1. Use the traceroute Command to Verify MPLS Labels

Action

To verify MPLS labels, enter the following JUNOS CLI operational mode command, where host-name is the IP address or the name of the remote host:

user@host> traceroute host-name

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)  0.861 ms  0.718 ms  0.679 ms

     MPLS Label=100048 CoS=0 TTL=1 S=1

 2  10.1.24.2 (10.1.24.2)  0.822 ms  0.731 ms  0.708 ms

     MPLS Label=100016 CoS=0 TTL=1 S=1

 3  10.1.46.2 (10.1.46.2)  0.571 ms !N  0.547 ms !N  0.532 ms !N

Sample Output 2

user@R1> traceroute 10.0.0.6 

traceroute to 10.0.0.6 (10.0.0.6), 30 hops max, 40 byte packets

 1  10.1.13.2 (10.1.13.2)  0.605 ms  0.548 ms  0.503 ms

 2  10.0.0.6 (10.0.0.6)  0.761 ms  0.676 ms  0.675 ms

What It Means

Sample Output 1 shows that MPLS labels are used to forward packets through the network. Included in the output is a label value (MPLS Label=100048), the time-to-live value (TTL=1), and the stack bit value (S=1).

The MPLS Label field is used to identify the packet to a particular LSP. It is a 20-bit field, with a maximum value of (2^^20-1), or approximately 1,000,000.

The TTL value contains a limit on the number of hops that this MPLS packet can travel through the network (1). It is decremented at each hop, and if the TTL value drops below one, the packet is discarded.

The bottom of the stack bit value (S=1) indicates that is the last label in the stack and that this MPLS packet has one label associated with it. The MPLS implementation in the JUNOS software supports a stacking depth of 3 on the M-series routers and up to 5 on the T-series platforms. For more information on MPLS label stacking, see RFC 3032, MPLS Label Stack Encoding.

MPLS labels appear in Sample Output 1 because the traceroute command is issued to a BGP destination where the BGP next hop for that route is the 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 shows that MPLS labels do not appear in the output for the traceroute command. If the BGP next hop does not equal the LSP egress address or the destination is an IGP route, the BGP traffic does not use the LSP. Instead of using the LSP, the BGP traffic is using the IGP (IS-IS, in this case) to reach the egress address (R6).