Network Working Group                                     George Swallow
Internet Draft                                       Cisco Systems, Inc.
Category: Standards Track
Expiration Date: May 2008
                                                            Jim Guichard
                                                     Cisco Systems, Inc.

                                                           November 2007

                  Network Scaling with Aggregate LSPs


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      This document defines a means for an Multiprotocol Label Switched
      network to summarize routes while maintaining end-to-end LSP
      connectivity thereby reducing the number of host routes that need
      to be carried within the interior gateway protocol.

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Internet Draft   draft-swallow-mpls-aggregate-fec-00.txt   November 2007


 1      Introduction  ..............................................   3
 1.1    Conventions  ...............................................   3
 1.2    Terminology  ...............................................   3
 2      Overview  ..................................................   4
 2.1    Inter-area LSP Construction  ...............................   4
 2.2    Label forwarding operation  ................................   6
 3      Aggregate FEC  .............................................   7
 3.1    Aggregate FEC Element  .....................................   7
 3.2    Label distribution procedures  .............................   7
 4      Algorithmically derived de-aggregation label  ..............   8
 5      Security Considerations  ...................................   8
 6      IANA Considerations  .......................................   8
 7      References  ................................................   8
 7.1    Normative References  ......................................   8
 7.2    Informative References  ....................................   9
 8      Authors' Addresses  ........................................   9

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1. Introduction

   The growth of next-generation Multiprotocol Label Switched
   (MPLS)-based services such as l2vpn, l3vpn and so on, have introduced
   an explosion in the number of edge devices that are needed to deploy
   such services. Typically these services require an end-to-end label-
   switched path LSP, from ingress Label-Switching Router (LSR) to
   egress LSR.  These LSPs are maintained between the host IP addresses
   of the LSRs. This requirement forces the operator to carry each host
   address for every edge device within their interior gateway protocol
   (IGP) which has a negative impact on the scale and convergence goals
   of their IGP.

   This document defines extensions to the Label Distribution Protocol
   (LDP) to provide a means for the end-to-end LSP path to be maintained
   between edge devices without the necessity of carry each and every
   host address within the IGP or to distribute labels throughout a
   domain for host addresses.  This is achieved by defining an Aggregate
   Forwarding Equivalence Class (FEC) and an algorithmically derived
   label per aggregated host route which stacked together form end-to-
   end LSPs.

1.1. Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [KEYWORDS].

1.2. Terminology

     ABR       Area border router
     FEC       Forwarding equivalence class
     IGP       Interior gateway protocol
     LDP       Label Distribution Protocol
     LIB       Label information base
     LSP       Label-switched path
     LSR       label-switching router
     NHLFE     Next-hop label forwarding entry
     PE        Provider edge [LSR]
     RIB       Routing information base
     VPN       Virtual private network

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2. Overview

   In a typical MPLS deployment today, Service Providers are either
   running a flat IGP or leaking the loopback addresses of their PEs
   between the area of their IGPs.  LDP then creates LSPs based on all
   of these host routes.  Our objective here is to eliminate the need
   for a full mesh of LSPs between all PE pairs.  This will enable
   Service Providers to scale and improve the convergence of their IGPs.

   The basic idea is to use an LSP bound to a summarized IP prefix to
   deliver MPLS packets from the ingress PE, across the core, to the
   ABRs serving the egress PEs.  Nested within these LSPs are LSPs for
   each of the specific hosts covered by that prefix.  This is
   accomplished by defining two new FECs, the Aggregate FEC and the De-
   aggregation FEC.

   The Aggregate FEC is exactly like the Prefix FEC as far as routing is
   concerned.  An Aggregate FEC differs in that at the end of the LSP,
   it forms the context for interpreting the next label which is bound
   to a De-aggregation FEC.  In that regard, an aggregate LSP never uses
   penultimate-hop popping.  Further, to ensure that an LSP exists all
   the way to an LSR capable of de-aggregating the FEC, labels bound to
   these FECs are always distributed in ordered mode.

   A De-aggregation FEC represents a specific host route.  It is exactly
   the same as a Host Address FEC except that labels bound to these FECs
   are not distributed in LDP.  Instead, the label value bound to a
   particular host is algorithmically derived from the host address and
   the prefix length of the aggregate FEC using a simple algorithm
   described in section 4 below.

   The use of these FECs is illustrated in the example below.

2.1. Inter-area LSP Construction

   To illustrate the construction of inter-area LSPs consider the
   following simple topology with one backbone area and two stub areas.
   Using the mechanisms described within this document, we describe an
   LSP is built from ingress PE4 to egress PE1 with traffic destined to in VPN_1:

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                          Area 0
            Area 1  |                |  Area 2
         +----------+                +----------+
         |          |                |          |
         |          |                |       +-----+        VPN_1
         |          |                |       | PE1 |------
         |          |                |       +-----+
         |          |                |          |
         |          |                |          |
      +-----+    +------+        +------+    +-----+
      | PE4 |    | ABR1 |        | ABR2 |    | PE2 |
      +-----+    +------+        +------+    +-----+
         |          |                |          |
         |          |                |          |
         |          |                |       +-----+
         |          |                |       | PE3 |
         |          |                |       +-----+
         |          |                |          |
         +----------+                +----------+
                    |                |

                 Figure 1: Example network

   All routers are MPLS enabled and MPLS connectivity is required
   between all PE routers to facilitate edge services.

   In the egress area (area 2), the records available are:

       RIB                  LIB         Labels bound to:                                  

   The area border router ABR2 advertises in the backbone area:

        - the IP prefix 10.10.2/24 (which covers all available
          PE routers in area 2) in the IGP

        - a label (say 51) bound to the Aggregate FEC 10.10.2/24
          in LDP to its neighbors in area 0.

   ABR2 algorithmically creates a label entry for each host route
   covered by the summary 10.10.2/24 (see section 4).  It then creates
   NHLFEs for each of these binding them to the next-hop labels it
   received for each of the specific routes.

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   In the backbone (area 0), the records available are:

       RIB                  LIB
        10.10.2/24           Labels bound to:  10.10.2/24

   The area border router ABR1 receives the route 10.10.2/24 via the IGP
   and labels from its neighbors in area 0 bound to the Aggregate FEC
   10.10.2/24 {22} via LDP from the next-hop router towards the route
   10.10.2/24.  It redistributes both the route and a label bound to the
   FEC into area 1.

   The routers in area 1, including PE4, receive the route 10.10.2/24
   via the IGP and and labels from their neighbors bound to the
   Aggregate FEC 10.10.2/24 via LDP.

   In the ingress area (area 1), the records available are:

       RIB                  LIB
        10.10.2/24           Labels bound to:  10.10.2/24

2.2. Label forwarding operation

   Using the information presented in the previous section, the label
   forwarding from ingress PE4 to egress PE1 is as follows:

   PE4 has a VPN destination reachable via remote PE1
   whose next-hop is which is bound to say 47.  PE4 only
   has a summary route 10.10.2/24 covering the more specific next-hop, but has label bindings for that aggregate FEC.  Using
   the same algorithm as the egress ABR2, PE4 determines that the de-
   aggregation label for PE1 is 17.  PE1 creates a VRF entry for which includes a three label stack consisting of its
   next-hop label for 10.10.2/24, say 22, stacked upon the de-
   aggregation label (17) stacked upon the VPN label (47).

   Thus to forward a packet in VPN_1, PE4 pushes on a
   label stack {22, 17, 47}.  This is forwarded via normal label-
   switching to ABR2 where it arrives with the label stack {51, 17, 47}.
   ABR2 recognizes label 51 as context label and pops it.  It then looks
   up label 17 in the context associated 51.  ABR2 then pushes onto the
   label stack the LDP derived label for the local PE. Forwarding
   continues at this point as per normal RFC4364 procedures.

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3. Aggregate FEC

   A FEC TLV is defined in [LDP] section 3.4.1. This TLV serves as a
   container for FEC Elements.  To enable distribution of the Aggregate
   FEC, we define a new FEC Element.

3.1. Aggregate FEC Element

      Aggregate FEC Element value encoding:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |  Prefix (TBD)   |     Address Family            |   PreLen    |
      |                     Prefix                                    |

      Address Family

         Two octet quantity containing a value from ADDRESS FAMILY
         NUMBERS in [RFC1700] that encodes the address family for the
         address prefix in the Prefix field.


         One octet unsigned integer containing the length in bits of
         the address prefix that follows.  A length of zero indicates
         a prefix that matches all addresses (the default
         destination); in this case the Prefix itself is zero octets).


         An address prefix encoded according to the Address
         Family field, whose length, in bits, was specified in the
         PreLen field, padded to a byte boundary.

3.2. Label distribution procedures

   LSRs MUST NOT assign a NULL label value to an Aggregate FEC.

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4. Algorithmically derived de-aggregation label

   In order to avoid having to distribute de-aggregation labels, they
   are algorithmically derived from the host address with the following

      The network mask is inverted and applied to mask off the
      high-order bits of the address.  The remaining bits are treated
      as an integer to which 16 is added.  This value represented as a
      20-bit integer becomes the label value.

   The value 16 is added in the algorithm in order to bypass the
   reserved label range.

   Issue:  This algorithm may not be appropriate for IPv6.

5. Security Considerations

   [to be written]

6. IANA Considerations

   [to be written]

7. References

7.1. Normative References

   [LDP]      Andersson, L. et al., "LDP Specification", RFC 3036,
              January 2001.

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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7.2. Informative References

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual
              Private Networks (VPNs)", RFC 4364, February 2006.

8. Authors' Addresses

      George Swallow
      Cisco Systems, Inc.
      1414 Massachusetts Ave.
      Boxborough, MA 01719


      Jim Guichard
      Cisco Systems, Inc.
      1414 Massachusetts Ave
      Boxborough, MA 01719


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Internet Draft   draft-swallow-mpls-aggregate-fec-00.txt   November 2007

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