DHCP Vendor Classes & Cisco IP Phones

Unanswered Question
Jun 3rd, 2008

Cisco IP Phones identify themselves using DHCP Option 60 - for example a 7940 sends 'Cisco Systems, Inc. IP Phone CP-7940G' in it's DHCP discover/requests. With other DHCP clients using vendor classes allows me to customise the DHCP offer/ACK for specific clients (using a Windows 2003 DHCP Server). I attempted to configure this up recently and it appeared to work - i.e. I was only replying with Option 150 to the Vendor Classes I had configured (7940, 7960 & 7970). However when I installed a new IP Phone it wouldn't register with CCM. I did a bit of playing around and found the TFTP server IP address wasn't in the phone, I could see the DHCP server replying correctly with the Option 150 embedded in Vendor Specific Info Option 43. I then modified DHCP to send Option 150 to all clients and the phone registered OK. I then changed DHCP back and the phone continued to work after several resets. I then did a factory reset on the phone and again it didn't work.

From this I can only conclude that Vendor Classes don't work with IP Phones, either that or I am configuring it wrong. The configuration of the DHCP Server is the same as I use for LWAPP AP's (obviously different vendor class & options). I am running the latest SCCP code on all the 7940/60 & 7970's.

Has anyone sucessfully used Vendor Classes with Cisco IP Phones?


I have this problem too.
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Paolo Bevilacqua Mon, 06/09/2008 - 06:29

Hi, for this very reason the best practice is to use VLAN and separated TFTP servers, namely CM or CME for the phones.

This will eliminate any conflicting issue like the ones you have observed.

andrew.butterworth Mon, 06/09/2008 - 06:35

Yes I fully understand that and that is how the network is set up (separate Voice & Data VLANs per switch). The (potential) issue is users who sniff the network and capture the CDP packets can then create 802.1q tagged packets and can receive a DHCP reply on the voice VLAN which will contain the Option 150 details. Plus coding this manually on every DHCP scope is a labourious task, configuring it once globally is much better.


iptuser55 Tue, 09/30/2008 - 02:31


Did you get this to work. We are looking to use Nortel switches which do not recognise Cisco Handsets and so the User Class may be good for us- what are they requirements?


andrew.butterworth Tue, 09/30/2008 - 02:40

I gave up in the end. My only conclusion was that Cisco IP Phones (or at least the 7940/7960 & 7970's I have) don't support DHCP Vendor Classes and Option 150 in the General Options must be configured.

What you are attempting to do though is something different - as far as I know Cisco IP Phones can only discover the Voice VLAN via CDP. Unlike other vendors IP Phones that can use DHCP there is no other automatic mechanism for voice VLAN discovery.

No, hang on, the latest IP Phones (7941, 7961 etc) support LLDP which is probably supported on the Nortel switches so this may work?


iptuser55 Tue, 09/30/2008 - 03:49


Thanks, My thinking is that

1. Phone when it broadcasts the DHCP request also sends out the vendor class " CISCO PHONE7941......" Option 60

2. DHCP responds to the request but assigns a " voice " scope based upon the option 60 which only voice scopes have been configured with

3. Voice scope also has option 150 configured as well

4. DHCP provides the voice IP range as well as the Option 150

In theory the Vendor class can be any "free text field" Video, Application x .....

Is this correct or completely rubbish

many thanks


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