Verifying an LSP on a Transit Router

Purpose

You can verify the availability of an LSP when it is up by examining the mpls.0 routing table on a transit router. MPLS maintains the mpls.0 routing table, 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.

Action

To verify an LSP on a transit router, enter the following Junos OS CLI operational mode command:

Sample Output

mpls.0: 5 destinations, 5 routes (5 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

0                    * [MPLS/0] 7w3d 22:20:56, metric 1
                      Receive
1                    * [MPLS/0] 7w3d 22:20:56, metric 1
                      Receive
2                    * [MPLS/0] 7w3d 22:20:56, metric 1
                      Receive
100064               * [RSVP/7] 2w1d 04:17:36, metric 1
                     > via so-0/0/3.0, label-switched-path R1-to-R6
100064 (S=0)          * [RSVP/7] 2w1d 04:17:36, metric 1
                    > via so-0/0/3.0, label-switched-path R1-to-R6

Meaning

The sample output from transit router R3 shows route entries in the form of MPLS label entries, indicating that there is only one active route, even though there are five active entries.

The first three MPLS labels are reserved MPLS labels defined in RFC 3032. Packets received with these label values are sent to the Routing Engine for processing. Label 0 is the IPv4 explicit null label. Label 1 is the MPLS equivalent of the IP Router Alert label and Label 2 is the IPv6 explicit null label.

The two entries with the 100064 label are for the same LSP, R1-to-R6. There are two entries because the stack values in the MPLS header may be different. The second entry, 100064 (S=0), indicates that the stack depth is not 1 and additional label values are included in the packet. In contrast, the first entry of 100064 has an inferred S=1 which indicates a stack depth of 1 and makes it the last label in the packet. The dual entry indicates that this is the penultimate router. For more information on MPLS label stacking, see RFC 3032, MPLS Label Stack Encoding.

The incoming label is the MPLS header of the MPLS packet, and is assigned by RSVP to the upstream neighbor. Juniper Networks routers dynamically assign labels for RSVP traffic-engineered LSPs in the range from 100,000 through 1,048,575.

The router assigns labels starting at label 100,000, in increments of 16. The sequence of label assignments is 100,000, 100,016, 100,032, 100,048, and so on. At the end of the assigned labels, the label numbers start over at 100001, incrementing in units of 16. Juniper Networks reserves labels for various purposes. Table 1 lists the various label range allocations for incoming labels.