Cisco phone was registered and now is unregistered

[jpinkney99]

Hello. I am having an issue with our CUCM where we had phones at a remote site with trusted subnets in our DHCP pool. The phones have been working fine, and registered for months now, but here recently, nothing has changed on either end, and the phones continue to say unregistered now. Do you know what could be causing this issue? Thanks in advance!

[Roger Kallberg]

What type of communication do you have with the remote site, MPLS, some VPN or SDWAN?

[jpinkney99]

It's a DMVPN Tunnel.

[Roger Kallberg]

Does that DMVPN tunnel allow traffic to and from the CM servers from the phone vlan on the remote site?

[jpinkney99]

It does. They had been registered before, multiple times. This is the only
time we have had this problem.

[Roger Kallberg]

Based on experience things do not just stop working without a reason. Likely there have been some sort of change that has caused this to happen.

[jpinkney99]

Other than a possible Latency issue, there have been no network or domain
changes. Some of the phones there worked, and some did not. Before, they
all worked.

[Roger Kallberg]

If you currently have a few working devices and a few none working my advice would be to look at one of each and compare them in detail to see what the difference is between them.

[jpinkney99]

I compared it to the same type of phone that we are using here, and they are both configured exactly the same, minus the number and MAC

[Roger Kallberg]

Down to the minimum level of details for network, vlan, IP address, DHCP scope options and so on? Have you checked the logs on the phone that doesn’t register?

[Daniel Bosch]

Do you have access to the switch the phones are connected to?  If so, the first thing I would look at is the 'show dhcp neighbor detail' output to see if the phones are getting an IP address or not (assuming Cisco switch).  Do some ping test from the voice vlan interface or SVI to CUCM subscribers and the DHCP server as Roger suggested.

If a Cisco router or switch is your dhcp server, I've seen the conflict table fill up over time and run out of IPs to give out.

Check 'show ip dhcp conflict'.  If it has a bunch of IPs, you can follow that up with 'clear ip dhcp conflict *'