Link-local IPv6 addresses are scoped to a link and duplicate addresses can be used on different links to the same or different peer LSR. When the duplicate addresses exist on the same LAN, routing detects them and block one of them. In all other cases, duplicate links are valid because they are scoped to the local link.
In this section, LLn refers to Link-Local address (n).
Figure: FEC resolution in LAN shows FEC resolution in a LAN.
LSR B resolves a mLDP FEC with the root node being Root LSR. The route lookup shows that best route to loopback of Root LSR is {interface if-B and next-hop LL1}.
However, LDP finds that both LSR A and LSR C advertised address LL1 and that there are hello adjacencies (IPv4 or IPv6) to both A and C. In this case, a change is made so that an LSR only advertises link-local IPv6 addresses to a peer for the links over which it established a Hello adjacency to that peer. In this case, LSR C advertises LL1 to LSR E but not to LSRs A, B, and D. This behavior applies with both P2P and broadcast interfaces.
Ambiguity also exists with prefix FEC (unicast FEC); the above solution also applies.
FEC Resolution over P2P links
---------(LL1)-[C]------
|
[Root LSR]-------[A]-(LL1)-----[B] ------(LL4)-[D]------
| |
|-(LL2)---------|
| |
|-(LL3)---------|
LSR B resolves an mLDP FEC with root node being Root LSR. The route lookup shows that best route to loopback of Root LSR is {interface if-B and next-hop LL1}.
case 1
LDP is enabled on all links. This case has no ambiguity. LDP only selects LSR A because the address LL1 from LSR C is discovered over a different interface. This case also applies to prefix FEC (unicast FEC) and there is no ambiguity in the resolution.
case 2
LDP is disabled on link A-B with next-hop LL1; LSR B can still select one of the two other interfaces to upstream LSR A as long as LSR A advertised LL1 address in the LDP session.