I had a cisco 3750 switch that i wanted to enable some vlans for voice on, but only having one DHCP server on a completely seperate vlan to the voice vlans.

vlan 1 = data (dhcp server on this range)

vlan 266 = voice (phone dhcp clients on this range)

PROBLEM:

The switch for some reason had aload of dhcp server config on it before i started the exercise, so i removed the pools and ran the command: no service dhcp

This was the problem, running this command (no service dhcp) disables dhcp but also disables dhcp relay to, this stops the ip helper-address command from working too.

So even when i put the ip-helper address on the vlan as below it didn't relay the dhcp broadcasts from the 266 vlan to the dhcp server on vlan 1.

interface Vlan266
 description *** VOIP-BBlock ***
 ip address 172.29.66.1 255.255.255.0
 ip helper-address 172.19.5.131

Where 172.19.5.131 is the DHCP server address.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.2_25_seb/configuration/guide/swdhcp82.html#wp1122942

RESOLUTION:

So to fix it, i ran the command: service dhcp

(making sure any dhcp pool configuration has been removed, and this fixed the problem)