RFC 2545 (rfc2545) - Page 2 of 5

Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing

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