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IP-Phone not getting DHCP

mohan mayilraj


We are having issue IP-phone 7940 some of them not getting IP address from DHCP server but some Phones are getting without issue.

We verified DHCP helper is configured correctly on Core-Swicthes .



Hi Mohan,

Please try performing factory reset once and see.

Hope it helps.


Please rate helpful posts !!


No, We did the same but still it is not getting IP address. guid eme.



did you check the firmware on all the phones. The ones that are not getting it?


We are using two model phones - 7911 & 7940. We have two voice vlan 38 & 39. Currently those IP-Phones are connected to vlan 38 and user moved to different location and those phones are connecetd to new voice vlan 39 but it is not getting Ip address from DHCP.

Firmware  - 7911- SCCP11.8-3-4sr1s & 7940 - P00308000800.



Robert Thomas
Rising star
Rising star

DHCP problem is hardly a problem related to the phone itself. If you configure an IP Address manually does the phone registers?

If you want to troubleshoot the DHCP allocation problem we would need a packet capture taken with a span session from the back of the phone

Thanks, I will capture and update.

I am new to VOIP environment and current setup has CCUM-6.1.0 Version , Cluster SUB, PUB,ICM,IEM. Could you provide basic document configuration and setup documents.


What is the configuratoin you have done in interface VLAN for the new VLAN 39. And is IP helper address command used if DHCP server not on same network and is DHCP server reachable from the VLAN 39.


Ronak patel

Rate helpful posts.

Regards Ronak Patel Rate all helpful post by clicking stars below the answer.

Hi Mohan.

Are you using voice vlan in your environment?

Is that port in Data VLAN & Voice Vlan?


What Robert is saying is correct. DHCP is far from being related to the phone, but could be in some defective firmwares.

try to plug your phone in a switch port where another phone is communicating with no problem with your DHCP server.

If it registers, double check the configuration on that port and apply it on all other ports.

Optionally, you can try a hard-reset and test again.

Yes. We tested Switch level. Same results but we did the packet capture we notice that Phone is sending DHCP discovery request but not getting DHCP (Windows Server) respond from server.

Configuration on Switch side Same as other Voice Vlan and Access port configured correctly.

Packet capture  Source -  Designation Protocol- DHCP  length - 368 Info- DHCP discover - Transaction ID 0X14703F1.

Please add the full .cap file so we can review. In any case if you don’t see any response coming from the server I think the phone will just sit there cycling.

Did you faced same kind of problem IP helper is not working correctly on different Vlan-SVI's? Do you have any documents to check IP helper and DHCP are working or not.

DHCP is providing Ip address some of the IP- PHONE but remaining 50% not getting DHCP IP address.

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