Label Distribution Protocol
7450 ESS MPLS Guide Page 609
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 will find that both LSR A and LSR C advertised address LL1 and that there are
hello adjacencies (IPv4 or IPv6) to both A an 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 will advertise LL1 to LSR E but not to LSRs A, B, and
D. This behavior will apply with both P2P and broadcast interfaces.
Note that ambiguity also exists with prefix FEC (unicast FEC) and the above solution also applies
in that situation.
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 will only select
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 thus 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.
IGP and Static Route Synchronization with LDP
The IGP-LDP synchronization and the static route to LDP synchronization features are modified
to operate on a dual-stack IPv4/IPv6 LDP interface as follows:
1. If the router interface goes down or both LDP IPv4 and LDP IPv6 sessions go down, IGP
sets the interface metric to maximum value and all static routes with the ldp-sync option
enabled and resolved on this interface will be de-activated.
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