[c-nsp] MAC / CAM flapping problem

[Bill Wichers]

I have a Catalyst 4000 with several trunks to other switches, and I've
been seeing a lot of this message:

2006 Jul 17 19:07:39 edt -04:00 %SYS-4-P2_WARN: 1/Host xx:xx:xx:xx:xx:xx
is flapping between port 2/6 and port 2/4

Where all the xx:xx: stuff is the MAC address of the gateway out, which is
on port 2/4. Port 2/6 is an uplink to another switch with some 100+
servers on it.

I've checked and there is no loop in the network (everything stars out
from the 4000), and the other switches are unfortunatly not Cisco (yet,
we're replacing them) so I have limited diagnostic tools available on
them.

The problem I'm having is that every time the flapping occurs it causes a
burst of packet loss, presumably while the switch relearns the correct MAC
address on the correct port. Both ports are 802.1q trunks carrying
numerous VLANs. I was looking at port security, but the docs imply that it
can't work on a trunk.

Does anyone know of a way to force the switch (running CatOS v8.1(3)) to
ignore the routers MAC address if it hears it as a source address on any
port except 2/4? I can't seem to find a way to do it that will work on the
trunk ports AND will still allow traffic to pass to the router using the
router's MAC address as a destination.

Any help much appreciated!

     -Bill

*****************************
Waveform Technology
Systems Engineer