[c-nsp] Re-thinking (remembering) how a switch operates

[Andre Beck]

On Wed, Apr 27, 2005 at 10:21:55PM -0400, Jeff Kell wrote:
> 
> [... unknown unicast going to flood the whole BD ...]
>
> Now afterwards, it has me thinking philosophically about the relatively
> short default mac-address table aging time (300 secs is default in IOS
> and CatOS, IIRC)

Which is a standard.

> versus the relatively long ARP cache timeout (which is what,
> 400 minutes?

4h (in words: *four* *hours*). And that's a Cisco specialty. Nobody
else is using such insane large ARP cache timeouts AFAIK.

> it's a real long time relative to mac-address
> aging).  Having the ARP cache saves you from having to do frequent ARPs,
> but if you *did* ARP a little more frequently, it would keep the
> mac-address tables loaded up when the answer was returned.  And if the
> device is down, but still in the ARP cache, anything sent to the device
> will be sent (layer-3) and broadcast (layer-2 due to the switches).

End stations of today usually have ARP cache timeouts of 120s to 300s.
It's just the Cisco side that breaks it. I'm configuring "ip arp timeout
300" on any Ethernet (sub)interface I can get hold of.
 
-- 
                  The _S_anta _C_laus _O_peration
  or "how to turn a complete illusion into a neverending money source"

-> Andre Beck    +++ ABP-RIPE +++    IBH Prof. Dr. Horn GmbH, Dresden <-