I made a phone call today. I know it doesn’t really sound like a big deal does it? Well, when you stop to think about it, everything that happens to make that call successful is really amazing. It’,s funny how people never stop to think about things like this and in all honesty, most people don’t need to worry about these things; that is our job.
A large part of making sure something works reliably or fixing it when it doesn’t work, is to understand what happens when it is working properly. I started this post planning on discussing call flow but before you can even setup a call you need a working phone. So the focus of today’s post will be the registration of a Cisco IP phone. We will discuss call flow in another article.
For the purposes of this article, we will assume the phone that is going to register is a Skinny Client Control Protocol (SCCP) device. The first thing that phone will need is power. There are two ways the phone will receive its power. The first is wall power. A power supply is plugged into the wall and attached to the phone. The second and, more common, is referred to as inline power or Power over Ethernet (PoE). There are two types of PoE that Cisco Phones use: Cisco pre-standard and 802.3af. The details on how each work will be the topic of a future article but for now understand the the phone negotiates with the switch to determine power requirements. Once the switch determines that the attached device is a PoE device, power is provided.
The following are the steps that a phone goes through once it has power:
1. The phone boots using its firmware.
2. The VLAN is determined. If a voice VLAN has been configured, the phone will learn that VLAN otherwise it will use the access VLAN.
3. If the phone is using DHCP, a DCHP request is sent. When the phone recieves the DHCP information, it will also receive the IP address of the TFTP server.
4. Once the phone learns the TFTP server address, it requests a configuration file. Since every phone has a unique configuration, the MAC address is used as part of the configuration file name.
5. If the phone has previously been configured within Communications Manager, a configuration file will exist in the TFTP server and will be sent to the phone.
Note: If the phone has not been configured in the Communication Manager, the phone will attempt to begin the auto-registration process. This will only be successful if auto-registration has been configured.
6. Once the phone has loaded its configuration file, it will complete the registration with Communications Manager.
You know the phone has registered when the correct extension number appears on it and you can receive dial tone.
When troubleshooting phone failure, a few common issues you may run into are:
- Switch not configured to supply power.
- Incorrect DHCP configuration.
- Non-functioning TFTP Server.
- Phone not configured in that Communications Manager.
- Improper setup of auto-registration.
Now that you know how the registration process should work, troubleshooting registration failures should be easier.