[c-nsp] Brief CPU spikes on 6500 Sup 720

[Phil Mayers]

On 14/07/10 15:51, Aaron Riemer wrote:
> Yes i have read all about unicast flooding:
>
> Can occur by:
>
> 1) Asymmetric routing
> 2) Spanning Tree TCN
> 3) MAC aging out
>
> I cannot see any TCN's or Asymmetric routing so i think we may have to
> adjust the mac aging as you suggested.

If you're running HSRP, the standby node has a route for the subnet as 
well as the active e.g. traffic might flow out through the active, and 
back through the standby as follows:

host
  |
  active-standby
  |        |
(cloud)   ^
  |        |
  router --/
  |
target

from   host->active->router-target
return target->router->standby->host

...if "standby" has an ARP entry (default 4 hours) but not MAC table 
entry (default 5 minutes) it will unknown-unicast-flood the return 
traffic. If it's a lot of traffic, that will burn a lot of bandwidth...

Whether this will happen will depend on your routing topology.

>
> I am just trying to work out why the hell this has only just started
> occurring!

Well, without knowing the source of the traffic it's impossible to tell.