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Label Distribution Protocol
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 given 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 will program the data path with both a primary
NHLFE and a backup NHLFE for each next hop of the FEC.
In order 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:
Refer to RFC 5286, Basic Specification for IP Fast Reroute: Loop-Free Alternates,
for more information on LFAs.
386
• 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.
Use subject to Terms available at: www.nokia.com
© 2022 Nokia.
MPLS Guide
3HE 18686 AAAB TQZZA
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