|Title||Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing
|Author||P. Marques, F. Dupont
Network Working Group P. Marques
Request for Comments: 2545 cisco Systems, Inc.
Category: Standards Track F. Dupont
Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing
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 (C) The Internet Society (1999). All Rights Reserved.
BGP-4 Multiprotocol Extensions [BGP-MP] defines the format of two BGP
attributes (MP_REACH_NLRI and MP_UNREACH_NLRI) that can be used to
announce and withdraw the announcement of reachability information.
This document defines how compliant systems should make use of those
attributes for the purpose of conveying IPv6 routing information.
The BGP-4 protocol [BGP-4] in particular, and path vector routing
protocols in general, are mostly independent of the particular
Address Family for which the protocol is being used.
IPv6 falls under the generic category of protocols for which BGP-4 is
suitable and, unless stated otherwise in this document, the BGP-4
procedures to apply when using BGP-4 to carry IPv6 reachability
information are those defined in [BGP-4] and in subsequent documents
that extend or update the BGP-4 specification.
In terms of routing information, the most significant difference
between IPv6 and IPv4 (for which BGP was originally designed) is the
fact that IPv6 introduces scoped unicast addresses and defines
particular situations when a particular address scope must be used.
This document concerns itself essentially with the necessary rules to
accommodate IPv6 address scope requirements.
2. IPv6 Address Scopes
IPv6 defines 3 unicast address scopes [ADDR-ARCH]: global, site-local
and link-local. Site-local addresses are non-link-local address which
are valid within the scope of a "site" and cannot be exported beyond
it. As this document makes no assumption on the characteristics of a
particular routing realm where BGP-4 is used, it makes no distinction
between global and site-local addresses and refers to both as
"global" or "non-link-local". Network administrators must however
respect address scope restrictions and should be aware that the
concepts of a BGP-4 routing domain and "site" are orthogonal notions
and that they may or may not coincide in a given situation.
Companion IPv6 specifications further define that only link-local
address can be used when generating ICMP Redirect Messages [ND] and
as next hop addresses in some routing protocols (eg. RIPng [RIP]).
This restrictions does imply that an IPv6 router must have a link-
local next hop address for all directly connected routes (routes for
which the given router and the next hop router share a common subnet
Link-local addresses are not, however, well suited to be used as next
hop attributes in BGP-4 given the rules defined for this attribute in
the protocol specification [BGP-4].
For the above reasons, when BGP-4 is used to convey IPv6 reachability
information it is sometimes necessary to announce a next hop
attribute that consists of a global address and a link-local address.
The following section describes the rules that should be followed
when constructing the Network Address of Next Hop field of an
3. Constructing the Next Hop field
A BGP speaker shall advertise to its peer in the Network Address of
Next Hop field the global IPv6 address of the next hop, potentially
followed by the link-local IPv6 address of the next hop.
The value of the Length of Next Hop Network Address field on a
MP_REACH_NLRI attribute shall be set to 16, when only a global
address is present, or 32 if a link-local address is also included in
the Next Hop field.
The link-local address shall be included in the Next Hop field if and
only if the BGP speaker shares a common subnet with the entity
identified by the global IPv6 address carried in the Network Address
of Next Hop field and the peer the route is being advertised to.
In all other cases a BGP speaker shall advertise to its peer in the
Network Address field only the global IPv6 address of the next hop
(the value of the Length of Network Address of Next Hop field shall
be set to 16).
As a consequence, a BGP speaker that advertises a route to an
internal peer may modify the Network Address of Next Hop field by
removing the link-local IPv6 address of the next hop.
TCP connections, on top of which BGP-4 messages are exchanged, can be
established either over IPv4 or IPv6. While BGP-4 itself is
independent of the particular transport used it derives implicit
configuration information from the address used to establish the
peering session. This information (the network address of a peer) is
taken in account in the route dissemination procedure. Thus, when
using TCP over IPv4 as a transport for IPv6 reachability information,
additional explicit configuration of the peer's network address is
Note that the information referred above is distinct from the BGP
Identifier used in the BGP-4 tie breaking procedure. The BGP
Identifier is a 32 bit unsigned integer exchanged between two peers
at session establishment time, within an OPEN message. This is a
system wide value determined at startup which must be unique in the
network and should be derived from an IPv4 address regardless of the
network protocol(s) a particular BGP-4 instance is configured to
convey at a given moment.
The use of TCP over IPv6 as transport protocol for IPv6 reachability
information also has the advantage of providing explicit confirmation
of IPv6 network reachability between two peers.
5. Security Considerations
The extensions defined in this document allow BGP to propagate
reachability information about IPv6 routes. As such, no new security
issues are raised beyond those that already exist in BGP-4 and its
use with IPv4.
This document derives from the work on BGP-4 Multiprotocol Extensions
by Tony Bates, Ravi Chandra, Dave Katz and Yakov Rekhter.
[ADDR-ARCH] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[BGP-4] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[BGP-MP] Bates, T., Chandra, R., Katz, D. and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 2283, February
[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[ND] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
[RIP] Malkin, G. and R. Minnear, "RIPng for IPv6",
RFC 2080, January 1997.
8. Author Information
Pedro R. Marques
cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
Phone: +1 408 527-5202
Fax: +1 408 526-4952
Domaine de Voluceau
78153 Le Chesnay CEDEX
Phone: +33 1 39 63 52 13
Fax: +33 1 39 63 58 66
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