BFD for MPLS LSPs monitors the LSP between its LERs, regardless of how many LSRs the LSP may traverse. Therefore, it enables local faults on individual LSPs to be detected, whether they also affect forwarding for other LSPs or IP packet flows. This makes BFD for MPLS LSPs ideal for monitoring LSPs carrying specific high-value services, where detecting forwarding failures in the minimal amount of time is critical. The system raises an SNMP trap, and indicates the BFD session state in show and tools dump commands if an LSP BFD session goes down. It can also optionally determine the availability of the tunnel in TTM for use by applications, or trigger a switchover of the LSP from the currently active path to a backup path.
The system supports LSP BFD on RSVP LSPs. See Label Distribution Protocol for information about using LSP BFD on LDP LSPs and the 7750 SR and 7950 XRS Segment Routing and PCE User Guide for information about Seamless BFD on SR-TE LSPs. BFD packets are encapsulated in an MPLS label stack corresponding to the FEC that the BFD session is associated with, as described in Section 7 of RFC 5884, Bidirectional Forwarding Detection (BFD) for MPLS Label Switched Paths (LSPs).
Because RSVP LSPs are unidirectional, a routed return path is used for the BFD control packets from the egress LER toward the ingress LER.