Q: Traceroute analysis in MPLS.
A: When traceroute packet enters MPLS domain, if TTL value of IP header is not processed, IP network will regard MPLS network as one hop. At this time, the result by traceroute is the same as the direct connection between ingress router and egress router. If necessary to display the status of middle MPLS, it needs to replicate the TTL in IP header when the packet enters MPLS domain. The traceroute packets enter MPLS domain have two types: public network and private network packets, so the process for them differs at ingress PE. If it is public network traceroute packet, the ingress router needs to configure the ttl propagate public command when it enters MPLS domain, and replicate the TTL in IP header to the TTL part in MPLS packet tag domain. Thus, each hop that traceroute packet passes in MPLS domain equals to the hop of IP datagram, and each hop in MPLS network could be displayed by traceroute. When a UDP packet in the reach of traceroute reaches a router and TTL is one, and the router transmits a packet to inform the source that TTL times out, with a processing course similar to IP. ttl propagagte public is enabled by default, so each hop in MPLS domain within the reach of traceroute could be displayed once without any configuration. If it is not required to display the status of MPLS domain, it needs to configure undo ttl propagate public command. Noticeably, all LSR must enable TTL replication; otherwise, falts may occur to result by traceroute, without reflecting the status of network. If it is private network traceroute packet, it needs to configure ttl propagate vpn command for igress router when the packet enters MPLS domain, replicating the TTL value of IP header to the TTL part in MPLS packet tag domain of IP header. It is defaulted to non-replication.