Host with same IP of default gateway. How to prevent?

[guibarati]

Hi,

I had a problem this week in the network. A host was plugged in the network with the same IP address of the default gateway of that Vlan.

Is there someway to prevent it? I know with 802.11x I could know who is doing that, but it would not avoid the problem to occur.

Is there anyway to force the hosts to use DHCP or something?

[darrenj]

Hey there. You want to look at DHCP snooping ;-) Make sure you have a DHCP server configured, and DHCP snooping enabled on your switch. If a device tries to use any static assigned IP addresses, the switch interface will block it (it must be DHCP assigned). For your router interface, make sure you trust the interface (as you will have a static IP address assigned).

Hope this helps, good luck

Dazzler

[guibarati]

Thanks, I didn't think dhcp snooping would work this way, i thought it would only prevent an unautoryzed DHCP server to be installed on the network.

I will test that and rate.

[darrenj · Accepted solution]

It will also prevent an unauthorised DHCP server on an untrusted interface. It ensures (on an untrusted interface) only DHCP client messages such as DHCP requests (denies DHCP server messages such as DHCP replies) and only allows source IP addresses from packets that it knows have been assigned by DHCP. Does that make sense?

Darren