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no mpls ip propogation-ttl

here is the topoogy:


command on A only: no mpls ip propagat..

now if a traceroute is run from x to y, it shows A and D..

My question is y is it showing D in traceroute?? is it b/c of PHP since D is not receiving Labeled pkt when destined for its directly connected subnet (link bw D and Y)..


Re: no mpls ip propogation-ttl


The question is not very clear.

But the no mpls ip propogation ttl is command is used whento hide the sp network.

The mpls ip propagate-ttl command causes a traceroute command to show all the hops traversed by the MPLS packet in the network.

The no form of the mpls ip propagate-ttl command causes a traceroute command to ignore all hops traversed by the MPLS packet in the network


shivlu jain

Re: no mpls ip propogation-ttl

Topology: x-A-B-C-D-y

MPLS domain A-B-C-D

no ip mpls propagate-ttl is configured on router A only..

1st question.(just need verification)

traceroute is run from x towards y

without PHP: the traceroute should show A and y, since D is receiving labeled packet with Label TTL ...

with PHP: the traceroute would show A,D and y, this is because C removes the label and what D receives is IP-TTL value in IP-Header..

and as u said it will ignore all hops, no it will ignore the core LSRs only not the edges.

2nd Question:

why do we need to issue the command no mpls ip propagation-ttl on all routers in MPLS domain, when disabling ttl-prop on edges serve the purpose..


Re: no mpls ip propogation-ttl


Enable ip mpls propataion ttl on all routers and run the traceroute from x to y. During trace route you will see the first two outputs with 2 labels where in one label will be changing . Now C has to do the PHP so the one label will be removed from C and it will forward the packet to D with one label which is of vpn label. Now D will remove the label and forward the ip packet to y.

The no format of command is recommended in the core so that CPE should not see the intermediate hops.


shivlu jain


Re: no mpls ip propogation-ttl

You have answered your questions.

Because of PHP, you get to see D in the traceroute. Note however, if you are running an MPLS VPN, you do not see D. This is because of the 2 labels, and only the upper label is popped.

For the second question, I do not think it is compulsory to disable ttl propagation on the core routers as well. Ttl propagation has an effect when the IP packet is being labelled. Once labelled, it has no effect.

However, consider if you have a broken LSPs. If you have a broken LSP on the core, the traceroute would show the hops from the 2nd LSP. The best rule of the thumb would be to disable it across board.

Re: no mpls ip propogation-ttl

I guess disabling ttl propagation on core routers won't help in case of broken LSPs too.

since it will any way receive IP ttl in Ip packet ...