Loop Free Alternate: IP Fast Reroute (FRR)

We can use the IPv4 Loop-Free Alternate Fast Reroute feature to reduce the Outages duration when the primary uplinks fail on a router.

When a link or a router fails, routing algorithms like ISIS or OSPF compute new routes that take into account the failure. The time taken for computation is called routing transition. Until the transition is complete and all routers are converged on a common view of the network, the connectivity between the source and destination pairs is interrupted. Routing transition depends on the numbers of routers present in that area. The bigger the area; more the time is. This may impact on revenue of the customer also. We can use the IPv4 Loop-Free Alternate Fast Reroute feature to reduce the routing transition time to less than 50 milliseconds using a pre-calculated alternate next hop. When a router is notified of a link failure, the router immediately switches over to the repair path to reduce traffic loss.

The repair path computation is done by the Intermediate System-to-Intermediate System (IS-IS) or OSPF and the resulting repair paths are sent to the Routing Information Base (RIB). The repair path installation is Forwarding Information Base(FIB).

Eigrp has it's own Loop Free Alternate.

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