[c-nsp] Forgetting switch

Vincent De Keyzer vincent at dekeyzer.net
Mon Nov 8 12:08:49 EST 2004

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?...

More information about the cisco-nsp mailing list