Network Working Group
Request for Comments: 5283
Category: Standards Track
B. Decraene
JL. Le Roux
France Telecom
I. Minei
Juniper Networks, Inc.
July 2008

LDP Extension for Inter-Area Label Switched Paths (LSPs)

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.


To facilitate the establishment of Label Switched Paths (LSPs) that would span multiple IGP areas in a given Autonomous System (AS), this document describes a new optional Longest-Match 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 Information Base (RIB). Matching is defined by an IP longest-match search and does not mandate an exact match.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................2
   3. Terminology .....................................................2
   4. Problem Statement ...............................................3
   5. Longest-Match Label Mapping Message Procedure ...................4
   6. Application Examples ............................................6
      6.1. Inter-Area LSPs ............................................6
      6.2. Use of Static Routes .......................................7
   7. Caveats for Deployment ..........................................8
      7.1. Deployment Considerations ..................................8
      7.2. Routing Convergence Time Considerations ....................8
   8. Security Considerations .........................................9
   9. References ......................................................9
      9.1. Normative References .......................................9
      9.2. Informative References .....................................9
   10. Acknowledgments ...............................................11

1. Introduction

Link state Interior Gateway Protocols (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] recommends that the IP address of the FEC Element should *exactly* match an entry in the IP Routing Information Base (RIB). According to [LDP], section ("Label Mapping Messages Procedures"):

An LSR [Label Switching Router] receiving a Label Mapping message from a downstream LSR for a Prefix SHOULD NOT use the label for forwarding unless its routing table contains an entry that exactly matches the FEC Element.

Therefore, MPLS LSPs between Label Edge Routers (LERs) in different areas/levels are not set up unless the specific (e.g., /32 for IPv4) loopback addresses of all the LERs are redistributed across all areas.

The problem statement is discussed in section 4. Then, in section 5 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 consists of allowing for longest-match-based Label Mapping.

2. 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 [RFC2119].

3. Terminology

IGP Area: OSPF Area or IS-IS level


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


        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.

4. Problem Statement

Provider-based MPLS (Multiprotocol Label Switching) networks are expanding with the success of Layer 3 Virtual Private Networks [L3-VPN] and the new deployments of Layer 2 VPNs ([VPLS-BGP], [VPLS-LDP]). Service providers' MPLS backbones are significantly growing both in terms of density with the addition of Provider Edge (PE) routers to connect new customers and in terms of footprint as traditional Layer 2 aggregation networks may be 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, service providers currently have three solutions: 1) LDP with IGP route leaking, 2) BGP [MPLS-BGP] over LDP with MPLS hierarchy, and 3) inter-area RSVP-TE (Resource Reservation Protocol-Traffic Engineering [RSVP-TE]).

IGP route leaking consists of redistributing all specific 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 IGP Link State Database (LSDB), RIB, and Forwarding Information Base (FIB) 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 set up 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 provider routers as it requires BGP on all ABRs. In addition, MPLS hierarchy does not allow locally protecting the LSP against ABR failures (IP/LDP Fast Reroute), and hence ensuring sub- 50ms recovery upon ABR failure. The resulting convergence time may not be acceptable for stringent Service Level Agreements (SLAs) required for voice or mission-critical applications. Finally, this solution requires a significant migration effort for service providers that started with LDP and IGP route leaking to quickly set up the first inter-area LSPs.

Service providers may also set up these inter-area LSPs by using inter-area RSVP-TE [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 that allows setting up contiguous inter-area LSPs while avoiding leaking of specific PE loopback addresses across area boundaries, thereby 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 while maintaining IP prefix aggregation on the ABRs. This procedure is similar to the one defined in [LDP] but performs an IP longest match when searching the FEC element in the RIB.

5. Longest-Match Label Mapping Message Procedure

This document defines a new Label Mapping Procedure for [LDP]. It is applicable to IPv4 and IPv6 prefix FEC elements (address families 1 and 2 as per the "Address Family Numbers" registry on the IANA site). It SHOULD 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, an LSR receiving a Label Mapping message from a neighbor LSR for a Prefix Address FEC Element FEC1 SHOULD use the label for MPLS forwarding if its routing table contains an entry that matches the FEC Element FEC1 and the advertising LSR is a next hop to reach FEC1. If so, it SHOULD advertise the received FEC Element FEC1 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 (i.e., if the FEC element is a superset of a RIB entry, it is not considered a match).

Note that LDP re-advertises to its peers the specific FEC element FEC1, and not the aggregated prefix found in the IP RIB during the longest-match search.

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 are distributed in an ordered way. In case of LER failure, the removal of reachability to the FEC occurs using LDP ordered label distribution mode procedures. As defined in [LDP] in section A.1.5, the FEC will be removed in an ordered way through the propagation of Label Withdraw messages. The use of this (un)reachability information by application layers using this MPLS LSP (e.g., [MP-BGP]) is outside the scope of this document.

As per [LDP], LDP already has 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 IP prefix disappears,
  • next-hop change when an existing IP prefix has a new next hop following a routing change.

With this Longest-Match Label Mapping Message Procedure, multiple FECs may be concerned by a single RIB prefix change. The LSR MUST check all the FECs that 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. For example, the FEC elements and used the IP RIB entry, and a new more specific IP RIB entry appears. This may result in changing the LSR used as next hop and hence the Next Hop Label Forwarding Entry (NHLFE) for this FEC.
  • When a prefix disappears in the RIB, the LSR MUST check all FEC elements that are using this RIB prefix as best match. For each 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 changes, the LSR MUST change the NHLFE of all the FEC elements using this prefix.

Future work may define new management objects to the MPLS LDP MIB modules [LDP-MIB] to activate/deactivate this Longest-Match Label Mapping Message Procedure, possibly on a per-prefix basis.

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 Area.

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 (i.e., an 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 in the IGP
- all the specific 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 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 ABRs.

6.2. Use of Static Routes

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

                              |         |
                             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 Bidirectional Forwarding Detection protocol [BFD] can be used to quickly detect loss of connectivity.

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.

7. Caveats for Deployment

7.1. Deployment Considerations

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 FECs are aggregated in the RIB, this specification must be implemented on all LSRs in all areas where IP aggregation is used. If an LSR on the path does not support this procedure, then the LSP initiated on the egress LSR stops at this non-compliant LSR. There are no other adverse effects.

This extension can be deployed incrementally:

  • It can be deployed on a per-area or per-routing-domain basis and does not necessarily require an AS-wide deployment. For example, if all specific 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.
  • Within each routing area, LSRs can be upgraded independently, at any time, in any order, and without service disruption. During deployment, if those LSPs are already used, one should only make sure that ABRs keep advertising the specific IP prefixes in the IGP until all LSRs of this area are successfully upgraded. Then, the ABRs can advertise the aggregated prefix only and stop advertising the specific ones.

A service provider currently leaking specific LER loopback addresses in the IGP and considering performing IP aggregation on ABR should be aware that this may result in suboptimal routing as discussed in [RFC2966].

7.2. Routing Convergence Time Considerations

IP and MPLS traffic restoration time is based on two factors: the Shortest Path First (SPF) calculation in the control plane and Forwarding Information Base (FIB) / Label FIB (LFIB) update time in the forwarding plane. The SPF calculation scales O(N*Log(N)) where N is the number of Nodes. The FIB/LFIB update scales O(P) where P is the number of modified prefixes. Currently, with most routers implementations, the FIB/LFIB update is the dominant component [IGP-CONV], and therefore the bottleneck that should be addressed in priority. The solution documented in this document reduces the link state database size in the control plane and the number of FIB entries in the forwarding plane. As such, it solves the scaling of pure IP routers sharing the IGP with MPLS routers. However, it does not decrease the number of LFIB entries so is not sufficient to solve the scaling of MPLS routers. For this, an additional mechanism is required (e.g., introducing some MPLS hierarchy in LDP). This is out of scope for this document.

Compared to [LDP], for all failures except LER failure (i.e., links, provider routers, and ABRs), the failure notification and the convergence is unchanged. For LER failure, given that the IGP aggregates IP routes on ABRs and no longer advertises specific prefixes, the control plane and more specifically the routing convergence behavior of protocols (e.g., [MP-BGP]) or applications (e.g., [L3-VPN]) may be changed in case of failure of the egress LER node. For protocols and applications which need to track egress LER availability, several solutions can be used, for example:

  • Rely on the LDP ordered label distribution control mode -- as defined in [LDP] -- to know the availability of the LSP toward the egress LER. The egress to ingress propagation time of that unreachability information is expected to be comparable to the IGP (but this may be implementation dependent).
  • Advertise LER reachability in the IGP for the purpose of the control plane in a way that does not create IP FIB entries in the forwarding plane.

8. Security Considerations

The Longest-Match Label Mapping procedure described in this document does not introduce any change as far as the Security Considerations section of [LDP] is concerned.

9. References

9.1. Normative References

   [LDP]         Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
                 "LDP Specification", RFC 5036, October 2007.
   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

9.2. Informative References

   [L3-VPN]      Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
                 Networks (VPNs)", RFC 4364, February 2006.
   [MP-BGP]      Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
                 "Multiprotocol Extensions for BGP-4", RFC 4760, January
   [MPLS-BGP]    Rekhter, Y. and E. Rosen, "Carrying Label Information
                 in BGP-4", RFC 3107, May 2001.
   [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., Ed., and Y. Rekhter, Ed., "Virtual
                 Private LAN Service (VPLS) Using BGP for Auto-Discovery
                 and Signaling", RFC 4761, January 2007.
   [VPLS-LDP]    Lasserre, M., Ed., and V. Kompella, Ed., "Virtual
                 Private LAN Service (VPLS) Using Label Distribution
                 Protocol (LDP) Signaling", RFC 4762, January 2007.
   [RFC2966]     Li, T., Przygienda, T., and H. Smit, "Domain-wide
                 Prefix Distribution with Two-Level IS-IS", RFC 2966,
                 October 2000.
   [RSVP-TE]     Farrel, A., Ed., Ayyangar, A., and JP. Vasseur,
                 "Inter-Domain MPLS and GMPLS Traffic Engineering --
                 Resource Reservation Protocol-Traffic Engineering
                 (RSVP-TE) Extensions", RFC 5151, February 2008.
   [LDP-MIB]     Cucchiara, J., Sjostrand, H., and J. Luciani,
                 "Definitions of Managed Objects for the Multiprotocol
                 Label Switching (MPLS), Label Distribution Protocol
                 (LDP)", RFC 3815, June 2004.
   [BFD]         Katz, D. and D. Ward, "Bidirectional Forwarding
                 Detection", Work in Progress, March 2008.
   [IGP-CONV]    Francois, P., Filsfils, C., and Evans, J., "Achieving
                 sub-second IGP convergence in large IP networks".  ACM
                 SIGCOMM Computer Communications Review, July 2005.
   [OSPFv2]      Moy, J., "OSPF Version 2", STD 54, RFC 2328, April

10. Acknowledgments

The authors would like to thank Yakov Rekhter, Stefano Previdi, Vach Kompella, Bob Thomas, Clarence Filsfils, Kireeti Kompella, Luca Martini, Sina Mirtorabi, Dave McDysan, Benoit Fondeviole, Gilles Bourdon, and Christian Jacquenet for the useful discussions on this subject, their reviews, and comments.

Authors' Addresses

Bruno Decraene
France Telecom
38 rue du General Leclerc
92794 Issy Moulineaux cedex 9


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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