IPv4 Loop-Free Alternate Fast Reroute
When a link or router fails, distributed routing algorithms compute new routes that take into account the change. The time
taken for computation is called the routing transition. Until the transition is complete and all routers are converged on
a common view of the network, connectivity between the source and destination pairs is interrupted. You can use the IPv4 Loop-Free
Alternate Fast Reroute feature to reduce the routing transition time to less than 50 milliseconds using a precomputed alternate
next hop. When a router is notified of a link failure, the router immediately switches over to the repair path to reduce traffic
IPv4 Loop-Free Alternate Fast Reroute focuses on the precomputation of repair paths. The repair path computation is done by
the IS-IS routing protocol and the results (the repair paths) are sent to the RIB. The repair path installation is done by
Cisco Express Forwarding.
In Cisco IOS XE Release 3.6S, this feature was introduced in ASR 903 Series Aggregation Services Routers.
The following commands were introduced or modified:
debug isis fast-reroute ,
fast-reroute load-sharing disable ,
fast-reroute per-prefix ,
fast-reroute tie-break ,
show isis fast-reroute .