IP Telephony Troubleshooting

Enterprise Networks & Servers, Dec 2004 by Trunk, Ronald

The DHCP server responds with a new IP address and a list of call server (IP PBX) addresses. The phone displays "Locating Server...", connects to the call server, optionally downloads updated firmware and attempts to register. If successful, you will either see a normal phone display or the prompt "Node: xx?" Either one indicates that the phone can communicate with the call server. From a networking perspective, everything is working properly.

Troubleshooting the Boot Process

Now that you have a good understanding of the boot process, you can isolate problems and identify them more quickly. To begin with, all the phones have some rudimentary diagnostic display that tells you what parameters the phone is working with. On Cisco phones, press the [options?, network config?] keys. You can then scroll though a list of parameters such as IP address, DHCP server address, call manager address, etc.

On Avaya phones, press 8439 # (8439 spells "view" on the phone keypad), and you can scroll through a similar list. On Nortel phones, press to scroll through the list. For instance, if a Cisco phone is "stuck" at the prompt "Configuring VLAN," check that the auxiliary VLAN is configured correctly and that CDP is enabled on that port. By default it is, but it can be turned off on individual parts as well as globally. If the phone displays "Configuring IP address, or an Avaya phone displays "DHCP xxx" (xxx is a counter; if it increases much beyond 15, there's usually a problem), you need to troubleshoot the DHCP server. Since the phone is an IP device, you can troubleshoot the DHCP problems just like any other network device.

If the phone indicates a problem accessing the TFTP server, check to see if the server is reachable from the phone. Phones in general do not allow you to ping other devices. Instead, try pinging the phone from the server. Verify that the TFTP service is running, and that there are no file or directory permissions getting in the way. Use a TFTP client on a PC to verify that the files can be read or written.

Finally, if the phone indicates that it is trying to register with the IP PBX but can't, follow the same troubleshooting steps as you did for the TFTP server. Can you ping the phone from the PBX? Is the PBX running? Check the phone's diagnostic to verify that it has the correct address for the PBX.

Remember that from a networking perspective, if the PBX rejects the phone's attempt to register, that is just as good as if it accepted it. It means that the phone and the PBX can communicate. The problem lies elsewhere, most likely in the PBX configuration.

As you can see, by understanding some of the details how IP phones communicate with other components of the IP telephony system during boot up, troubleshooting the process becomes much simpler. Next time, I'll explain how calls between phones (and gateways) are set up so you can quickly troubleshoot problems with them as well.

Ronald Trunk

Ronald Trunk (CCIE #5942, CISSP, CHP) is a senior consultant with Chesapeake NetCraftsmen, a high-end consulting firm and Cisco Premier Partner. NetCraftsmen has 10 CCIEs with expertise including large network high-availability routing/switching and design, VoIP, QoS, MPLS, IPSec VPN, wireless LAN and bridging, network management, security, IP multicast, and other areas. Ronald can be reached at rtrunk@netcraftsmen.net