Many-to-One Link Protection (Facility Backup) Overview
Many-to-one (facility backup) is based on interface rather than on LSP. While fast reroute protects interfaces or nodes along the entire path of a LSP, many-to-one protection can be applied on interfaces as needed, as shown in Figure 1. In Figure 1, a bypass path is set up around the link to be protected (10.0.12.14) using an alternate interface to forward traffic. The bypass path is shared by all protected LSPs traversing the failed link (many LSPs protected by one bypass path).
In Figure 1, two LSPs (lsp1-r6-to-r0 and lsp2-r1-to-r5) are protected by one preestablished bypass path from R1 to R2 through R7. Both LSPs have strict paths configured that go through interface fe-0/1/0. On R1, the interface 10.0.12.13 has link protection configured that protects the next hop 10.0.12.14.
Link protection (many-to-one or facility backup) allows a router immediately upstream from a link failure to use an alternate interface to forward traffic to its downstream neighbor. This is accomplished by preestablishing a bypass path that is shared by all protected LSPs traversing the failed link. A single bypass path can safeguard a set of protected LSPs. When an outage occurs, the router immediately upstream from the link outage switches protected traffic to the bypass link, then signals the link failure to the ingress router.
Like fast reroute, link protection provides local repair and restores connectivity faster than the ingress router switching traffic to a standby secondary path. However, unlike fast reroute, link protection does not provide protection against the failure of the downstream neighbor.
Link protection is appropriate in the following situations:
The number of LSPs to be protected is large.
Satisfying path selection criteria (priority, bandwidth, and link coloring) for bypass paths is less critical.
Control at the granularity of individual LSPs is not required.