[c-nsp] Forgetting switch

[Michael Axelrod]

good writeup on the subject:
http://www.cisco.com/en/US/customer/products/hw/switches/ps700/products_tech_note09186a00801d0808.shtml#cause1

Take a look at the itneresting troubelshooting  steps with Cat2948G.
Cisco suggested a couple of remedies, preferred is:
a..
  1.. the MAC aging time and ARP timeout to the same timeout value.
  2.. The preferable method is to change the MAC aging time to 14,400
seconds. The configuration guidelines are listed below.
  a..
    1.. CatOS
  set cam agingtime vlan agingtime_in_msec
  b.. IOS/2900 XL/3500 XL

  mac-address-table aging-time seconds [vlan vlan]

Mike
----- Original Message ----- 
From: "Vincent De Keyzer" <vincent at dekeyzer.net>
To: <cisco-nsp at puck.nether.net>
Sent: Monday, November 08, 2004 9:08 AM
Subject: [c-nsp] Forgetting switch


> Hello,
>
> 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.
>
> The complete set-up is as follows (it a standard two-routers +
two-switches
> HSRP config):
>
> * Router A is connected with POS back-to-back to Router B
> * Switch A is connected to router A, switch B is connected to router B
> * Switch A and switch B are connected to each other
> * Router A and B play HSRP, router A is the active router
> * Host A is connected to switch A, host B is connected to switch B and
> they are in the same VLAN
> * UDP (RTP) traffic for host A arrives on router B, which makes an ARP
> look-up, finds out the MAC address of host A, and sends frames to switch B
> * Switch B, who was listening the ARP requests, has understood that
> host A is behind switch A, so it forwards the frames towards it
> * Host A sends back UDP traffic towards router... A (which is the
> active HSRP router) - so this return traffic is not seen by switch B
> * So after 300 seconds (its aging-time), switch B forgets where the
> MAC address of host A is, and starts broadcasting frames on the host B
port
> also (which host B does not seem to appreciate (but that's my next
problem))
> * Router B keeps sending frames to switch B, because its ARP entry
> hasn't aged yet
>
> 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 have the option of increasing
> the aging-time of the switch, but I am not sure it's the best way? And
> anyway, I wouldn't know which value to pick?...
>
> Vincent
>
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