As all blogs should, I’ll start this one with a personal experience…

Just recently (sound familiar yet?)..

I had configured the CX500 as per usual as a common area phone. It signed in beautifully once I added the telephone number (without the + of course) and the accompanying PIN.

When I rebooted the phone it showed the boot cycle and then got stuck on a screen saying that there was no network detected. It then presented me with the configure device options as if it had forgotten the telephone number and PIN. Once I re-entered these it logged in as before.

Right so if I have a power outage my device isn’t going to automatically login with the cached credentials…not good.

For giggles I first updated the firmware on the CX500, same behaviour. Checking the device I noticed that the DHCP request was going unanswered and the CX500 wasn’t getting an IP..DHCP problem?? NO!

After much packet capturing and head scratching the culprit was discovered. The switch of course, a Spanning Tree Setting was delaying the DHCP request from reaching the DHCP Server. That should cause the error we are getting you may say…

The CX500 sends only 4 DHCP requests (in rather quick succession), normally 1 will suffice and get a response. In my case the delay injected by the switch prevented a timely answer. The CX500 then goes into “I give UP” mode, and then sits there waiting on the “credentials” page. Once the Credentials are re-entered further DHCP requests are sent.

Changing the switch port solved the issue.