In order for the presented method to work, it is important that network topology and policy allow the establishment of a backup LSP between the endpoint switches of the protected LSP tunnel such that, with the exception of the tunnel endpoint switches, the backup LSP does not share any resources with the path that it intends to protect. These extensions are not defined in this document. The main idea behind the presented method is to reverse traffic at the point of failure of the protected LSP back to the source switch of the protected LSP such that the traffic flow can be then redirected via a parallel LSP between source and destination switches of the protected LSP tunnel. Referring to Figure 1, there is an MPLS network consisting of 7 interconnected switches. Only failures within the protected path segment, which may include the entire primary path, are subject to fast reroute to the alternative path.
Arrows indicate direction of the data traffic.
The switch at the ingress endpoint of the protected path segment is referred as 'the source switch'. Switch 1 in Figure 1 is the source switch in our example of a protected path. Haskin, et al. The switch at the egress endpoint of the protected path segment is referred as 'the destination switch'. Switch 7 in Figure 1 is the destination switch in our example of a protected path.