Labeled-BGP route tunnels pose a challenge to traceroute capability because there are two labels used in the transport layer: an inner BGP label identifying the far-end node and the regular label identifying the next hop for that far end.
Traceroute and TTL monitoring on the 7705 SAR have been enhanced to be able to identify and report every PE, ABR, ASBR, and P node along the path of a Layer 2 or Layer 3 service built partially or wholly over labeled-BGP route tunnels.
Both the MPLS tunnel TTL and the labeled-BGP route tunnel TTL are monitored for TTL expiry, which causes an ICMP TTL expired message to be sourced. Depending on the role of the intermediate nodes along the path, monitoring both TTL values is the most comprehensive way to ensure proper TTL handling.