Configuring Explicit-Path LSPs

If you disable constrained-path label-switched path (LSP) computation, as described in Disabling Constrained-Path LSP Computation, you can configure LSPs manually or allow the LSPs to follow the IGP path.

When explicit-path LSPs are configured, the LSP is established along the path you specified. If the path is topologically not feasible, either because the network is partitioned or insufficient resources are available along some parts of the path, the LSP will fail. No alternative paths can be used. If the setup succeeds, the LSP stays on the defined path indefinitely.

To configure an explicit-path LSP, follow these steps:

1. Configure the path information in a named path, as described in Creating a Named Path. To configure complete path information, specify every router hop between the ingress and egress routers, preferably using the strict attribute. To configure incomplete path information, specify only a subset of router hops, using the loose attribute in places where the path is incomplete.

   For incomplete paths, the MPLS routers complete the path by querying the local routing table. This query is done on a hop-by-hop basis, and each router can figure out only enough information to reach the next explicit hop. It might be necessary to traverse a number of routers to reach the next (loose) explicit hop.

   Configuring incomplete path information creates portions of the path that depend on the current routing table, and this portion of the path can reroute itself as the topology changes. Therefore, an explicit-path LSP that contains incomplete path information is not completely fixed. These types of LSPs have only a limited ability to repair themselves, and they tend to create loops or flaps depending on the contents of the local routing table.

2. To configure the LSP and point it to the named path, use either the primary or secondary statement, as described in Configuring the Primary and Secondary LSPs.
3. Disable constrained-path LSP computation by including the no-cspf statement either as part of the LSP or as part of a primary or secondary statement. For more information, see Disabling Constrained-Path LSP Computation.
4. Configure any other LSP properties.

Using explicit-path LSPs has the following drawbacks:

• More configuration effort is required.
• Configured path information cannot take into account dynamic network bandwidth reservation, so the LSPs tend to fail when resources become depleted.
• When an explicit-path LSP fails, you might need to manually repair it.

Because of these limitations, we recommend that you use explicit-path LSPs only in controlled situations, such as to enforce an optimized LSP placement strategy resulting from computations with an offline simulation software package.

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