To avoid loss of traffic, Fast Reroute (FRR) protects the LSP and allows a broken LSP to be repaired immediately at the point of failure.

A Label Switched Path (LSP) set up across a MPLS network is used to switch traffic across MPLS network. The path used by an LSP across the network is based upon network resources or any other traffic engineering constraints provided by you. Based on TE-constraints, the ingress MPLS router computes the path to be taken by LSP and signals it using RSVP protocol.

By nature, nodes and links in a MPLS network are prone to failure. It is likely that the link or the nodes through which LSP is traversing can fail. In the event of a failure of a node or link, RSVP protocol has mechanisms that inform the ingress node about the failure. On receipt of failure message for LSP across the path, the ingress router re-signals the LSP using a new path.

Due to messaging and other network delays, the ingress router cannot respond fast enough to minimalize the loss of traffic. Traffic is lost from the moment the failure occurs and until the new path is setup for the LSP, which is quite large in quantum for service provider networks.

To avoid loss of traffic, Fast Reroute (FRR) protects the LSP and allows a broken LSP to be repaired immediately at the point of failure. The point of failure is termed as "Point of local repair" (PLR), where the LSP can be repaired locally without intimating or waiting for the ingress router. PLR is the MPLS router which detects the failure and redirects the traffic appropriately to its backup path with minimal loss.

Typically at the PLR, two types of protection can be provided to LSP:

Link Protection: In this protection, the backup is selected in such a way that it avoids the failed link which was used earlier by the LSP. Traffic merges back to the main stream from the backup on the very next MPLS router. Refer to following Link protection for FRR illustrating link protection provided at R2 to LSP ingressing from R1 to R4.

Node Protection: In this protection, backup is selected in such a way that it avoids the failed link along with router to which this link connects. The node which was responsible for link failure is avoided altogether in its entirety, which was used earlier by the LSP. Traffic merges back to main stream from backup on somewhere downstream from the node, which is being avoided. Refer to Link protection for FRR illustrating node protection provided at R1 to LSP ingressing from R1 to R4.

As part of link protection for FRR, ingress routers are allowed to expose this property of MPLS RSVP LSP to you and lets you choose between link protection or node protection. Once the node protection is chosen, PLR first tries to establish a backup LSP, which provides node protection. When node protection is not possible, it attempts to fall back to link protection.

When you choose link protection over node protection, this is communicated to all routers participating in LSP. Each PLR. in this case. limits its search for backup LSP, which provides link protection. In cases where link protection cannot be offered, PLR falls back to node protection.

Link protection for FRR provides options to you to set a preferential method requested for local protection. When RSVP LSP is enabled with FRR (local protection), you are able to configure either link protection or node protection. Link protection is the default.