I have an eight port Netgear GS108Tv2 between my firewalls and their gateways. One switch serves my two firewall and the two ADSL and SDSL routers.
One of the firewalls (pfSense) pings its gateway every second. About 50% of the time it reports the gateway being down, for long periods. So, several hours it works, then several hours it doesn't. Sometimes I can still reach the WAN interface of this firewall though the router it considers down (ping and remote HTTPS admin).
It turns out that the replies from the ICMP ECHOs are being sent out of the wrong port by the switch. I see them arriving on the WAN interface of the other firewall. Rebooting the switch makes the problem go away for a while.
The switch is changed from defaults in that port 1 has a PVID of 1 and emits tagged VLAN 9 frames. The other ports are all PVID 9. This allows me access to the WAN network elsewhere on the LAN.
Is it possible my configuration has messed this up, or it is just a broken switch? Maybe they are all like this if VLANs are actually used. The web interface is pretty poor, so I don't have much confidence in it.