Secondary paths (also known as secondary LSPs) are optional and protect against link and transit node failures. If the primary path can no longer reach the egress router, the alternative, secondary path is used, as shown in Figure 3.
In Figure 3, a secondary path R1-R7-R9-R5 is activated when the primary path R1-R2-R4-R5 fails. R2 notifies R1 of the outage and R1 switches traffic to the precomputed secondary path.
Two types of secondary paths, standby and non-standby, can become active when a primary path fails, depending on which is configured. A standby secondary path, configured with the standby statement, is precomputed and pre-signaled. A non-standby secondary path, configured without the standby statement, is precomputed but is not pre-signaled.
Secondary paths configured with the standby statement consume more resources because the router must maintain state when the secondary path is not active. However, standby secondary paths do reduce recovery time by eliminating the call-setup delay that is required to establish a new physical path for the LSP.
If the problem with the primary path is corrected, after a few minutes of hold-down to ensure that the primary path remains stable, the ingress router switches traffic from the secondary path back to the primary path. It may not be always prudent for the router to switch back to the primary path. For information on how to keep the router from switching back to the primary path, see Preventing Use of a Path That Previously Failed.
To configure and verify a secondary path, follow these steps: