Want to use Microsoft DHCP instead of the native 520 DHCP

Unanswered Question
Sep 22nd, 2009

I would like to disable DHCP on the UC520 but I dont see any options to turn it off, it appears I can just remove the scope but then the phones hang on ftp download. Are than any option that are required on the Microsoft DHCP Server to let the phones know how to get to the second vlan?

I have this problem too.
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Marcos Hernandez Tue, 09/22/2009 - 17:14

Page 14 in this document tells you how to correctly remove the DHCP server FOR THE DATA SCOPE:


For the VOICE VLAN, we recommend that you do not delete the DHCP server. If you absolutly have to, then your new DHCP server needs to be able to communicate with the voice VLAN and must be abale to offer the UC500's voice vlan IP as option 150 (TFTP server) to the IP phones (DHCP clients).

If your DHCP server sits on the data VLAN, you can configure the voice VLAN to relay the DHCP broadcast packets as unicast traffic using "ip helper-address":




acole10000 Tue, 09/22/2009 - 17:49

So I understand the DHCP removal part, I need a little help on the ip helper address, my customers dhcp server sits at (not my network design), would the command be:

interface ethernet 1

ip helper-address

Marcos Hernandez Tue, 09/22/2009 - 17:54

No, it would be

interface bvi100

ip helper-address x.x.x.x

That is the voice VLAN.

Let us know how it goes. Also, do not forget to specify option 150 on your DHCP server for the phone DHCP scope. Option 150 should be equal to the Voice VLAN IP,


acole10000 Tue, 09/22/2009 - 18:03

1) interface bvi100

ip helper-address x.x.x.x

I assume x.x.x.x is equal to

2) I think you meant to say set option 150 to ?

Marcos Hernandez Tue, 09/22/2009 - 18:04

no, x.x.x.x is your DHCP server IP. And is correct (if you haven't change dthe default voice subnet)

sethschmautz Thu, 09/24/2009 - 17:14

Hi Adam,

Our data VLAN DHCP server is our Windows SBS server, whereas our voice VLAN is using the UC500 as our DHCP server.  So far, everything is working great.  The only thing I've had to do is set static TFTP servers on my softphones.  If you would like me to send you any information about our system, let me know.  I'd be happy to send screenshots in CCA or anything else that might be helpful.


acole10000 Fri, 09/25/2009 - 01:54


Thank you for the reply. The whole process was quite simple in retrospect.

We moved the DHCP to MS server and added option 150 pointing to (voice server) to the scope, Deleted the data vlan dhcp (make sure you keep the voice vlan dhcp). The only gotcha part was making sure the ethernet port role for our server in CCA was desktop (not phone desktop).

Worked like a charm. Thanks for all the help


John Platts Tue, 09/22/2009 - 17:55

I understand that you might not want to disable the Microsoft DHCP server. Whenever we deploy a UC500 or 800 series ISR, we generally use the DHCP server on the UC500 or ISR, even if the customer has servers running Windows Server or using Active Directory. Cisco IOS has the necessary features, and we can add settings to the DHCP server on the UC520 without breaking CCA compatibility.

Here is how DHCP is configured on the UC520:

  • IP address range must be in the subnet of the Data VLAN
  • Default gateway is set to address of Data VLAN interface on UC520
  • DNS servers are usually set to the ISP-assigned DNS servers, but would be set to the IP address of the Windows Server if you are using Windows Server and need to support Active Directory
  • Option 150 is set to Voice VLAN IP address on subnets used to connect IP phones

I understand that some SOHO routers lack the capabilities of IOS-based devices. However, the new SA500 in the Cisco Small Business Pro line actually allows you to override the DNS servers used by LAN devices, and can be used on networks running Windows Server.

acole10000 Tue, 09/22/2009 - 18:08

When using MS DHCP it auto-populates the associated forward and reverse lookup records on the MS DNS server. Am I mssing why you would not want to find internal devise by host names as oppsoed to IP address?


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