We have two Draytek Vigor 2930 (firmware v 3.2.1) firewall routers on the same subnet, call them A and B. A has DHCP service enabled and all our LAN clients get their IPs from it, it's also the default gateway. B has DHCP service disabled. Both routers have 2 external ISP connections, although most LAN clients use only A's.

Recently we've seen strange behaviour where Draytek B appears to register its MAC address against an IP address used by one of our LAN clients.

In the DHCP table on A, we can see

192.168.1.113 00-23-DF-97-70-20

that's the IP and MAC address of the LAN client. However, in the ARP cache on the same router we see

192.168.1.113 00-50-7F-C7-60-04

the LAN client's IP address is mapped to a MAC address that actually belongs to Draytek router B. But Draytek B has a static IP of 192.168.1.2. It seems router B is responding to ARP broadcasts for the .113, and registering its MAC address in the ARP cache of router A.

As a result, the LAN client cannot access external Internet - even pings to the default gateway fail about 80% of the time.

The problem only goes away when we reboot Draytek A - clearing its ARP cache doesn't seem to help.

This has been getting more frequent recently - does anyone have any ideas why it's happening?