Cisco IP Phone Boot Process

Once IP phone is connected to a network, it goes to following standard steps to get registered to Call Manager and to get a  directory number. A figure below shows the overview of Cisco IP phone startup process when IP phone is connected to Cisco Catalyst switch, capable of providing PoE.

IP Phone Startup process

1. Obtain power from the switch: When an IP phone is connected to Cisco Catalyst switch, it is able to provide in-line power or Power Over Ethernet.The switch will first send Fast Link Pulse (FLP) signal.The switch will use FLP to determine if attached phone is unpowered IP phone.If a unpowered state is determined, Cisco IPphone loops back FLP, signalling the switch to send -48 V DC power down the line.

2. Load the stored phone image: The Cisco IP Phone has non-volatile Flash memory in which it stores firmware images and user-defined preferences. At startup, the phone runs a bootstrap loader that loads a phone image stored in Flash memory. Using this image, the phone initializes its software and hardware

3. Configure VLAN: After receiving power and loading image file in IP Phone, a switch will send the Cisco Discovery Protocol (CDPv2) trigger packet to the IP phone.This CDP packet provides VLAN information to the IP phone.The IP phone will then tag all the with appropriate Voice VLAN information.

4. Obtain IP address and TFTP server address: As the IP phone learn VLAN information the IP Phone broadcasts a request to a DHCP server. The DHCP server responds to the IP Phone with an IP address, a subnet mask, and the IP address of the Cisco TFTP server.

DHCP server can provide the location of TFTP server to the Cisco IP phones in two different ways.

1. DHCP option 66: It gives the Cisco IP phone the hostname of the TFTP server.
2. DHCP option 150: It gives the Cisco IP phone the IP address of the TFTP server.

5. Contact TFTP server for configuration: The TFTP server has configuration files for  IP phones, which define parameters for connecting to Cisco CallManager. The TFTP server sends the configuration information for that IP Phone, which contains an ordered list of up to three Cisco CallManagers.

6. Register with Cisco CallManager: After obtaining the file from the TFT P server, the phone attempts to make a TCP connection to the highest priority Cisco CallManager on the list.After registration, it receives extension number and become operational.