bgp: want support for link-local only ipv6 next-hops (bgp unnumbered)

[taspelund]

This documents the request for BGP unnumbered, i.e. support for advertising/installing routes with only a link-local address in an IPv6 next-hop.

RFC 2525 defines the use of MP-BGP extensions to carry IPv6 Unicast NLRI. In this spec, it defines the next-hop format to be either:
A) 16-bytes carrying just a routable unicast IPv6 address
or
B) 32-bytes carrying a routable unicast IPv6 address followed by a link-local unicast IPv6 address

It does not specify the encoding for the use of just a link-local IPv6 address without a routable unicast IPv6 address.

draft-ietf-idr-linklocal-capability attempts to standardize the encoding and handling of link-local only IPv6 next-hops via the Link-Local Next-Hop (LLNH) capability. However, this capability was implemented by several routing stacks (e.g. FRR, Arista, Cisco) before the IETF draft was ever created.

Implementing this will likely require support rx of non-standard encoding methods, and the ability to exchange link-local next-hops without having negotiated the LLNH capability.

Some known encoding formats:

1. 16-byte IPv6 next-hop carrying only the link-local address
2. 32-byte IPv6 next-hop pair carrying the link-local address in both address fields
3. 32-byte IPv6 next-hop pair carrying the link-local address in the LL field with the routable next-hop field zeroed