rfc3107.txt

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Network Working Group                                         Y. Rekhter
Request for Comments: 3107                              Juniper Networks
Category: Standards Track                                       E. Rosen
                                                     Cisco Systems, Inc.
                                                                May 2001


                  Carrying Label Information in BGP-4

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.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This document specifies the way in which the label mapping
   information for a particular route is piggybacked in the same Border
   Gateway Protocol (BGP) Update message that is used to distribute the
   route itself.  When BGP is used to distribute a particular route, it
   can be also be used to distribute a Multiprotocol Label Switching
   (MPLS) label which is mapped to that route.

Table of Contents

    1      Specification of Requirements  ..........................   2
    2      Overview  ...............................................   2
    3      Carrying Label Mapping Information  .....................   3
    4      Advertising Multiple Routes to a Destination  ...........   4
    5      Capability Advertisement  ...............................   4
    6      When the BGP Peers are not Directly Adjacent  ...........   5
    7      Security Considerations  ................................   5
    8      Acknowledgments  ........................................   6
    9      References  .............................................   6
   10      Authors' Addresses  .....................................   7
   11      Full Copyright Statement  ...............................   8








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1. Specification of Requirements

   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 RFC 2119.

2. Overview

   When BGP is used to distribute a particular route, it can also be
   used to distribute an MPLS label that is mapped to that route [MPLS-
   ARCH].  This document specifies the way in which this is done.  The
   label mapping information for a particular route is piggybacked in
   the same BGP Update message that is used to distribute the route
   itself.

   This can be useful in the following situations:

      -  If two immediately adjacent Label Switched Routers (LSRs) are
         also BGP peers, then label distribution can be done without the
         need for any other label distribution protocol.

      -  Suppose one's network consists of two "classes" of LSR:
         exterior LSRs, which interface to other networks, and interior
         LSRs, which serve only to carry traffic between exterior LSRs.
         Suppose that the exterior LSRs are BGP speakers.  If the BGP
         speakers distribute MPLS labels to each other along with each
         route they distribute, then as long as the interior routers
         support MPLS, they need not receive any of the BGP routes from
         the BGP speakers.

         If exterior router A needs to send a packet to destination D,
         and A's BGP next hop for D is exterior router B, and B has
         mapped label L to D, then A first pushes L onto the packet's
         label stack.  A then consults its IGP to find the next hop to
         B, call it C.  If C has distributed to A an MPLS label for the
         route to B, A can push this label on the packet's label stack,
         and then send the packet to C.

   If a set of BGP speakers are exchanging routes via a Route Reflector
   [BGP-RR], then by piggybacking the label distribution on the route
   distribution, one is able to use the Route Reflector to distribute
   the labels as well.  This improves scalability quite significantly.
   Note that if the Route Reflector is not in the forwarding path, it
   need not even be capable of forwarding MPLS packets.

   Label distribution can be piggybacked in the BGP Update message by
   using the BGP-4 Multiprotocol Extensions attribute [RFC 2283].  The
   label is encoded into the NLRI field of the attribute, and the SAFI



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RFC 3107          Carrying Label Information in BGP-4           May 2001


   ("Subsequent Address Family Identifier") field is used to indicate
   that the NLRI contains a label.  A BGP speaker may not use BGP to
   send labels to a particular BGP peer unless that peer indicates,
   through BGP Capability Advertisement, that it can process Update
   messages with the specified SAFI field.

3. Carrying Label Mapping Information

   Label mapping information is carried as part of the Network Layer
   Reachability Information (NLRI) in the Multiprotocol Extensions
   attributes.  The AFI indicates, as usual, the address family of the
   associated route.  The fact that the NLRI contains a label is
   indicated by using SAFI value 4.

   The Network Layer Reachability information is encoded as one or more
   triples of the form <length, label, prefix>, whose fields are
   described below:

      +---------------------------+
      |   Length (1 octet)        |
      +---------------------------+
      |   Label (3 octets)        |
      +---------------------------+
      .............................
      +---------------------------+
      |   Prefix (variable)       |
      +---------------------------+

   The use and the meaning of these fields are as follows:

      a) Length:

         The Length field indicates the length in bits of the address
         prefix plus the label(s).

      b) Label:

         The Label field carries one or more labels (that corresponds to
         the stack of labels [MPLS-ENCAPS]).  Each label is encoded as 3
         octets, where the high-order 20 bits contain the label value,
         and the low order bit contains "Bottom of Stack" (as defined in
         [MPLS-ENCAPS]).

      c) Prefix:

         The Prefix field contains address prefixes followed by enough
         trailing bits to make the end of the field fall on an octet
         boundary.  Note that the value of trailing bits is irrelevant.



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RFC 3107          Carrying Label Information in BGP-4           May 2001


   The label(s) specified for a particular route (and associated with
   its address prefix) must be assigned by the LSR which is identified
   by the value of the Next Hop attribute of the route.

   When a BGP speaker redistributes a route, the label(s) assigned to
   that route must not be changed (except by omission), unless the
   speaker changes the value of the Next Hop attribute of the route.

   A BGP speaker can withdraw a previously advertised route (as well as
   the binding between this route and a label) by either (a) advertising
   a new route (and a label) with the same NLRI as the previously
   advertised route, or (b) listing the NLRI of the previously
   advertised route in the Withdrawn Routes field of an Update message.
   The label information carried (as part of NLRI) in the Withdrawn
   Routes field should be set to 0x800000.  (Of course, terminating the
   BGP session also withdraws all the previously advertised routes.)

4. Advertising Multiple Routes to a Destination

   A BGP speaker may maintain (and advertise to its peers) more than one
   route to a given destination, as long as each such route has its own
   label(s).

   The encoding described above allows a single BGP Update message to
   carry multiple routes, each with its own label(s).

   In the case where a BGP speaker advertises multiple routes to a
   destination, if a route is withdrawn, and a label(s) is specified at
   the time of withdrawal, only the corresponding route with the
   corresponding label is withdrawn.  If a route is withdrawn, and no
   label is specified at the time of withdrawal, then only the
   corresponding unlabeled route is withdrawn; the labeled routes are
   left in place.

5. Capability Advertisement

   A BGP speaker that uses Multiprotocol Extensions to carry label
   mapping information should use the Capabilities Optional Parameter,
   as defined in [BGP-CAP], to inform its peers about this capability.
   The MP_EXT Capability Code, as defined in [BGP-MP], is used to
   advertise the (AFI, SAFI) pairs available on a particular connection.

   A BGP speaker should not advertise this capability to another BGP
   speaker unless there is a Label Switched Path (LSP) between the two
   speakers.






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