AirPort Extreme between two VLANS - intermittent response to ping
I have the same issue. After putting wireshark on the switch it is clear regardless of the VLAN config the management IP is being arped on all ports. that means the apple extreme is seeing the same arp on both its lan and wan port. It basically puges seeing that,cannot handle it and you have your issue. putting the router in between prevents the one arp hitting the apple.