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Larry G


Tracing interfaces

I have two routers on the network and I'm having some difficulty tracing the traffic. This is probably a basic question, but is there a way for me to check which interfaces a ping is going through?

Here's why - one of the routers is connected to a P2P T-1 between us and a customer while the other router is our primary router for external internet access and hosting. When I attempt to ping a machine that exists on the customer network from my external router, the ping traverses the P2P router (expected) and succeeds; however, when I attempt the same thing from the P2P router, it fails.

Performing a traceroute on the external facing router yields a full set of results that clearly show it going through the P2P router, but a traceroute on the P2P router die on the P2P interface.

I'm trying to determine why this is happening. I'm suspecting that somehow my NAT on the external router is actually ALLOWING it to traverse the P2P router and the nonexistent NAT on the P2P is somehow blocking it, but I can't tell because I can't figure out which interface is being used during the route traversal.

Any help is appreciated - even if you could point me in the right direction, I'd appreciate it.



ping x.x.x.x source y.y.y.y or ping x.x.x.x source interface s0/0

Replace x.x.x.x with your target IP and y.y.y.y with the source IP or interface.
I do not, have not, and will not work for AT&T/Comcast/Verizon/Charter or similar sized company.

Larry G


Thanks for the fast response! Definitely helped.