RFC 2545 (rfc2545) - Page 2 of 5
Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing
Alternative Format: Original Text Document
RFC 2545 BGP-4 Multiprotocol Extensions for IPv6 IDR March 1999
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
prefix).
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
MP_REACH_NLRI attribute.
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.
Marques & Dupont Standards Track