The TTL Propagation, when transferring MPLS from IP NW NW or conversely from MPLS NW, the IP NW, between TTL Field in TTL Field and MPLS header in the IP header, is the ability to take over the value of each other .
For example, it works as follows.
However, if the TTL Propagation is enabled (often, ISP skilled owned, managed) by Traceroute such configuration information MPLS NW, (the user of the example MPLS NW) person other than own skill even made somewhat grasped put away, there is a problem.
If you disable the TTL Propagation, TTL value of MPLS header is always set to the set value (255 usually), you can so that the problem as described above does not occur.
In this case, TTL value of the IP header does not change on the MPLS NW.