RFC 3107 (rfc3107) - Page 2 of 8

Carrying Label Information in BGP-4

RFC 3107          Carrying Label Information in BGP-4           May 2001


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



Rekhter & Rosen             Standards Track