When LSP Self-ping is enabled, destination UDP port 8503 is opened and a unique session ID is allocated for each RSVP LSP path. When an RESV message is received following a resignaling event, LSP Self-ping packets are sent at configurable periodic intervals until a reply is received from the far end for that session ID.
LSP Self-ping applies in cases where the active path is changed, while the previous active path remains up, whether it is FRR/MBB or pre-empted. These cases are as follows:
Primary in degraded state -> standby or secondary path
Standby or secondary path -> primary path (reversion)
Standby or secondary path -> another standby or secondary path (tools>perform>router>mpls>switch-path command or path preference change)
Degraded standby/secondary path -> degraded primary path (degraded primary is preferred to degraded standby/secondary path)
MBB on active path
A path can go to a degraded state either because of FRR active (only on the primary path), soft pre-emption, or LSP BFD down (when the failure action is failover).
The system does not activate a candidate path until the first LSP Self-ping reply is received, subject to the timeout. The LSP Self-ping timer is started when the RESV message is received. The system then periodically sends LSP Self-ping packets until the timer expires or the first LSP Self-ping reply is received, whichever comes first. If the timeout expires before an LSP Self-ping reply has been received and the timeout-action is set to retry, then the system tears down the candidate path (in the case of switching between paths) and go back to CSPF for a new path. The system then starts the LSP Self-ping cycle again after a new path is obtained. In the case of switching between paths, the system retries immediately and increments the retry counter. In the case of MBB, the system retries immediately, but does not increment the retry counter, which has the effect of continuously repeating the retry/LSP Self-ping cycle until a new path is successfully established.
If the configured timeout value is changed for an LSP with an in-progress LSP Self-ping session, the previous timer completes, and the new value is not used until the next lsp-self-ping session.
If no timeout is configured, then the default value is used.