Carrying Label Information in BGP-4
RFC 3107
| Document | Type |
RFC - Proposed Standard
(May 2001; Errata)
Obsoleted by RFC 8277
Updated by RFC 6790
|
|
|---|---|---|---|
| Last updated | 2020-01-21 | ||
| Stream | IETF | ||
| Formats | plain text html pdf htmlized with errata bibtex | ||
| Stream | WG state | (None) | |
| Document shepherd | No shepherd assigned | ||
| IESG | IESG state | RFC 3107 (Proposed Standard) | |
| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | (None) | ||
| Send notices to | (None) | ||
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
Rekhter & Rosen Standards Track [Page 1]
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 [Page 2]
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
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