Peer at link-local addresses: fe80::1%eth0

A peer's endpoint may be a link-local IPv6 address with the interface it
is on. Such a peer is the address and the interface together, wherever
peers are told apart: admission, collisions, hold-downs, the refusal log,
the status page. The interface is looked up by name each time it is
needed, so one that appears later or is made anew is found.

Our routes go out with our link-local address in both places of
MP_REACH_NLRI's next hop, as FRR sends its own. A peer's link-local next
hop is taken alone, twice, or after "::" (BIRD), and resolved in the
kernel on the interface of the session rather than by looking fe80::/64
up, which every interface has.

The lab has a new phase, linklocal: sessions with FRR (by interface, with
radvd beside the daemon) and BIRD over a LAN with link-local addresses
only, both families, pings across, the interface down and up.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/bench/benches/full_table.rs

@@ -162,6 +162,7 @@ fn into_table(wire: &[Vec<BytesMut>], routes: usize, threads: usize) {
                     asn: 64512 + peer as u32 + 1,
                     router_id: Ipv4Addr::from(peer as u32 + 1),
                     address: Ipv4Addr::new(192, 0, 2, peer as u8 + 1).into(),
+                    interface: 0,
                     ibgp: false,
                 })
                 .expect("room for the peer")

crates/bench/benches/real_table.rs

@@ -281,6 +281,7 @@ fn add_peers(table: &RibTable, dump: &Dump, peers: &[usize]) -> Vec<PeerIndex> {
                     asn: dump.peers[peer].asn,
                     router_id: Ipv4Addr::from(peer as u32 + 1),
                     address: dump.peers[peer].address,
+                    interface: 0,
                     ibgp: false,
                 })
                 .expect("room for the peer")