Step 2: Verify the LSP Route on the Transit Router

Purpose

If the LSP is up, the LSP route should appear in the mpls.0 routing table. MPLS maintains an MPLS path routing table (mpls.0), which contains a list of the next label-switched router in each LSP. This routing table is used on transit routers to route packets to the next router along an LSP. If routes are not present in the output for the transit router, check the MPLS protocol configuration on the ingress and egress routers.

Action

To verify the LSP route on the transit router, enter the following command from the transit router:

user@host> show route table mpls.0

Sample Output 1

user@R3> show route table mpls.0                  

mpls.0: 3 destinations, 3 routes (3 active, 0 holddown, 0 hidden)

+ = Active Route, - = Last Active, * = Both

0                  *[MPLS/0] 16w2d 21:52:40, metric 1

                      Receive

1                  *[MPLS/0] 16w2d 21:52:40, metric 1

                      Receive

2                  *[MPLS/0] 16w2d 21:52:40, metric 1

                      Receive

Sample Output 2

user@R3> show route table mpls.0 

mpls.0: 7 destinations, 7 routes (7 active, 0 holddown, 0 hidden)

+ = Active Route, - = Last Active, * = Both

0                  *[MPLS/0] 16w2d 22:26:08, metric 1

                      Receive

1                  *[MPLS/0] 16w2d 22:26:08, metric 1

                      Receive

2                  *[MPLS/0] 16w2d 22:26:08, metric 1

                      Receive

100864             *[RSVP/7] 00:07:23, metric 1

                    > via so-0/0/2.0, label-switched-path R6-to-R1

100864(S=0)        *[RSVP/7] 00:07:23, metric 1

                    > via so-0/0/2.0, label-switched-path R6-to-R1

100880             *[RSVP/7] 00:07:01, metric 1

                    > via so-0/0/3.0, label-switched-path R1-to-R6

100880(S=0)        *[RSVP/7] 00:07:01, metric 1

                    > via so-0/0/3.0, label-switched-path R1-to-R6

What It Means

Sample Output 1 from transit router R3 shows three route entries in the form of MPLS label entries. These MPLS labels are reserved MPLS labels defined in RFC 3032, and are always present in the mpls.0 routing table, regardless of the state of the LSP. The incoming labels assigned by RSVP to the upstream neighbor are missing from the output, indicating that the LSP is down. For more information on MPLS label entries, see Verifying LSP Use.

In contrast, Sample Output 2 shows the MPLS labels and routes for a correctly configured LSP. The three reserved MPLS labels are present, and the four other entries represent the incoming labels assigned by RSVP to the upstream neighbor. These four entries represent two routes. There are two entries per route because the stack values in the MPLS header may be different. For each route, the second entry 100864 (S=0) and 100880 (S=0) indicates that the stack depth is not 1, and additional label values are included in the packet. In contrast, the first entry, 100864 and 100880 has an inferred S=1 value which indicates a stack depth of 1 and makes each label the last label in that particular packet. The dual entries indicate that this is the penultimate router. For more information on MPLS label stacking, see RFC 3032, MPLS Label Stack Encoding.