I am not well versed in Telephony, so any answers, please keep it simple.
I had an event occur recently where phones began to de-register from Call Manager. No configuration changes were being made, nor had there been any recently. We noticed in our DHCP server (Windows) that the phones were registering with the primary VLAN (76)’s subnet and not the Voice VLAN (6)’s subnet. So we had a bunch of phones attempting to pull a dynamic IP from an address pool that was about 90% full. Once these phones began registering on VLAN 76 they quickly maxed out the subnet.
From what we could observe, the phone would fail to get an IP from VLAN 6 (for whatever reason) and so attempted to acquire an address from the primary VLAN 76. Once the phone failed to get an IP from this maxed out address pool, it would revert back to its cached IP from VLAN 6. Though by this time, another phone would have taken that VLAN 6 address and so the loop would continue.
What finally fixed the issue was statically assigning IP addresses to each symptomatic phone (≈12). About 5 minutes after this was completed, all of the 40+ phones that were successfully registered using 76, swapped back and re-registered with VLAN 6. We haven’t had any issues since we made these static assignments and we have not reverted the phones back to DHCP.
Any ideas on why this might occur?
Also, do VoIPs use the primary VLAN (76) if they are unable to reach the configured Voice VLAN (6)?
Thank you for any assistance!
What switches and software version are you using? Are you using cdp.lldp and is it working okay on concerned switches?
Try to capture traffic from a phone that was not working to understand what is happening. You want to see how the cdp and dhcp is negotiating