Monday, February 2, 2009

"Unknown DHCP problem.. No allocation possible"

Configuring DHCP client on a Cisco IOS router interface is pretty straight forward:

interface fastethernet1/0

ip address dhcp

But when your DHCP server is assigning addresses with option 150 for IP phones, you will not be able to get your address from the DHCP server for your interface and you will see the error message in debug dhcp:

"Unknown DHCP problem.. No allocation possible"

What you'll need to do is to ask your router interface (the DHCP client) to stop requesting option 150, and there is a per-interface command allows you to do that:

interface fastethernet1/0

ip address dhcp

no ip dhcp client request tftp-server-address

You should be able to get your address by then!