TTL

[amms68958]

Hi,

An IP packet is traversing thru an MPLS network.I know that as it passes thru each router the MPLS TTL value is decremented..But just wanted to know what happens to the IP TTL value..?

Thanks in advance..

[f.geldner]

Hi,

Label TTL value is decremented in the MPLS backbone and copied back into IP TTL on egress PE router.

Frank

[mheusinger]

Hello,

the correct behaviour and all options are described in RFC 3443. Mainly there are two options you can configure in a Cisco PE:

R(config)#mpls ip propagate-ttl

This is the default. The reduced IP TTL will be used in all imposed labels by the PE. Then in the MPLS network, label TTL are decreased and IP TTL is unmodified until the IP packet is forwarded without labels. Then the last lael TTL decremented by one will be used in IP header. The main idea is, that a traceroute will still show all hops in the proper order.

The second option is:

R(config)# no mpls ip propagate-ttl forwarded

This will set the label TTL to 255 for Customer traffic and thus transparently tunnel the IP TTL through the MPLS domain. This allows to hide MPLS core routers from customers. Nice feature for a SP.

Hope this helps! Please rate all posts.

Regards, Martin