Administrator Guide

BGP 1419
Network Address of Next Hop
When advertising IPv6 routes, the Network Address of Next Hop field in
MP_REACH_NLRI is set according to RFC 2545. Under conditions specified
in this RFC, both a global and a link local next-hop address may be included.
The primary purpose of the global address is an address that can be re-
advertised to internal peers. The primary purpose of the link local address is
for use as the next hop of routes.
We expect interfaces to external peers will normally have both a link local and
a global IPv6 address. Both addresses are included in the Network Address of
Next Hop field when sending MP_REACH_NLRI to the peer. Normally,
internal peers are not on a common subnet (even when they are, peering is
normally to addresses on loopback interfaces) and the Network Address of
Next Hop field includes only a global address. Even when the peer address of
an internal peer is on a local link, Dell EMC Networking BGP only advertises
a global next-hop IPv6 address.
Identifying Local IPv6 Addresses
In some situations, a router sets the next-hop addresses to addresses
configured on one of its own interfaces. When local IPv6 addresses are
needed, Dell EMC Networking BGP uses IPv6 addresses on the local end of
the TCP connection to the peer. If the peering session uses IPv4, BGP finds
IPv6 addresses on the same routing interface as the IPv4 address that
terminates the TCP connection. If there are multiple global addresses on the
interface, BGP uses the first one (essentially, a random choice). If the set of
IPv6 addresses on the interface changes, BGP may alter its choice of local
IPv6 addresses.
Using Policy to Specify Next Hop
The network administrator can override the normal rules for selecting a next
hop address by configuring IPv6 next hops with outbound policy (a neighbor-
specific route map with a set ipv6 next-hop term). When configuring an IPv6
next hop, the network administrator should ensure the neighbor can reach
the next-hop address. For example, a link local next-hop address should not
be configured to an internal peer not on a local link. Using per-neighbor
outbound policy to set the IPv6 next hop has the disadvantage of putting
each neighbor in a different outbound update group, thus losing the
efficiency advantages of sharing an Adj-RIB-Out and of building an UPDATE
message once and sending it to many peers.