Bidirectional forwarding detection (BFD) for MPLS LSPs monitors the LSP between its LERs, irrespective of how many LSRs the LSP may traverse. This feature enables the detection of faults that are local to individual LSPs, irrespective of whether they also affect forwarding for other LSPs or IP packet flows. BFD for MPLS LSPs is ideal for monitoring LSPs that carry high-value services, and for which the quick detection of forwarding failures is critical. If an LSP BFD session goes down, the system raises an SNMP trap and indicates the BFD session state in the show and tools dump commands.
SR OS supports LSP BFD on RSVP and LDP LSPs. See MPLS and RSVP for information about using LSP BFD on RSVP LSPs. BFD packets are encapsulated in an MPLS label stack corresponding to the FEC that the BFD session is associated with, as described in RFC 5884, Section 7. SR OS does not support the monitoring of multiple ECMP paths that are associated with the same LDP FEC which is using multiple LSP BFD sessions simultaneously. However, LSP BFD still provides continuity checking for paths associated with a target FEC. LDP provides a single path to LSP BFD, corresponding with the first resolved lower interface index next-hop, and the first resolved lower TID index for LDP-over-RSVP cases. The path may potentially change over the lifetime of the FEC, based on resolution changes. The system tracks the changing path and maintains the LSP BFD session.
Because LDP LSPs are unidirectional, a routed return path is used for the BFD control packets traveling from the egress LER to the ingress LER.