I have a test lab that I support that is a flat network that consists of a couple hubs, 2 Cisco 3500XL switches and a PIX 515 firewall. The issue is that everything will work fine for 10-15 minutes and then IP communication between hosts will cease. I was able to pinpoint the issue, just not the cause. In the switch that has the host in question directly connected I checked the arp table.
The table when everything is working is as follows (other extries removed):
Internet 192.168.3.1 10 0002.b35f.da1f ARPA VLAN1
Internet 192,168.3.166 4 OOla.646d.3ece ARPA VLAN1
when not working the arp table is this (other extries removed):
Internet 192.168.3.1 3 0002.b35f.da1f ARPA VLAN1
Internet 192,168.3.166 3 0002.b35f.da1f ARPA VLAN1
192.168.3.1 is the default gateway for the subnet and is the PIX firewall. 192.168.3.166 is a win2k server.