LDP can only track an LDP peer using the Hello and Keep-Alive timers. If an IGP protocol registered with BFD on an IP interface to track a neighbor, and the BFD session times out, the next-hop for prefixes advertised by the neighbor are no longer resolved. This however does not bring down the link LDP session to the peer because the LDP peer is not directly tracked by BFD.
To properly track the link LDP peer, LDP needs to track the Hello adjacency to its peer by registering with BFD.
The user effects Hello adjacency tracking with BFD by enabling BFD on an LDP interface:
config>router>ldp>if-params>if>enable-bfd [ipv4][ipv6]
The parameters used for the BFD session, that is, transmit-interval, receive-interval, and multiplier, are those configured under the IP interface:
config>router>if>bfd
The source or destination address of the BFD session is the local or remote address of link Hello adjacency. When multiple links exist to the same LDP peer, a Hello adjacency is established over each link. However, a single LDP session exists to the peer and uses a TCP connection over one of the link interfaces. Also, a separate BFD session should be enabled on each LDP interface. If a BFD session times out on a specific link, LDP immediately brings down the Hello adjacency on that link. In addition, if there are FECs that have their primary NHLFE over this link, LDP triggers the LDP FRR procedures by sending to IOM and line cards the neighbor/next-hop down message. This results in moving the traffic of the impacted FECs to an LFA next-hop on a different link to the same LDP peer or to an LFA backup next-hop on a different LDP peer depending on the lowest backup cost path selected by the IGP SPF.
As soon as the last Hello adjacency goes down as a result of the BFD timing out, the LDP session goes down and the LDP FRR procedures are triggered. This results in moving the traffic to an LFA backup next-hop on a different LDP peer.