Fast reroute or one-to-one backup is a short-term solution to reduce packet loss associated with a particular LSP. One-to-one backup is appropriate under the following circumstances:
- Protection of a small number of LSPs relative to the total number of LSPs.
- Path selection criteria, such as bandwidth, priority, and link coloring for detour paths is critical.
- Control of individual LSPs is important.
In one-to-one backup, the ingress router adds the fast reroute object to the RSVP Path message requesting that downsteam routers establish detours. Downstream routers generate Path messages and establish detours to avoid the downstream link or node. Detours are always calculated to avoid the immediate downstream link and node, providing against both link and node failure, as shown in Figure 5.
Figure 5 shows a network with one LSP configured from the ingress router R1 to the egress router R5, transiting R2 and R4. The following detours are established:
- R1 creates a detour to R5 via R7 and R9
- R2 creates a detour to R5 via R7 and R9
- R4 creates a detour to R5 via R9
Each detour is dedicated to a particular LSP traversing the router (one detour to one LSP). If the network topology has insufficient links and nodes, it may be impossible to establish a detour. Also, detour paths are not meant for long-term use because they may provide inadequate bandwidth and can result in congestion on the links. As soon as the ingress router calculates a new path avoiding the failure, traffic is redirected along the new path, detours are torn down, and new detours established.