Network Working Group Yakov Rekhter
Internet Draft Cisco Systems
Expiration Date: February 1999 Eric Rosen
Cisco Systems
Carrying Label Information in BGP-4
draft-ietf-mpls-bgp4-mpls-01.txt
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,
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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
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.
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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
speakers.
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
1771) http://ds.internic.net/rfc/rfc1771.txt
[BGP-CAP] "Capabilities Negotiation with BGP-4", R. Chandra, et al,
Work in progress, http://www.internic.net/internet-drafts/draft-
ietf-idr-bgp4-cap-neg-00.txt
[BGP-MP] "Multiprotocol Extensions for BGP-4", T. Bates, et al (RFC
2283) http://ds.internic.net/rfc/rfc2283.txt
[MPLS-ARCH], "A Proposed Architecture for MPLS", E. Rosen, et al,
Work in progress, http://www.internic.net/internet-drafts/draft-
ietf-mpls-arch-00.txt
[MPLS-ENCAPS], "MPLS Label Stack Encoding", E. Rosen, et al, Work in
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Internet Draft draft-ietf-mpls-bgp4-mpls-01.txt August 1998
progress, http://www.internic.net/internet-drafts/draft-ietf-mpls-
label-encaps-00.txt
10. Author Information
Yakov Rekhter
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
email: yakov@cisco.com
Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
email: erosen@cisco.com
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