Link Ldp Hello Adjacency Tracking With Bfd - Alcatel-Lucent 7450 Manual

Label Distribution Protocol
BFD tracking of an LDP session associated with a T-LDP adjacency allows for faster detection of
the liveliness of the session by registering the peer transport address of a LDP session with a BFD
session. The source or destination address of the BFD session is the local or remote transport
address of the targeted or link (if peers are directly connected) Hello adjacency which triggered
the LDP session.
By enabling BFD for a selected targeted session, the state of that session is tied to the state of the
underneath BFD session between the two nodes. The parameters used for the BFD are set with the
BFD command under the IP interface which has the source address of the TCP connection.

Link LDP Hello Adjacency Tracking with BFD

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 since the LDP peer is not directly tracked by BFD.
In order 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>interface-parameters>interface>enable-bfd [ipv4][ipv6]
The parameters used for the BFD session, i.e., transmit-interval, receive-interval, and multiplier,
are those configured under the IP interface:
config>router>interface>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 but only a single LDP session will exist to the peer and will use 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 will immediately bring down the Hello
adjacency on that link. In addition, if the there are FECs which have their primary NHLFE over
this link, LDP triggers the LDP FRR procedures by sending to IOM the neighbor/next-hop down
message. This will result 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 due to BFD timing out, the LDP session goes down
and the LDP FRR procedures will be triggered. This will result in moving the traffic to an LFA
backup next-hop on a different LDP peer.
7450 ESS MPLS Guide Page 531