP-4", RFC 4760, January 2007.

     [L3-VPN]   Rosen, E., Rekhter, Y. ," BGP/MPLS IP Virtual Private
          Networks (VPNs) ", RFC 4374, February 2006

     [MPLS-BGP] Rekhter, Y., Rosen, E., "Carrying Label Information in
          BGP-4", RFC 3107, May 2001

     [OSPFv2]   Moy, J.,"OSPF Version 2", RFC 2328, April 1998

     [IS-IS]    Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and
          Dual Environments", RFC 1195, December 1990

     [VPLS-BGP] Kompella, K., Rekhter, Y., "Virtual Private LAN Service
          (VPLS) Using BGP for Auto-discovery and Signaling", RFC 4761,
          January 2007.

     [VPLS-LDP] Lasserre, M., Kompella, V., "Virtual Private LAN Service
          (VPLS) Using Label Distribution Protocol (LDP) Signaling", RFC
          4762, January 2007.

     [ID-RSVP-TE] Farrel, Ayyangar, Vasseur, " Inter domain MPLS and
          GMPLS Traffic Engineering - RSVP-TE extensions", draft-ietf-
          ccamp-inter-domain-rsvp-te, work in progress.

10.    Acknowledgments

   Authors would like to thank Yakov Rekhter, Stefano Previdi, Vach
   Kompella, Benoit Fondeviole, Gilles Bourdon and Christian Jacquenet
   for the useful discussions on this subject, their review and

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

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