[c-nsp] Forgetting switch

[lee.e.rian at census.gov]

On 11/08/2004 "Vincent De Keyzer" <vincent at dekeyzer.net> wrote:


> I have got problem with a switch that forgets a certain MAC address after
> 300 seconds, and starts forwarding frames on all ports of the switch that
> are in that VLAN, which disturbs one host.

on a cat 6000
set cam aging 1-1000 21600

> The complete set-up is as follows
      <.. snip details ..>
> According to a friend, this is a classical problem, but it's new to me
(and
> my friend forgot the solution!).
>
> How does this sort of problem ("router still has ARP entry, but switch
has
> forgotten MAC address") usually get fixed?

I dunno.  The Cisco people I talked to really didn't like the idea of
changing the cam aging time to 6 hours

> I have the option of increasing
> the aging-time of the switch, but I am not sure it's the best way?

Me either.  The only solutions I could think of were
- redesign the network so it can't happen again
- change the arp timeout on the routers to something < cam aging time on
the switches
- change the cam aging time on the switches to something > arp timeout on
the routers

I couldn't think of a network redesign that would eliminate all of the
possibilities for unicast flooding & I didn't like the idea of reducing the
arp timeout on the routers.  So that left increasing the cam aging time...

> And anyway, I wouldn't know which value to pick?...

Any value greater than the routers arp timeout value

HTH
Lee