[c-nsp] Cisco 7600 TTL and MTU Failures

[Blake Willis]

On Fri, 7 Apr 2006, Palis Michalis wrote:

> I think that MTU and TTL failures get hardware dropped from the router? Is 
> it true?

Yassas Michalis,

 	The PFC does do the acutal dropping, but the packets have to be 
punted up to the MSFC process-switched level in order to have TTL or MTU 
exceeded message generated.  This is why the rate-limiter is very useful for 
preventing high cpu utilisation by the "IP Input" process.

 	You can use 'sh ip traffic' to see how many messages your MSFC is 
actually generating, which is a good way to tune the rate-limiter.  In order 
to find the actual traffic that's being process-switched, 'sh int stats' 
will show you the interface counters for the various switching paths.  'sh 
buffers input-interface Xn/n header' will then show the header details of 
any packets held in the buffer while waiting for the cpu.

 	See also cisco.com/warp/public/473/6k_high_cpu.pdf and 
cisco.com/warp/public/63/ts_inputdrops_12000_18004.html.

  -Blake

---
  Blake Willis
  Network Engineer
  blake at 2112 dot net