Node-link protection (many-to-one or facility backup) extends the capabilities of link protection and provides slightly different protection from fast reroute. While link protection is useful for selecting an alternate path to the same router when a specific link fails, and fast reroute protects interfaces or nodes along the entire path of an LSP, node-link protection establishes a bypass path that avoids a particular node in the LSP path.
When you enable node-link protection for an LSP, you must also enable link protection on all RSVP interfaces in the path. Once enabled, the following types of bypass paths are established:
- Next-hop bypass LSP—Provides an alternate route for an LSP to reach a neighboring router. This type of bypass path is established when you enable either node-link protection or link protection.
- Next-next-hop bypass LSP—Provides an alternate route for an LSP through a neighboring router en route to the destination router. This type of bypass path is established exclusively when node-link protection is configured.
Figure 7 illustrates the example MPLS network topology used in this section. The example network uses OSPF as the interior gateway protocol (IGP) and a policy to create traffic.
The MPLS network in Figure 7 illustrates a router-only network that consists of unidirectional LSPs between R1 and R5, (lsp2-r1-to-r5) and between R6 and R0 (lsp1-r6-to-r0). Both LSPs have strict paths configured that go through interface fe-0/1/0.
In the network shown in Figure 7, both types of bypass paths are preestablished around the protected node (R2). A next-hop bypass path avoids interface fe-0/1/0 by going through R7, and a next-next-hop bypass path avoids R2 altogether by going through R7 and R9 to R4. Both bypass paths are shared by all protected LSPs traversing the failed link or node (many LSPs protected by one bypass path).
Node-link protection (many-to-one or facility backup) allows a router immediately upstream from a node failure to use an alternate node 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.
When an outage occurs, the router immediately upstream from the outage switches protected traffic to the bypass node, and then signals the failure to the ingress router. Like fast reroute, node-link protection provides local repair, restoring connectivity faster than the ingress router can establish a standby secondary path or signal a new primary LSP.
Node-link protection is appropriate in the following situations: