IP Fast ReRoute (IP FRR)

IP networks transport packets in unconnected mode: the path taken by the packets is not pre-established before the packets are sent; each packet is routed independently of other packets in the same flow; each node (router) in the network routes the packet independently of the routing performed by the following or preceding nodes. This is known as hop-by-hop routing.

Each packet is routed by consulting a routing table, which indicates the next hop (router) to be used. This routing table is calculated by one or more routing protocols. Within an operator, or autonomous system, the internal routing protocol, such as OSPF or IS-IS, discovers the current network topology and then determines the best route. If this topology changes, due to a failure for example, the routing protocol detects the failure, announces the new topology to the whole network and calculates a new routing table.

This article introduces the problem of fast rerouting and explains why the Fast Reroute solution is faster than the internal routing protocol. It then describes the different Fast Reroute solutions applicable in IP networks: Loop Free Alternate (LFA), Remote Loop Free Alternate (RLFA) and Topology Independent Loop Free Alternate (TI-LFA). Each solution is described in terms of functionality, algorithm and applicability.