IP Traffic - bad hop counts

[srego4]

Hello

On one of my routers, when I do the show ip traffic command I keep seeing the "bad hop count " counter increasing. I also note that this router has gone (and still does) through 10 - 30 minutes of 99%cpu util (ip input)

Any idea on where I can start to trouble shoot this problem? My first thought was an application error where the ttl was set wrongly ... but I know no way to verify or check this ... hence the question..

Thanks in advance

P.S. I do not have multicasting enabled on any interface on this router

[raymong]

The bad hop count counter is increased when the router receives a packet with a TTL of zero. This might be normal if you have multicast traffic sent with low TTLs. Since you don't have multicast on this router, you can do a "debug ip error" on this router to determine what traffic is being discarded. Look for debug entries with the line "dispose ip.hopcount" in it.

Refer to the following url for more information on the debug ip error command:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12supdoc/debug_r/dipdrp.htm#7680

[ChristopheLucas]

It can also be a routing loop ;)

[andrus_d]

I had this exact problem which was causing the CPU utilization to max out on a site edge switch.    

"show ip traffic"  showed thousands of bad hop counts.

I ran "debug ip error"  briefly then checked the log.  

Log was filled with entries ending with "dispose ip.hopcount" which indicates TTL expired. 

-- The IP addresses in the entries were all "default" routed - no RIB entry

Further investigation showed a faulty default route which was being propagated to the upstream router resulting in a loop.

Resolved the faulty route and the CPU utilization dropped immediately.

[Richard Burts]

Thank you for sharing your experience of how you had this problem and how you approach troubleshooting it. Glad you were able to resolve your issue. Hopefully other participants in the community will read your post and benefit from it.