3.4 BGP Next-Hop Information

BGP link-state information for both IPv4 and IPv6 networks can be

carried over either an IPv4 BGP session or an IPv6 BGP session. If

an IPv4 BGP session is used, then the next hop in the MP_REACH_NLRI

SHOULD be an IPv4 address. Similarly, if an IPv6 BGP session is

used, then the next hop in the MP_REACH_NLRI SHOULD be an IPv6

address. Usually, the next hop will be set to the local endpoint

address of the BGP session. The next-hop address MUST be encoded as

described in [RFC4760]. The Length field of the next-hop address

will specify the next-hop address family. If the next-hop length is

4, then the next hop is an IPv4 address; if the next-hop length is

16, then it is a global IPv6 address; and if the next-hop length is

32, then there is one global IPv6 address followed by a link-local

IPv6 address. The link-local IPv6 address should be used as

described in [RFC2545]. For VPN Subsequent Address Family Identifier

(SAFI), as per custom, an 8-byte Route Distinguisher set to all zero

is prepended to the next hop.

The BGP Next Hop attribute is used by each BGP-LS speaker to validate

the NLRI it receives. In case identical NLRIs are sourced by

multiple originators, the BGP Next Hop attribute is used to tiebreak

as per the standard BGP path decision process. This specification

doesn't mandate any rule regarding the rewrite of the BGP Next Hop

attribute.