Step 4: Verify MPLS Labels with the traceroute Command

Purpose

Display the route packets take to a BGP destination where the BGP next hop for that route is the LSP egress address. By default, BGP uses the inet.0 and the inet.3 routing tables to resolve the next-hop address. When the next-hop address of the BGP route is not the router ID of the egress router, traffic is mapped to IGP routes, not to the LSP. Use the traceroute command as a debugging tool to determine whether the LSP is being used to forward traffic.

Action

To verify MPLS labels, enter the following command from the ingress router:

user@host> traceroute hostname

Sample Output 1

user@R1> traceroute 100.100.6.1 

traceroute to 100.100.6.1 (100.100.6.1), 30 hops max, 40 byte packets

 1  10.1.12.2 (10.1.12.2)  0.627 ms  0.561 ms  0.520 ms

 2  10.1.26.2 (10.1.26.2)  0.570 ms !N  0.558 ms !N  4.879 ms !N

user@R6> traceroute 100.100.1.1 

traceroute to 100.100.1.1 (100.100.1.1), 30 hops max, 40 byte packets

 1  10.1.26.1 (10.1.26.1)  0.630 ms  0.545 ms  0.488 ms

 2  10.1.12.1 (10.1.12.1)  0.551 ms !N  0.557 ms !N  0.526 ms !N

Sample Output 2

user@R1> traceroute 100.100.6.1    

traceroute to 100.100.6.1 (100.100.6.1), 30 hops max, 40 byte packets

 1  10.1.13.2 (10.1.13.2)  0.866 ms  0.746 ms  0.724 ms

     MPLS Label=100912 CoS=0 TTL=1 S=1

 2  10.1.36.2 (10.1.36.2)  0.577 ms !N  0.597 ms !N  0.546 ms !N

user@R6> traceroute 100.100.1.1    

traceroute to 100.100.1.1 (100.100.1.1), 30 hops max, 40 byte packets

 1  10.1.36.1 (10.1.36.1)  0.802 ms  0.716 ms  0.688 ms

     MPLS Label=100896 CoS=0 TTL=1 S=1

 2  10.1.13.1 (10.1.13.1)  0.570 ms !N  0.568 ms !N  0.546 ms !N

What It Means

Sample Output 1 shows that BGP traffic is not using the LSP, consequently MPLS labels do not appear in the output. Instead of using the LSP, BGP traffic is using the IGP (IS-IS, in the example network in Figure 22) to reach the BGP next-hop LSP egress address. The JUNOS software default behavior uses LSPs for BGP traffic when the BGP next hop equals the LSP egress address.

Sample Output 2 is an example of output for a correctly configured LSP. The output shows MPLS labels, indicating that BGP traffic is using the LSP to reach the BGP next hop.