Depending on how the network is built on elements in the topology, a node along the path might be operating based on the outer MPLS tunnel label or the inner BGP label. In order to ensure that all the nodes along the path are displayed correctly, the 7705 SAR sets the TTL of the outer MPLS transport tunnel and the inner labeled-BGP route tunnel to identical values. The next node is therefore identified and displayed correctly in a traceroute regardless of which label it is operating on.
For a self-generated traffic traceroute, both the MPLS and the labeled-BGP TTLs are set to 1 in order to identify the first node. At the next hop, both TTL values are incremented to 2. This pattern continues at each hop in the path.