Traceroute

[sergeyrar]

For some time I've been wondering on some topics related to traceroute -

1. Should I care what is the return path to me while using traceroute? - Or the router just replies with an ICMP packet from whichever path that he chosen to return to me..?

2. How does traceroute work in an MPLS L3VPN environment .?.. where the P router doesn't even know how to return to me..


Thanks!!

[Reggle]

1. You should care. Assymetric routing makes traceroute very unpredictable. Not sure what you mean: an iCMP TTL expired is a packet as any other so it can be affected by routing issues in the return path.

2. Assuming you decrement TTL over P-hops: ICMP extensions for MPLS. Basically, the ICMP unreachable will be encapsulated with the same labels as the expired packet, then forwarded along the original path. Until it reaches a VRF-aware router (the PE), which takes away the last MPLS label, sees the IP packet, and forwards it according to the VRF routing table. For this reason, traceroute over MPLS will typically have a number of hops with the same latency: the ICMP packet travelled to the PE router 'on the other side' every time.

[Putney]

Have a read of this document: https://www.nanog.org/meetings/nanog47/presentations/Sunday/RAS_Traceroute_N47_Sun.pdf

It's helped me explain results to customers many times.

[Fred]

Have a read of this document: https://www.nanog.org/meetings/nanog47/presentations/Sunday/RAS_Traceroute_N47_Sun.pdf

It's helped me explain results to customers many times.

+1

Great presentation. Worth sending to the engineers and helpdesk in your organization as traceroute is commonly misinterpreted.