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advertized the binding, as identified by its LSR-id. When the next-hop is no longer available, LDP
de-activates the FEC and de-programs the NHLFE in the data path. LDP will also immediately
withdraw the labels it advertised for this FEC and deletes the ILM in the data path unless the user
configured the label-withdrawal-delay option to delay this operation. Traffic that is received
while the ILM is still in the data path is dropped. When routing computes and populates the
routing table with a new next-hop for the prefix, LDP resolves again the FEC and programs the
data path accordingly.
When LDP FRR is enabled and an LFA backup next-hop exists for the FEC prefix in RTM, or for
the longest prefix the FEC prefix matches to when aggregate-prefix-match option is enabled in
LDP, LDP will resolve the FEC as above but will program the data path with both a primary
NHLFE and a backup NHLFE for each next-hop of the FEC.
In order perform a switchover to the backup NHLFE in the fast path, LDP follows the uniform
FRR failover procedures which are also supported with RSVP FRR.
When any of the following events occurs, LDP instructs in the fast path the on the line cards to
enable the backup NHLFE for each FEC next-hop impacted by this event. The line cards do that
by simply flipping a single state bit associated with the failed interface or neighbor/next-hop:
1. An LDP interface goes operationally down, or is admin shutdown. In this case, LDP sends
2. An LDP session to a peer went down as the result of the Hello or Keep-Alive timer
3. The TCP connection used by a link LDP session to a peer went down, due say to next-hop
4. A BFD session, enabled on a T-LDP session to a peer, times-out and as a result the link
5. A BFD session enabled on the LDP interface to a directly connected peer, times-out and
The tunnel-down-dump-time option or the label-withdrawal-delay option, when enabled, does not
cause the corresponding timer to be activated for a FEC as long as a backup NHLFE is still
available.
7950 SR OS MPLS Configuration Guide
a neighbor/next-hop down message to the line cards for each LDP peer it has adjacency
with over this interface.
expiring over a specific interface. In this case, LDP sends a neighbor/next-hop down
message to the line cards for this LDP peer only.
tracking of the LDP transport address in RTM, which brings down the LDP session. In
this case, LDP sends a neighbor/next-hop down message to the line cards for this LDP
peer only.
LDP session to the same peer and which uses the same TCP connection as the T-LDP
session goes also down. In this case, LDP sends a neighbor/next-hop down message to the
line cards for this LDP peer only.
brings down the link LDP session to this peer. In this case, LDP sends a neighbor/next-
hop down message to the line cards for this LDP peer only. BFD support on LDP
interfaces is a new feature introduced for faster tracking of link LDP peers. See Section
1.2.1 for more details.
Label Distribution Protocol
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