If you have a clear idea regarding the process/steps involved on an IP phone registration, you can easily troubleshoot issues related to the registration. In this topic I have explained steps involved for both SIP and SCCP phones registration with the Cisco Unified Communications Manager.
SCCP Phone Registration Process
1. SCCP phone obtains the Power (PoE or AC adapter).
2. The phone loads its locally stored firmware image.
3. The phone learns the Voice VLAN ID via CDP from the switch.
4. The phone uses DHCP to learn its IP address, subnet mask, default gateway and TFTP server address.
5. The phone contacts the TFTP server and requests its configuration file. Each phone has a customized configuration file named SEP<mac_address>.cnf.xml created by CUCM and uploaded to TFTP when the administrator creates or modifies the phone.
6. The phone registers with the primary CUCM server listed in its configuration file. CUCM then sends the softkey template to the phone using SCCP messages.
SIP Phone Registration Process
SIP Phones use a different set of steps to achieve the same goal. Steps 1 to 4 are the same as SCCP Phones, refer the steps as illustrated in the figure: SCCP Phone Boot Process .
1. The phone contacts the TFTP server and requests the Certificate Trust List file (only if the cluster is secured).
2. The phone contacts the TFTP server and requests its SEP<mac-address>.cnf.xml configuration file.
3. If the SIP Phone has not been provisioned before boot time, the SIP Phone downloads the default configuration XMLDefault.cnf.xml file from the TFTP server.
4. The SIP phone requests a firmware upgrade (Load ID file), if one was specified in the configuration file. This process allows the phone to upgrade the firmware image automatically when required for a new version of CUCM.
5. The phone downloads the SIP dial rules configured for that phone.
6. The phone Establish connection with the primary CUCM and the TFTP server end to end.
7. The phone Registers with the primary CUCM server listed in its configuration file.
8. The phone downloads the appropriate localization files from TFTP.
9. The phone downloads the softkey configurations from TFTP.
10. The phone downloads custom ringtones (if any) from TFTP.
About TFTP server:
TFTP is a critical service for IP Phones. The Phone use TFTP to download their config files, firmware and other data. Without TFTP, the phones simply do not function properly. When you make a configuration change to a device, CUCM creates or modifies a config file for the device and uploads it to the TFTP server. TFTP service much therefore provided by one or more CUCM servers in the cluster.