Change DHCP server?

[Allan Kjaer]

Hi all,

This is my first question in this forum, so please don't be to hard on me :-):

I have installed two new DHCP servers in our German branch. My laptops are using the new server like they should (if I disable the old DHCP server service, they connect to the new one when renewing the IP address), but the Cisco 7975 phones don't. Why not? How do I force them to use the new DHCP server. Option 150 in DHCP server is created.

[Aaron Harrison]

Hi Allan

Your phones typically go into a different VLAN/Subnet to the PCs.

Your DHCP server will most likely be in the PC subnet.

Since DHCP is broadcast based, the broadcast requests for a DHCP server will reach from PC to the DHCP server, but will not reach between VLANs (as a VLAN is a broadcast domain).

On the switches or router that routes between the PC and Voice VLANs, you will typically have an 'ip helper-address' configured. This picks up the DHCP broadcast and unicasts it to the DHCP server. There's probably one set up for the IP of the old DHCP server, and you need to add the new server in there. You can have more than one for the migration period if you want..

Aaron