Have you ever wondered how a Cisco IP phone knows how to get its configuration and will keep that configuration even if you move it from one site to another? This is how:
When a Cisco IP Phone is plugged into a Cisco PoE switch it negotiates power consumption with the switch. (I will cover how this happens in more detail at a later date). Once power has been supplied, the phone and switch will exchange information through Cisco Discovery Protocol (CDP). Using CDP the phone will learn the tagged VLAN number (used for voice) and the untagged VLAN number (used for data). Once the phone knows the Voice VLAN number it will send out a DHCP discovery packet. A DHCP server will answer, instructing the phones to use a valid IP address on the voice VLAN. The DHCP server will also give the phone the IP address of a TFTP server, which is the Communications Manager server (CallManager). The phone will then send a packet to the CallManager and ask for its configuration. The CallManager will use TFTP to deliver the configuration to the phone. The phone reboots and loads the configuration.
As long as the phone can reach the CallManager server it can be moved anywhere in your IP network and still load its configuration and operate. Make sense?
Michael
