mpls ldp session protection作用

當ldp router之間的連接down的時候，mpls ldp session protection用來維持LDP綁定信息。因爲LSR之間的信息是通過hello messages交互來得到的，當連接down了的時候，LSR之間不能在交互hell messages，所以LSR之間的信息在hold time到期的時候，LSR將自動清除關於neighbor的信息。MPLS LDP Session Protection uses LDP Targeted Hellos to protect LDP sessions，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.