Cisco IP Phones requesting DHCP in data VLAN

Unanswered Question
Jul 17th, 2009

We detected on a customer that Cisco IP Phones are requesting DHCP in the data VLAN (as well as requesting DHCP in the voice VLAN).

We see the logs on the DHCP server of those requests on the data VLAN (which get replied and an IP address and lease is assigned to the IP Phone).

However, we are not able to ping the IP Phone on the data VLAN and the IP Phone is correctly registering on the CallManager with the IP address of the voice VLAN (which it is also getting from the DHCP server).

We are using CallManager 6.1.

The configuration of the switches ports is as follows:

interface FastEthernet0/x

switchport access vlan 10

switchport mode access

switchport voice vlan 20

srr-queue bandwidth share 10 10 60 20

srr-queue bandwidth shape 10 0 0 0

mls qos trust device cisco-phone

mls qos trust cos

no snmp trap link-status

auto qos voip cisco-phone

spanning-tree portfast

Has anyone faced this issue?


I have this problem too.
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iptuser55 Fri, 07/17/2009 - 03:02

If the phone is receiving a data IP address check that CDP is enabled on the switch, manually enter it in a Voice IP address and TFTP via the phone if this works then it is either CDP not enabled or if so then check the IP range in the Voice VLAN on the DHCP to confirm that is giving the correct range out.

Rui Carlos Antunes Fri, 07/17/2009 - 06:19

Hi. Thanks for the reply.

CDP is enabled on the switch.

The fact is that the IP Phone is able to successfuly get an IP address (from the DHCP server) of the Voice VLAN and is able to register with the CallManager.

However, apparently, before it gets the IP address on the Voice VLAN, it requests an IP address on the Data VLAN, to which the DHCP server replies. Afterwards, it gets the IP address on the Voice VLAN and starts using that IP address (from the Voice VLAN), and does not reply to the IP address on the Data VLAN.

I'm going to double-check the DHCP Server (a Windows server) to see if there is any problem with the DHCP range, but it looks more like some strange behaviour on the IP Phone (sending a DHCP Request on the Data VLAN before it learns, through CDP, the Voice VLAN).

James Hawkins Wed, 08/19/2009 - 07:05


Did you get a resolution to this. We have a CUCM 6.1 site with similar issues.

Rui Carlos Antunes Wed, 08/19/2009 - 07:09


No, we are still facing the same problem.

Apparently it happens only when the phones are restart/reset (including after changing the phones' configuration).

We suspect the phones' firmware but have no confirmation.

Good luck...


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