Subject: Different TTLs

From: John Smith <[email-masked]>
Date: Mon, 30 May 2011 23:57:45 +0100 (BST)

Hi,

I would like to understand the relation between the TTL carried in the BGP label 
and the IP payload in the L3 VPN scenarios. As per my understanding it will 
always be the same as IP TTL. Is there any scenario when it will be different 
from the IP TTL. I understand that there are different models for carrying the 
transport TTL. In one model the IP TTL is copied to the transport label, which 
is reduced by each P hop and is finally copied back to the IP TTL field at the 
egress PE router. In this case the IP TTL gets decremented by as many hops as 
exist in the service provider network. In another, the TTL of the transport 
label is kept as 255 and is decremented at each hop. The IP TTL is preserved and 
the customer is oblivious to the service provider topology.

Is my understanding of the two models correct?

Now, my question is what do we do about the TTL carried in the second label (the 
BGP label)? Should it be kept as 255 or should we copy the IP TTL. Whats the use 
of setting any TTL there as this label is never exposed (unless someone is using 
implicit null at the egress that is).

Thanks, John