MPLS missing a hop, but packet still can reach ? Bug ?

[Thomas Leong]

 

This is the Topology 

Hi Expect,

A Hops Missing, but still can reach destination, why ya ? 

Source 1.1.1.1 to Destination 6.6.6.6

Below i share a YAML File (Cisco Modeling Lab)

Thanks,

Thomas

[Sergey Lisitsin]

It doesn't look to be skipping a hop, you get 5 hops, which is correct as per your topology. What the reason might be is that your hosts in the MPLS network don't have the routes back to 1.1.1.1 and 6.6.6.6 to return ICMP unreachable packets to, that's why one of the hops just times out. If your core is BGP-free, that is normal.

[Thomas Leong]

Hi @Sergey Lisitsin 

Thanks for your respond !!!

1. In traceroute * * * also count as 1 hop?
2. Could it be because MPLS TTL expired, and dont know how to send back to the original source ? because they are P and dont have the route to send back ICMP Message to the original packet. (so traceroute show this ***), thats why they just forward the packet to PE, and when PE receive the packet, they just go the the next hops. am i right ?

[Sergey Lisitsin]

Yes, * * * in traceroute means that there was no response received for that packet. But you still get the number of hops listed and their respective numbers.

[MHM Cisco World]

https://www.youtube.com/watch?v=LSA0FshJpwk

please see video and in end of it the mention about the bug cause missing hop.