Clearly understanding the Cisco IP phone boot process is a very important part of the troubleshooting process for any Cisco network engineer. I will describe the Cisco IP phone boot process in a few steps below.
- The PoE switch (using standard 802.3af or Cisco inline power mechanism) will detect that there is an unpowered IP device (PD) connected to its port and go ahead and provide power to the IP phone. The phone will then boot-up its firmware image
- The switch will provide the Voice VLAN information to the phone using CDP (Cisco Discovery Protocol) or possibly standards based LLDP (Link Layer Discovery Protocol). This will inform the IP phone what VLAN it belongs to for sending voice traffic.
- The IP phone will now send a DHCP request within the Voice VLAN. It will receive its IP address, subnet mask, default gateway and the CUCM TFTP server IP address (via DHCP option 150)
- The IP phone will contact the TFTP server to retrieve its configuration (.cnf) file. This configuration file is provided based on the requesting IP phone’s MAC address. This configuration file (among other things) will include the ordered list of CUCM servers which the phone should attempt to register to (i.e. primary, secondary, tertiary CUCM servers).
- The IP phone will now contact its primary CUCM server listed within its retrieved configuration file. It will register and become available for use.