LDP Session Protection
LDP session protection is based on the LDP targeted hello functionality defined in RFC 5036, LDP Specification, and is supported by the Junos OS as well as the LDP implementations of most other vendors. It involves sending unicast User Datagram Protocol (UDP) hello packets to a remote neighbor address and receiving similar packets from the neighbor router.
If you configure LDP session protection on a router or switch, the LDP sessions are maintained as follows:
- An LDP session is established between a router or switch and a remote neighboring router or switch.
- If all of the direct links between the routers or switches go down, the LDP session remains up so long as there is IP connectivity between the routers based on another connection over the network.
- When the direct link between the routers or switches is reestablished, the LDP session is not restarted. They simply exchange LDP hellos with each other over the direct link. They can then begin forwarding LDP-signaled MPLS packets using the original LDP session.
By default, LDP targeted hellos are set to the remote neighbor so long as the LDP session is up, even if there are no more link neighbors to that router or switch. You can also specify the duration you would like to maintain the remote neighbor connection in the absence of link neighbors. When the last link neighbor for a session goes down, the Junos OS starts an LDP session protection timer. If this timer expires before any of the link neighbors come back up, the remote neighbor connection is taken down and the LDP session is terminated. If you configure a different value for the timer while it is currently running, the Junos OS updates the timer to the specified value without disrupting the current state of the LDP session.
