The main advantages of path protection are control over where the traffic goes after a failure and minimum packet loss when combined with fast reroute (one-to-one backup or link protection). Path protection is the configuration, within a label-switched path (LSP), of two types of paths: a primary path, used in normal operations, and a secondary path used when the primary fails, as shown in Figure 1.
In Figure 1, an MPLS network consisting of eight routers has a primary path between R1 and R5 which is protected by the secondary path between R1 and R5. When a failure is detected, such as an interface down event, an Resource Reservation Protocol (RSVP) error message is sent to the ingress router which switches traffic to the secondary path, maintaining traffic flow.
If the secondary path is pre-signaled or on standby, recovery time from a failure is faster than if the secondary path is not pre-signaled. When the secondary path is not pre-signaled a call-setup delay occurs during which the new physical path for the LSP is established, extending the recovery time. If the failure in the primary path is corrected, and after a few minutes of hold time, the ingress router switches traffic back from the secondary path to the primary path.
Because path protection is provided by the ingress router for the entire path, there can be some disadvantages, for example, double-booking of resources and unnecessary protection of links. By protecting a single resource at a time, local protection can remedy these disadvantages.