Information About MPLS LDP Session Protection
MPLS LDP Session Protection maintains LDP bindings when a link fails. MPLS LDP sessions are
protected through the use of LDP Hello messages. When you enable MPLS LDP, the label switched
routers (LSRs) send messages to find other LSRs with which they can create LDP sessions.
• If the LSR is one hop from its neighbor, it is directly connected to its neighbor. The LSR sends out
LDP Hello messages as User Datagram Protocol (UDP) packets to all the routers on the subnet. The
hello message is called an LDP Link Hello. A neighboring LSR responds to the hello message and
the two routers begin to establish an LDP session.
• If the LSR is more than one hop from its neighbor, it is not directly connected to its neighbor. The
LSR sends out a directed hello message as a UDP packet, but as a unicast message specifically
addressed to that LSR. The hello message is called an LDP Targeted Hello. The nondirectly
connected LSR responds to the Hello message and the two routers establish an LDP session. (If the
path between two LSRs has been traffic engineered and has LDP enabled, the LDP session between
them is called a targeted session.)
MPLS LDP Session Protection uses LDP Targeted Hellos to protect LDP sessions. Take, for example,
two directly connected routers that have LDP enabled and can reach each other through alternate IP
routes in the network. An LDP session that exists between two routers is called an LDP Link Hello
Adjacency. When MPLS LDP Session Protection is enabled, an LDP Targeted Hello Adjacency is also
established for the LDP session. If the link between the two routers fails, the LDP Link Adjacency also
fails. However, if the LDP peer is still reachable through IP, the LDP session stays up, because the LDP
Targeted Hello Adjacency still exists between the routers. When the directly connected link recovers, the
session does not need to be reestablished, and LDP bindings for prefixes do not need to be relearned
https://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/fssespro.pdf