Checking the MPLS Layer

Purpose

After you have configured the label-switched path (LSP), issued the show mpls lsp command, and determined that there is an error, you might find that the error is not in the physical, data link, Internet Protocol (IP), interior gateway protocol (IGP), or Resource Reservation Protocol (RSVP) layers. Continue investigating the problem at the MPLS layer of the network.

Figure 21 illustrates the MPLS layer of the layered MPLS model.

Figure 21: Checking the MPLS Layer

With the MPLS layer, you check whether the LSP is up and functioning correctly. If the network is not functioning at this layer, the LSP does not work as configured.

Figure 22 illustrates the MPLS network used in this chapter.

Figure 22: MPLS Network Broken at the MPLS Layer

The network shown in Figure 22 is a fully meshed configuration where every directly connected interface can receive and send packets to every other similar interface. The LSP in this network is configured to run from ingress router R1, through transit router R3, to egress router R6. In addition, a reverse LSP is configured to run from R6 through R3 to R1, creating bidirectional traffic.

However, in this example, the reverse LSP is down without a path from R6 to R1.

The cross shown in Figure 22 indicates where the LSP is broken. Some possible reasons the LSP is broken might include an incorrectly configured MPLS protocol, or interfaces that are incorrectly configured for MPLS.

In the network shown in Figure 22, a configuration error on egress router R6 prevents the LSP from traversing the network as expected.

Steps To Take

To check the MPLS layer, follow these steps:

1. Verify the LSP
2. Verify the LSP Route on the Transit Router
3. Verify the LSP Route on the Ingress Router
4. Verify MPLS Labels with the traceroute Command
5. Verify MPLS Labels with the ping Command
6. Verify the MPLS Configuration
7. Take Appropriate Action
8. Verify the LSP Again