Thanks for replying, how can I reproduce this issue?
Because we didn't change anything recently.
Could this be device malfunctioning?
Nope - this isn't a malfunction. Here's how to reproduce the results. (not an issue):
from some host whose traffic goes through the router, but is not in the same network as 10.0.0.X - ping any IP address that you know is not in use.
You will see the address appear in the ARP table with all-zero MAC address.
Any time any packet comes into the router with a destination address that is not in the ARP table, the router must send an ARP request to learn the MAC address of that host. Usually, the host is really there and so it replies to the ARP, and the MAC changes from all-zeros to being the actual MAC of the host.
So - if you see all-zero MAC addresses, that means there are packets going through the router with destination addresses that don't correspond to a real host. What could be some causes of this?
Someone mis-types an address.
The missing host WAS online, and was talking to some other host, and then disconnected, but the remote host is still trying to reach the disconnected host. (are you still there? Hello? If you're there, I can't hear you... hello???)
Yes, I typed network scans twice. Try looking at the ARP table on an interface with a public IP range attached to it, and you'll see tons of these.