MPLS TTL Propogation

Hi @Network_Eric
where are you?

R3(config)#no mpls ip propagate-ttl ?***

forwarded Propagate IP TTL for forwarded traffic
local Propagate IP TTL for locally originated traffic

when do we need to use

forwarded keyword
local keyword

i need example for this

where are you?

Hey Major,

This is basically used to hide the MPLS core from the customer. Normally MPLS takes the TTL from the packet it is forwarding and applies it to the MPLS network, then at the far side, it updates the base IP packet with the reduced TTL. So, if you TTL runs out in the middle of the MPLS network, say with a traceroute, the router will respond to your traceroute. By running this command, when a packet is taken in by the router, it puts on the MPLS header and instead of copying the TTL it places a value of 255 in the field, once on the far side, the MPLS header is stripped and the original packet is unmodified. This basically makes your MPLS “invisible” to a traceroute.

The forwarded and local keywords do exactly what they say, you can restrict it to only impact locally originated traffic, or leave it at the default of also impacting forwarded traffic.

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