LDP Fast Reroute (FRR) provides local protection for an LDP FEC by precalculating and downloading a primary and a backup NHLFE for the FEC to the LDP FIB. The primary NHLFE corresponds to the label of the FEC received from the primary next hop as per the standard LDP resolution of the FEC prefix in the RTM. The backup NHLFE corresponds to the label received for the same FEC from a Loop-Free Alternate (LFA) next hop.
LDP FRR protects against single link or single node failure. SRLG failure protection is not supported.
Without FRR, when a local link or node fails, the router must signal the failure to its neighbors via the IGP providing the routing (OSPF or IS-IS), recalculate primary next-hop NHLFEs for all affected FECs, and update the FIB. Until the new primary next hops are installed in the FIB, any traffic destined for the affected FECs is discarded. This process can take hundreds of milliseconds.
LDP FRR improves convergence in case of a local link or node failure in the network, by using the label-FEC binding received from the LFA next hop to forward traffic for a specific prefix as soon as the primary next hop is not available. This means that a router resumes forwarding LDP packets to a destination prefix using the backup path without waiting for the routing convergence. Convergence times should be similar to RSVP-TE FRR, in the tens of milliseconds.
OSPF or IS-IS must perform the Shortest Path First (SPF) calculation of an LFA next hop, as well as the primary next hop, for all prefixes used by LDP to resolve FECs. The IGP also populates both routes in the RTM.
When LDP FRR is enabled and an LFA backup next hop exists for the FEC prefix in the RTM, or for the longest prefix the FEC prefix matches to when the aggregate-prefix-match option is enabled, LDP programs the data path with both a primary NHLFE and a backup NHLFE for each next hop of the FEC.
To perform a switchover to the backup NHLFE in the fast path, LDP follows the standard FRR failover procedures, which are also supported for RSVP-TE FRR.
When any of the following events occurs, the backup NHLFE is enabled for each affected FEC next hop:
an LDP interface goes operationally down or is administratively shut down
In this case, LDP sends a neighbor/next hop down message to each LDP with which it has an adjacency over the interface.
an LDP session to a peer goes down because the Hello timer or keepalive timer has expired over an interface
In this case, LDP sends a neighbor/next hop down message to the affected peer.
the TCP connection used by a link LDP session to a peer goes down
In this case, LDP sends a neighbor/next hop down message to the affected peer.
See RFC 5286, Basic Specification for IP Fast Reroute: Loop-Free Alternates, for more information about LFAs.