Strange behaviour

[Umberto]

I'm trying to setup a carp for redundancy, everything is working but I have something strange.
I use a cisco catalyst 2948G switch.
In the console I get warnings : Host xx:xx:xx:xx:xx:xx is flapping between port 2/3 and 2/2 (both the ports where the master and slave is connected) and also a lot of timeouts on a ping to both hosts.
When I use a cheapo hub, there is nothing wrong (because it doesn't check anything)

I'm using pfsense 2.0 beta atm, but I think this is a common problem, isn't it?