Network Working Group                                Yakov Rekhter
Internet Draft                                       Cisco Systems
Expiration Date: October 1998                           Eric Rosen
                                                     Cisco Systems

                  Carrying Label Information in BGP-4


1. Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   To view the entire list of current Internet-Drafts, please check
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   Shadow Directories on (Africa),
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   (Pacific Rim), (US East Coast), or
   (US West Coast).

2. Abstract

   When a pair of Label Switch Routers (LSRs) that maintain BGP peering
   with each other exchange routes, the LSRs also need to exchange label
   mapping information for these routes. The exchange is accomplished by
   piggybacking the label mapping information for a route in the same
   BGP Update message that is used to exchange the route. This document
   specifies the way in which this is done.

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3. Overview

   The Multiprotocol Label Switching (MPLS) architecture [MPLS-ARCH]
   identifies situations in which the mapping between a label and a
   route must be distributed between BGP peers. This document specifies
   how this label mapping information is distributed. The label mapping
   information is included in the BGP Update message that is used to
   distribute the route.  This is done by utilizing BGP-4 Multiprotocol
   Extensions attribute [BGP-MP].

4. Carrying Label Mapping Information

   Label mapping information is carried as part of the Network Layer
   Reachability Information (NLRI) in the Multiprotocol Extensions
   attributes. Such NLRI is identified by using Sub-AFI TBD.

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

      |   Label (3 octets)        |
      |   Length (1 octet)        |
      |   Prefix (variable)       |

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

      a) 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 bit contains "Bottom of Stack" (as
         defined in [MPLS-ENCAPS]). The following high-order three bits
         must be zero.  The remaining 20 bits contain the label value.

      b) Length:

         The Length field indicates the length in bits of the address

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         prefix. A length of zero indicates a prefix that matches all
         (as specified by the address family) addresses (with prefix,
         itself, of zero octets).

      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.

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

   The label(s) specified for a particular route (and associated with it
   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.

   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

   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

5. Capability Negotiation

   BGP-4 speakers using Multiprotocol Extensions to carry label mapping
   information should use the Capabilities Optional Parameter as defined
   in [BGP-CAP].  The MP_EXT Capability Code, as defined in [CAP-MP], is
   used to negotiate the (AFI, SAFI) pairs available on a particular

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

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6. Security Considerations

   Security issues are not discussed in this document.

7. Acknowledgements

   To be supplied.

8. References

   [BGP-4] "A Border Gateway Protocol 4 (BGP-4), Y. Rekhter, T.  Li (RFC

   [BGP-CAP] "Capabilities Negotiation with BGP-4", R. Chandra, et al,
   Work in progress,

   [BGP-MP] "Multiprotocol Extensions for BGP-4", T. Bates, et al (RFC

   [CAP-MP] "BGP-4 Capabilities Negotiation for BGP Multiprotocol
   Extensions", P. Marques, Work in progress,

   [LDP] "LDP Specification", N. Feldman, et al, Work in progress

   [MPLS-ARCH], "A Proposed Architecture for MPLS", E. Rosen, et al,
   Work in progress,

   [MPLS-ENCAPS], "MPLS Label Stack Encoding", E. Rosen, et al, Work in

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9. Author Information

   Yakov Rekhter
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134

   Eric Rosen
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA 01824

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