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IP Phone Registration Process

All IP phones follow these normal bootup and registration steps.  If the administrator has elected to perform an image change, there will be an additional step that involves getting the new image onto the phone.  This process is "outside the scope" of this document. 

Note:  Status Messages on the IP Phone can be of great help if you must troubleshoot a failure in the registration process.
  1. Phone Image:  Phone loads it's image and starts the process
  2. DHCP:  Get IP information
  3. TFTP:  Get configuration file
  4. TFTP:  Get other configuration parameters and files
  5. Registration Completes:  Cisco CallManager server sees phone as registered

If you have your Cisco CallManager servers set up in a cluster, every server has the configuration files for every phone that is in the Publisher database. Therefore, any Cisco CallManager server can serve as a TFTP server for the phones. The device pools to which you have assigned the phones determine the server with which the phones register. A phone can obtain the configuration file from a different server than the server with which the phone registers.

Step 1: Phone Image:  Phone starts the boot process

When you plug in an IP phone, the phone will attempt to boot and configure itself. The LCD screen provides an indication of the current phase of the bootup process as the bootup progresses. The phone cannot successfully complete the bootup process until the phone connects to the Ethernet network and registers with a Cisco CallManager server - note that "Auto-Registration" is disabled by default on the CallManager.  The phone will need to pre-configured within the CallManager otherwise.

If the phone LCD screen does not light up, you could have a faulty phone. The phone also could be faulty if the message the phone displays never changes after you plug in the phone.

Step 2: DHCP:  Get IP Information

Assuming that the phone is configured to use DHCP, it sends out a DHCP request.

If you'd like to verify that the phone is configured properly, see the following steps:

Cisco 7940 and 7960
  1. Choose Settings.
  2. Choose 3 (Network).
  3. Scroll down to the DHCP Enabled parameter.   Should be "Yes".
Cisco 7910
  1. Choose Settings.
  2. Choose 6 (Network).
  3. Scroll down to the DHCP Enabled parameter.  Should be "Yes".
  • Cisco 7910G supports only 10 MB speed, but 7910G+SW supports 10/100. If you have a 7910G, be sure to set the switch port that connects to the phone to 10 MB or Auto.
  • Any IP parameters that you have hard coded on the phones override the parameters that the DHCP server provides. In particular, the Alternate TFTP Server option overrides the TFTP server IP address that the DHCP provides.
  • Refer to RFC 1541: Dynamic Host Configuration Protocol in order to better understand DHCP.
The DHCP response contains the phone IP address and the IP address of the TFTP server (Option 150, which is usually a Cisco CallManager server). The response can also contain any of or all these common options:
  • IP address of the default router (gateway)
  • IP address of the Domain Name System (DNS) server
  • Domain name

    Step 3: TFTP:  Get the Configuration File

    The configuration file contains several pieces of information that a phone requires in order to function. At this stage in the bootup and registration process, the most important configuration elements are the list of Cisco CallManager servers with which the phone can register and the device pool to which the phone belongs. In this way, a phone can obtain the configuration from a different Cisco CallManager (TFTP) server than that with which the phone ultimately registers.

    The phone requests a specific configuration file.  For example, the file name for a phone with the MAC address 0030.94C2.D5CA is SEP003094C2D5CA.cnf. (SIP Phones begin with "SIP" instead of "SEP")

    If the phone is not in the Cisco CallManager database, the request for the specific configuration file results in a TFTP File Not Found response from the TFTP server. The phone then requests the file with the name SEPDEFAULT.cnf.

    Otherwise, the TFTP server of the Cisco CallManager server sends another File Not Found TFTP response. At this point, the phone restarts the configuration process.

    If you have configured the Cisco CallManager server for Auto-Registration, it sends the SEPDEFAULT.cnf file in response to the phone request. After the Cisco CallManager server database adds a phone by Auto-Registration, the phone has a SEPMAC-Address.cnf file. It does not reference the SEPDEFAULT.cnf again.

    Step 4: TFTP:  Get Other Configuration Parameters 

    The configuration file contains several parameters for the phone. These include the device pool, the Cisco CallManager servers to use, configured speed dials, and other parameters. In general, any time you make a change in Cisco CallManager that requires the phone (device) to be reset, you have made a change to the phone configuration file.

    Step 5:  Registration Completes

    The Cisco CallManager server sends the phone additional configuration elements during the final phases of the registration process. In general, the registration process must complete successfully if the process goes this far.

    More details here.

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