Network Working Group                                Yakov Rekhter
Internet Draft                                       Cisco Systems
Expiration Date: February 1999                          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
   the "1id-abstracts.txt" listing contained in the Internet-Drafts
   Shadow Directories on (Africa),
   (Northern Europe), (Southern Europe),
   (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:

             |   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

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                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.

             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 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.

          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.

       5. 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

          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.

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       6. Capability Negotiation

          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
          negotiate 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

          A BGP speaker that is capable of handling multiple routes to a
          destination (as described above) should use the Capabilities Optional
          Paramter, as defined in [BGP-CAP], to inform its peers about this
          capability. The value of this capability is TBD.

       7. Security Considerations

          Security issues are not discussed in this document.

       8. Acknowledgements

          To be supplied.

       9. 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

          [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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       10. 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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