MPLS LDP Session Protection maintains Label Distribution Protocol (LDP) bindings when a link fails. MPLS LDP sessions are
protected through the use of LDP hello messages. When you enable Multiprotocol Label Switching (MPLS) LDP, the label switch
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 devices on the subnet. The hello message is called an LDP Link Hello. A neighboring
LSR responds to the hello message, and the two devices 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 specific LSR. The hello message is called
an LDP Targeted Hello. The nondirectly connected LSR responds to the Hello message and the two devices 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. For example, two directly connected devices
have LDP enabled and can reach each other through alternate IP routes in the network. An LDP session that exists between two
devices 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 devices 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 devices. 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.
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