Each primary LSP can be protected by up to two secondary LSPs. When the LER detects a primary LSP failure, it signals its secondary LSPs, if any have been configured, and automatically switches to the first one that is available. LSP redundancy supports shared risk link groups (SRLG). See Shared Risk Link Groups for more information on SRLG.
LSP redundancy differs from the Fast Reroute (FRR) feature in that LSP redundancy is controlled by the LER that initiated the LSP, whereas FRR uses the node that detects the failure to take recovery action. This means that LSP redundancy takes longer to reroute traffic than FRR because failure messages need to traverse multiple hops to reach the LER and activate LSP redundancy, whereas an FRR-configured node responds immediately to bypass the failed node or link. See RSVP-TE Fast Reroute (FRR) for more information on FRR.
The following parameters can be configured for primary and secondary LSPs:
bandwidth — the amount of bandwidth needed for the secondary LSP can be reserved and can be any value; it does not need to be identical to the value reserved by the primary LSP. Bandwidth reservation can be set to 0, which is equivalent to reserving no bandwidth.
inclusion and exclusion of nodes — by including or excluding certain nodes, you can ensure that the primary and secondary LSPs do not traverse the same nodes and therefore ensure successful recovery. Each secondary LSP can have its own list of included and excluded nodes.
hop limit — the hop limit is the maximum number of LSRs that a secondary LSP can traverse, including the ingress and egress LERs.
standby (secondary LSPs only) — when a secondary LSP is configured for standby mode, it is signaled immediately and is ready to take over traffic the moment the LER learns of a primary LSP failure. This mode is also called hot-standby mode.
When a secondary LSP is not in standby mode, then it is only signaled when the primary LSP fails. If there is more than one secondary LSP, they are all signaled at the same time (upon detection of a primary LSP failure) and the first one to come up is used.
If a path-preference priority value is configured for standby secondary LSP paths, the standby secondary LSP configured with the highest path priority becomes the active LSP when the primary LSP fails.