QUOTE(nonamer @ Jun 21 2024, 05:33 PM)
In this case, the TTL will be correct, with caveat. In fact depending on deployment it might not work.
If the ICMP message is generated, it will have to continue traveling in the tunnel. The reason is that in BGP-Free Core, there is no routing table to resolve the source address of the packet. So the packet has to reach the exit and the node will route it. What this means is that you will see high latency in your traceroute because the packet actually travel the whole tunnel and come back.
In deployment that this might not work is when you try to encapsulate anything other than IP. Eg: Ethernet. When the ICMP message reach the end of the tunnel, the node expect Ethernet packet, not ICMP, hence it will be dropped.
You need every AS in the path to enable TTL propagation.
Even if they all enable it, traceroute might still fail because they configure their core router not to generate any ICMP message combined with other control plane protection mechanism.
Let's say none of all the above is true, we still might have a situation where MPLS tunnel is encapsulated inside another MPLS tunnel. This is a common scenario called "carrier of carrier".
Say all of that is not true, then yes, your packet TTL will be accurate. However, remember there is no routing information in Layer 3 and you still have asymmetric routing.
This post has been edited by kwss: Jun 21 2024, 07:45 PM