I have 3 x VLAN's with associated DHCP pools.   VLAN2 =  10.1.2.0  VLAN4 = 10.1.4.0  VLAN5 = 10.1.5.0. 

For some reason my 10.1.2.X network seem to be allocating the entire DHCP range to an unknown device with mac address 00:00:00:00:00:00  t other two VLAN's are configure exactly the same, and no such issues - not even sure where to start to look.

As soon as I release any of the leases, it gets immediatly populated with another 00:00:00:00:00:00 

Cheers

Keith

Did you do a Packet Sniff to see what may be going on?

U can block access to the MAC address. Just go to IP DHCP-SERVER LEASES, add manually and then block access to the suspect MAC address.

is vlan2 connected to wireless access point which is open?? may be a dhcp attack or some device in vlan2 have bad configuration.

Nope Wireless - Its a VERY closed and secure network - there are about 100 hosts on it, all with Static IP's.  Just wanted to set up a pool of 10 DHCP addresses, that can be used by new equipment that defaults to DHCP, so we can then configure them to their Static IP's.  It was only when I set up the DHCP server on that VLAN we discoverd the 'Issue'.

It may be a defective or badly configured piece of equipment. Try to locate it e.g. by segmenting the network,
sniffing the traffic and look in the request, etc.

What firmware are you running on? If so anything above 6.33.3 firmware causes that issue. it's a bug..

What firmware are you running on? If so anything above 6.33.3 firmware causes that issue. it's a bug..

I had this problem on my home's RB951Ui after changing network netmask on interface and DHCP server. I could't solve it until when rebooted my RB and everything came to work properly. What a crappy bug, MikroTik!