Verifying the Physical Layer
Purpose
After you have configured the LSP, issued the show mpls lsp extensive command, and determined that there is an error, you can start investigating the problem at the physical layer of the network.
Figure 10 illustrates the physical layer of the layered MPLS model.
With this layer, you must ensure that the routers are connected, and that the interfaces are up and configured correctly on the ingress, egress, and transit routers.
If the network is not functioning at this layer, the label-switched path (LSP) does not work as configured.
Figure 11 illustrates the MPLS network and the problem described in this chapter.
The network shown in Figure 11 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, traffic does not use the configured LSP. Instead traffic uses the alternate route from R1 through R2 to R6, and in the reverse direction, from R6 through R5 to R1.
When you become aware of a situation where an alternate route is used rather than the configured LSP, verify that the physical layer is functioning correctly. You might find that routers are not connected, or that interfaces are not up and configured correctly on the ingress, egress, or transit routers.
The cross shown in Figure 11 indicates where the LSP is broken because of a configuration error on ingress router R1.