Repair paths forward traffic during a routing transition. When a link or a router fails due to the loss of a physical layer
signal or the failure of a Bidirectional Forwarding Detection (BFD) session, initially only the neighboring routers are aware
of the failure. All other routers in the network are unaware of the nature and location of this failure until information
about this failure is propagated through a routing protocol, which may take several hundred milliseconds. Therefore, packets
affected by the network failure need to be steered to their destination.
A router adjacent to the failed link employs a set of repair paths for packets that would have used the failed link. These
repair paths are used from the time the router detects the failure until the routing transition is complete. By the time the
routing transition is complete, all routers in the network revise their forwarding data and the failed link is eliminated
from the routing computation.
Repair paths are precomputed in anticipation of failures so that they can be activated the moment a failure is detected.
When a protected element fails, a repair node carries traffic around it toward the destination. When the protecting node detects
this failure, it directs traffic around the protected element towards the repair node. In general, a repair node may be directly
connected to the protecting node.
The IS-IS remote LFA FRR feature uses the following repair paths:
Equal Cost Multipath (ECMP) uses a link as a member of an equal cost path-split set for a destination. The other members
of the set can provide an alternative path when the link fails.
LFA is a next-hop route that delivers a packet to its destination without looping back. Downstream paths are a subset of LFAs.