Verifying an LSP on the Ingress Router

Purpose

You can verify the availability of an LSP when it is up by examining the inet.3 routing table on the ingress router. The inet.3 routing table contains the host address of each LSP's egress router. This routing table is used on ingress routers to route BGP packets to the destination egress router. BGP uses the inet.3 routing table on the ingress router to help resolve next-hop addresses.

Action

To verify an LSP on an ingress router, enter the following JUNOS command-line interface (CLI) operational mode command:

user@host> show route table inet.3

Sample Output

user@R1> show route table inet.3 

inet.3: 1 destinations, 1 routes (1 active, 0 holddown, 0 hidden)

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

10.0.0.6/32        *[RSVP/7] 4w0d 22:40:57, metric 20

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

What It Means

The sample output shows the inet.3 routing table. By default, only BGP and MPLS virtual private networks (VPNs) can use the inet.3 route table to resolve next-hop information. One destination is listed in the route table, 10.0.0.6. This destination (10.0.0.6) is signaled by RSVP, and is the current active path, as indicated by the asterisk (*). The protocol preference for this route is 7, and the metric associated with it is 20. The label-switched path is R1-to-R6, through interface so-0/0/2.0, which is the physical next-hop transit interface.

Typically, the penultimate router in the LSP either pops the packet's label or changes the label to a value of 0. If the penultimate router pops the top label and an IPv4 packet is underneath, the egress router routes the IPv4 packet, consulting the IP routing table inet.0 to determine how to forward the packet. If another type of label (such as one created by Label Distribution Protocol (LDP) tunneling or VPNs, but not IPv4) is underneath the top label, the egress router does not examine the inet.0 routing table. Instead, it examines the mpls.0 routing table for forwarding decisions.

If the penultimate router changes the packet's label to a value of 0, the egress router strips off the 0 label, indicating that an IPv4 packet follows. The packet is examined by the inet.0 routing table for forwarding decisions.

When a transit or egress router receives an MPLS packet, information in the MPLS forwarding table is used to determine the next transit router in the LSP or whether this router is the egress router.

When BGP resolves a next-hop prefix, it examines both the inet.0 and inet.3 routing tables, seeking the next hop with the lowest preference; for example, RSVP preference 7 is preferred over OSPF preference 10. The RSVP signaled LSP is used to reach the BGP next hop. This is the default when the BGP next hop equals the LSP egress address. Once the BGP next hop is resolved through an LSP, the BGP traffic uses the LSP to forward BGP transit traffic.