To fully understand how the Cisco IP Phone operates, you should understand deeply the Cisco IP Phone Boot Process. Fine below step by step the Cisco IP Phone boot process:
- Connect the Cisco IP Phone to an Ethernet port of Cisco Switch. If the Cisco IP phone and Switch support PoE, the Cisco IP Phone draws power through either 802.3af PoE or Cisco-proprietary PoE.
- As the Cisco IP Phone receives power it is switched on, the Cisco switch sends VLAN information to the IP phone using Cisco Discovery Protocol as an information delivery mechanism. The Cisco IP Phone finds information about its VLAN from this information sent from switch.
- The Cisco IP Phone then sends a DHCP request to server asking for an IP address on its voice VLAN.
- The DHCP server responds back with an IP address offer. If Cisco IP Phone accepts this offer, it receives all the DHCP services that go with the DHCP request. DHCP services include items such as domain name information, DNS server information, default gateway, and so on. For a Cisco IP Phone, a unique DHCP option is included known as Option 150 that directs the IP phone to a TFTP server.
- When the Cisco IP Phone receives the IP address of the TFTP server, it connects with the TFTP server and downloads its configuration file. This configuration file includes a list of valid call processing agents (such as Cisco Unified Communications Manager Express CME or Cisco Unified Communications Manager agents).
- The Cisco IP Phone attempts to connect with the first call processing server configured on its configuration file to register. If it fails to connect with the primary server, the IP phone moves to the next server configured in the configuration file. This process continues until the IP phone registers successfully with the configured call processing agents.