[c-nsp] Cisco 7600 TTL and MTU Failures

[Palis Michalis]

Thanks for your uselull info.. But what is actually the cause of TTL and MTU 
failures? In my case it seems tha their is a large number of TTL MTU 
failures.

Thanks again
----- Original Message ----- 
From: "Blake Willis" <cnsp at 2112.net>
To: <security at cytanet.com.cy>; <cisco-nsp at puck.nether.net>
Sent: Friday, April 07, 2006 12:29 PM
Subject: Cisco 7600 TTL and MTU Failures


> 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