• 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.
• MPLS LDP Session Protection Customizations, page 32
MPLS LDP Session Protection Customizations
You can modify MPLS LDP Session Protection by using the keywords in the mpls ldp session protection
command.
Specifying How Long an LDP Targeted Hello Adjacency Should Be Retained
The default behavior of the mpls ldp session protection command allows an LDP Targeted Hello
Adjacency to exist indefinitely following the loss of an LDP Link Hello Adjacency. You can issue the
duration keyword to specify the number of seconds (from 30 to 2,147,483) that the LDP Targeted Hello
Adjacency is retained after the loss of the LDP Link Hello Adjacency. When the link is lost, a timer starts.
If the timer expires, the LDP Targeted Hello Adjacency is removed.
Specifying Which Routers Should Have MPLS LDP Session Protection
The default behavior of the mpls ldp session protection command allows MPLS LDP Session Protection for
all neighbor sessions. You can issue either the vrfor for keyword to limit the number of neighbor sessions
that are protected.
Enabling MPLS LDP Session Protection on Specified VPN Routing and Forwarding Instances
If the router is configured with at least one VPN routing and forwarding (VRF) instance, you can use the
vrf keyword to select which VRF is to be protected. You cannot specify more than one VRF with the mpls
ldp session protection command. To specify multiple VRFs, issue the command multiple times.
Enabling MPLS LDP Session Protection on Specified Peer Routers
You can create an access list that includes several peer routers. You can specify that access list with the for
keyword to enable LDP Session Protection for the peer routers in the access control list.
MPLS LDP Session Protection Customizations
Information About MPLS LDP Session Protection
MPLS LDP Configuration Guide, Cisco IOS Release 12.4
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