Step 7: Check That BGP Traffic Is Using the LSP Again

Action

To verify that BGP traffic is using the LSP, enter the following JUNOS CLI operational mode command from the ingress router:

user@host> traceroute hostname

Sample Output

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)  0.858 ms  0.740 ms  0.714 ms

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

 2  10.1.36.2 (10.1.36.2)  0.592 ms !N  0.564 ms !N  0.548 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)  0.817 ms  0.697 ms  0.771 ms

     MPLS Label=100000 CoS=0 TTL=1 S=1

 2  10.1.13.1 (10.1.13.1)  0.581 ms !N  0.567 ms !N  0.544 ms !N

What It Means

The sample output shows that MPLS labels are used to forward packets through the LSP. Included in the output is a label value (MPLS Label=100016), 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), approximately 1,000,000.

The time-to-live (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 routing platforms. For more information on MPLS label stacking, see RFC 3032, MPLS Label Stack Encoding.

MPLS labels appear in the sample output 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 by default uses LSPs for BGP traffic when the BGP next hop equals the LSP egress address.

If the BGP next hop does not equal the LSP egress address, the BGP traffic does not use the LSP, and consequently MPLS labels do not appear in the output for the traceroute command, as indicated in the sample output in Check BGP Sessions.