[c-nsp] Monday morning brain teaser

John Neiberger jneiberger at gmail.com
Mon Apr 1 11:36:21 EDT 2013

This is one of the strangest things I've ever seen. We have an ASR9K
(Router A) connected to a 7600 (Router B) via simple L3 link with no ACLs.
We can ping from Router A to Router B, and we can ping from Router A to a
different L3 interface on Router B. However, we cannot trace from Router A
to that other L3 interface on Router B. That alone is weird because this is
simple routing and no ACLs. That's enough of a brain teaser. However, it
gets worse.

We have a network management station that is polling these routers. While
the traceroute is running (and failing), the NMS can't poll the ASR9K and
starts to report it as down. The NMS is polling the loopback address of the
router. As soon as we stop the trace, the NMS starts getting replies from
Router A again.

So, two brain teasers:

1. Why would trace fail where ping succeeds? There are no ACLs, so this
really stumps me. We do not have CoPP configured.

2. While the trace is failing, why would our NMS stop getting replies from
the ASR9K?

I honestly don't know what to think about this. I don't think I've ever
seen anything like it.

Any thoughts?

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