Receiving DHCP address on a VLAN that is not configured for that.

[jdleon]

I have a VLAN, 101, which is the only vlan on the network that is not configured to receive dhcp ip on the nodes.  Yet, for some reason, it is receiving ip address, sent by the dhcp server.  The ip address that are coming in are from another vlan.  I have checked the core switch where the vlan interface for 101 is configured and there is no "ip helper address" setup. I check the dchp server and checked the scope, that vlan 101 is receiving the ip addresses, to make sure that the router scope option is not set to the vlan 101 interface, and it's not.  As a temporary solution I am only allowing vlan 101 to enter into vlan 101.  But can anyone tell me why all of a sudden IP address are coming into the vlan 101, which is not configured to receive ip addresses?  I also had the dchp server rebooted.

[andrew.prince]

DHCP is a UDP broadcast.  VLAN to VLAN traffic requires a layer 3 interface.

Do you have "IP Directed Broadcast" enabled ?

Do you have any "ip forward-protocol udp ###"  configured anywhere?

[jdleon]

No two both questions.

[Nagaraja Thanthry]

Hello,

Can you check the core switch to see if the MAC address of the client in

VLAN 1 is registered in its MAC address table? If yes, please check which

port it is registered to. Do the same on the downstream switches as well. It

could be that there is a leak (physical loop) in your network that is

sending traffic from VLAN 101 to other VLAN.

Regards,

NT