Prevent DECT IP phone from getting DHCP IP from Cisco router

[shenmue232]

Good morning,

We have an nec phone system which uses IP phones, the nec phone system has its own DHCP server which the phones need to get an IP address for.

Problem is the cisco primary router also hands out an IP via dhcp and the phones often get an IP from the Cisco instead of the nec DHCP phone system.

Can we prevent the Cisco router from replying to the DHCP request or relay specific Mac address for the phone handset to the nec phone system to get it's correct IP?

Hope this makes sense.

Regards

Shenmue232

[marce1000]

 - It's partially an architectural problem ; I don't like DHCP servers on routers, since  I believe L2 and L3 services should be separated from DHCP services. Assigning the DECT phones to their own vlan , with the DECT DHCP server on it could then resolve the problem. Possibly using DHCP snooping to futher restrict which DHCP servers can be reached from  a particular VLAN.

 M.

[Georg Pauwen]

Hello,

I guess you could try and use a database for the IOS DHCP server. The database is just a txt (text) file (I have attached an example) stored in flash, and that is used to bind IP addresses to MAC addresses. Use TFTP to upload it to the flash in your router.

R1(config)#ip dhcp database dhcp.txt

The thing I am not sure about is whether you have to actually NOT include the MAC addresses of the DECT phones in the text file, in order to prevent them from getting an IP address, or to add the MAC address WITHOUT an IP address. You would need to test this...

[pieterh]

you can create a separate VLAN for phones and NEC-DHCP server where your primary router is not routing

(primary router no interface in this vlan)

then create a secondary router (maybe a VRF will work?)  that routes between this vlan and the rest of the network, 

do NOT configure ip-helper here so no dhcp-forwarding to your primary router.

if you use the "voice vlan" option, you may need to remove or configure a different voice vlan on ports where the DECT systems enter your LAN (some DECT-gateway I presume).