Checking the Data Link Layer
Purpose
After you have configured the label-switched path (LSP), issued the show mpls lsp extensive command, and determined that there is an error, you might find that the error is not in the physical layer. Continue investigating the problem at the data link layer of the network.
Figure 12 illustrates the data link layer of the layered MPLS model.
With this layer, you must check the encapsulation mode, for example, Point-to-Point Protocol (PPP) or Cisco High-level Data Link Control (HDLC); PPP options, for example, header encapsulation; frame check sequence (FCS) size; and whether keepalive frames are enabled or disabled. Also, check the ingress, egress, and transit routers.
If the network is not functioning at this layer, the LSP does not work as configured.
Figure 13 illustrates the MPLS network used in this chapter.
The network shown in Figure 13 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 LSP is down without a path in either direction, from R1 to R6 or from R6 to R1.
When you verify that the data link layer is not functioning correctly, you might find a mismatch with PPP or Cisco HDLC encapsulation, PPP options, or keepalive frames.
The cross shown in Figure 13 indicates where the LSP is broken because of a configuration error on ingress router R1 that prevents the LSP from traversing the network as expected.