IP Telephony Troubleshooting

Enterprise Networks & Servers, Dec 2004 by Trunk, Ronald

The phone now connects to the TFTP server and downloads a configuration file that lists the Cisco Call Manager (IP PBX) address as well as some other phone configuration info.

Now the phone will attempt to register with the Call Manager. If the configuration file from the TFTP server listed more than one Call Manager, the phone will try each address until it can register with one. During this step, the phone will display "Configuring CM list," then "Opening a.b.c.d" which is the address of the Call Manager."

If the Call Manager accepts the phones registration request, the phone will display the extension and time. You should be able to pick up the phone and dial a number. At this point, it doesn't matter if the call goes through or not. Whether you hear a ringing tone, fast busy, or reorder tone, you know that the phone has successfully registered with the Call Manager and the two can communicate. If the call cannot be completed, it is probably a problem with the dial plan or codec selection.

Avaya

When an Avaya phone powers up, it shows a test pattern on the phone's display and the hold lamp is lit. Not surprisingly, Avaya does not use the Cisco Discovery Protocol to determine the voice VLAN. Instead, the phone requests an IP address from the DHCP server on the native (data) VLAN. The DHCP server offers an address and also a string of configuration information that among other things, tells the phone which VLAN to use.

The phone then reboots and begins tagging Ethernet frames with the proper VLAN information. The phone will display "DHCP xxx."

Next, the phone requests an address from the DHCP server, this time on the voice VLAN. The DHCP server responds with an IP address on the voice VLAN, and additional configuration information including the address of the TFTP server and the C-LAN (IP PBX interface) card. The phone will display "DHCP."

Now the phone connects to the TFTP server, downloads configuration information and firmware upgrade (if needed). The phone will display "TFTP server a.b.c.d."

Finally, the phone will connect to the C-LAN card and attempt to register with the Communication Manager (IP PBX application). If the DHCP server offered several C-LAN addresses, the phone will try them in order until registration is successful. During this process, the phone will display "Discovering..."

When the phone connects to the PBX, it will either display the extension and time, or it will display the prompt "Ext=", meaning it is waiting to be told what extension to use. In either case, the phone has successfully completed the boot process. From a networking perspective, everything is working properly.

Nortel

Like Avaya, Nortel phones do not use the Cisco Discovery Protocol to determine the voice VLAN, but instead relies on configuration information supplied by the DHCP server. When the Nortel phone boots, it displays a test pattern and the speakerphone and message waiting lamps light. The phone requests an IP address (on the native VLAN) from the DHCP server. The server responds with an IP address and a VLAN ID. The phone reboots and requests a new IP address on the voice VLAN by tagging all Ethernet frames with the VLAN ID.