All phones reset immediatley when dhcp server is offline

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Mar 22nd, 2010
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I recently had an unrelated network outage where we had to take the DHCP server offline for a few hours and had an interesting thing happen.  As soon as the server was shut down all f my DHCP registered phones said "IP Address Released" and then would not come back up until the DHCP server was back online.  Intersetingly also, there was one live voice conversation going when this happened it that call stayed up until the phone was hung up and then it too released it's IP and no longer functioned.

All of the leases and then been renewed for time lasting well past this planned outage.  I am wondering if this is because of some keepalive failing?  If so what keepalive would it be and how would I prevent this from being a problem again?

Thanks for any asistance.

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Overall Rating: 3.7 (3 ratings)
Tommer Catlin Mon, 03/22/2010 - 11:04
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Phones do not rely on heartbeats to the DHCP server unless the DHCP server was your CUCM server.    Take for example if my DHCP server was my Cisco IOS router at a remote site.  If I reboot this, the phones do not reboot.

If the network path between the phone and the CUCM server (s) listed on the phones Primary, Secondary list are not available, the phones will try and fall into SRST mode if available, or simply go "Registering..." and will keep trying to its last know TFTP server.  If no TFTP server is available, it may keep rebooting if I recall.

Sounds like you had a break in your network between the phones and the CUCM servers, unrelated to a DHCP server being down.

The only other reason, would be the lease about to expire on the phoens and the phones were getting ready to request another lease.  If that was cause, it could have been just bad timing.  Its hard to say now because the lease has been renewed and the phones are online.   Check the lease renweal time and keep an eye on it.

nstereo Mon, 03/29/2010 - 05:34
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Thanks for the reply!

My DHCP server is not the same server as my CUCM server.  Completely seperate boxes actually.  The leases were nowhere near expiration.  There was no interuption between CUCM and the phones, only the DHCP server.  This was actually a scheduled outage for the DHCP server, and we extended all the phone DHCP leases up to 8 days which left them all with at least 3 days left before they expired.  So I know it wasn't just a bad timing issue, I wish it was that simple.

Any other ideas would be greatly appreciated.

Tommer Catlin Mon, 03/29/2010 - 08:12
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Well, unless there is a bug in the release times on the phones firmware and it goes out to contact the DHCP server?  You may have to run a wireshark on the phones connection.  If you see does infact contact the DHCP server when it is not supposed to... then that would be your answer.   Could be a bug...?

CrowleyFleck Fri, 02/08/2013 - 09:35
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Looks like quite an old thread, but we are having a similar problem. Our dhcp server BSOD'd and instantly, about 120 out of 130 phones (model 8945) on site went unregistered. Even had a user come down to say he was in the middle of an active call and it terminated. When I checked my phone, the IP was When the server came back up, all phones registered. And, it wasn't a coincidental lease expiration. Our lease time is 5 days, and they had renewed 3 days prior. When I checked leases on the dhcp server after reboot, the phones still have the original expiration date of 2 days from that point. Really weird and I don't understand how a dhcp server could take down almost an entire office, or why there was a handful of phones that stayed registered. The dhcp server is also a secondary dns, and a domain controller.

Any info would be helpful.


nstereo Fri, 02/08/2013 - 17:38
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Unfortunately I never was able to get an answer as to why this happened.  It's not easy to simulate swapping a live DHCP server at the scale of that one.  We were swapping from an old DHCP server to a new one and as soon as the old server shut down, all the phones dropped.  All the books and certs say that it shouldn't happen, but it did.  I have not seen it happen again, not have I had to swap DHCP servers like that again.  So I just treat it as a cautionary tale at this point.

Even though I never got the full answer why it happened, it did take away a lesson from it.  I now, whenever possible host my DHCP scopes for phones from the nearest access or distro layer switch.  If the switch goes down, they will lose power anyway.


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