Tracerouting in MPLS Networks

in mpls network the recommand command :"no mpls ip propagate-ttl" are to  add the "forwarded" for better troubleshooting for the service  provider.

no mpls ip propagate-ttl:

When the IP packet first becomes labeled on the ingress PE router, the following rule is observed:

■ When an IP packet is first labeled, the TTL field is copied from the IP header to the TTL fields

    of all the labels in the label stack after being decremented by 1.

You can change that default behavior with the command no mpls ip propagate-ttl .

 The command no mpls ip propagate-ttl stops the copying of the IP TTL to the TTL fields

in the MPLS labels. In that case, the TTL fields in the labels are set to 255. The result of this is

that for a traceroute on the local CE router to the remote CE router, the topology of the MPLS

network is hidden from the customer because the MPLS routers (except the ingress PE) are skippe.

If you configure no mpls ip propagate-ttl on the PE routers, the output of the traceroute looks

like Example 1. The P routers and egress PE router are removed from the traceroute. As such,

the customer in the VPN cannot see the P routers when tracerouting through the MPLS network.

Example1:

A drawback of this command is that when the service provider performs a traceroute in his

network (from ingress PE to egress PE), he has the same result and sees his own network as only

one hop. This obviously makes troubleshooting a bit painful. Therefore, it might be better for the

service provider to configure no mpls ip propagate-ttl forwarded on his PE routers. Disabling

TTL propagation of forwarded packets allows only the structure of the MPLS network to be

hidden from customers, but not the service provider in an MPLS VPN network. If no mpls ip

propagate-ttl forwarded is used, the TTL value from the IP header is not copied into the TTL

fields of the labels for the packets that are switched through the ingress LSR. The TTL value is,

however, copied for the locally generated packets on the ingress LSR. An illustrative example of

the latter case is an MPLS VPN network with no mpls ip propagate-ttl forwarded configured on

the ingress PE. The TTL value is not copied for packets that are received from the CE router, but

it is copied into the labels for packets that are locally generated on the ingress PE router, such as

for a traceroute in the VRF on the ingress PE router.

Example 2 illustrates this. The first traceroute is what the customer sees from the CE router, and the second traceroute is what the service provider sees from the PE router.

source:Mpls Fundamentals book.