[c-nsp] 3550-12 interrupts out of control, possibly hardware?

Tóth András diosbejgli at gmail.com
Thu Aug 16 18:07:49 EDT 2012

Hi Andy,

One idea is different SDM templates being used. The SDM template is
not showing up in running-config, and changing it requires a reload as
well. I would compare them with 'sh sdm prefer' command. You might be
running out of IPv4 routes, which causes rest of routes to be applied
in software, so packets are software switched by the CPU which can
cause high utilization.



Best regards,

On Thu, Aug 16, 2012 at 6:36 PM, Andy Dills <andy at xecu.net> wrote:
> I've got a customer with a weird situation.
> They have a pretty straightforward setup, two 7200s fronting two cisco
> 3550-12s, distributing to a series of 48 port 3550s. It's a bit dated, but
> works very well for their needs.
> They have one special network attached to (only) one of the copper gige
> ports on (one of) the 3550-12s which gets a decent amount of traffic
> (~100mbps or so). It's a layer 3 connection.
> Well, one of their 3550-12s died, taking down that network. They moved the
> IP configuration of the port and moved the cable immediately, restoring
> service, and racked/configured a replacement switch, but left that network
> on the second 3550-12, as it seemed fine.
> However, once it began to come under load this morning, the CPU pegged
> (80-99%, normally at 1-2%), causing packet drops and latency.
> At that point I got involved, and for the life of me I can't figure out
> why this happened. Clearly it's interrupts, as there were no processes in
> the "sh proc cpu" that had more than 1% of CPU. However, cef was working
> fine, everything looked normal in terms of the traditional interrupt-based
> troubleshooting.
> So, after scratching our heads for a bit, I had them move the connection
> back to the original, newly-replaced switch. Note that these switches are
> configured 100% identically with the exception of IP address and hostname.
> Same IOS versions. I mean literally, if you diff the two in rancid, those
> are the only config changes.
> Zero problems from the point they moved the connection off of the switch
> in question, both switches now have 1-2% CPU and things are humming along
> fine.
> So, my question is: What could be the possible causes of this? Could this
> be a symptom of failing hardware, perhaps some bad memory requiring
> constant CPU corrections?
> Thanks,
> Andy
> ---
> Andy Dills
> Xecunet, Inc.
> www.xecu.net
> 301-682-9972
> ---
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