Working of Cisco IP Phone

--> The Cisco IP phone connects to an Ethernet switch port. If the IP phone and switch support PoE, the IP phone receive power via PoE.

-->  As the Cisco IP phone powers on, the Cisco switch delivers voice VLAN information to the IP phone using CDP. The Cisco IP phone now knows what VLAN it should use for its voice traffic.

--> The Cisco IP phone broadcasts a DHCP request (within the voice VLAN) asking for an IP  address on its voice VLAN.

--> The DHCP server responds with an IP address offer. Cisco IP phone receives all the DHCP options that go along with the DHCP request. DHCP options include items such as default gateway, DNS server information, domain name information, and so on.

--> In the case of Cisco IP phones, a specific DHCP option is required, known as Option 150. This option directs the IP phone to a TFTP server.

--> After the Cisco IP phone has the IP address of the TFTP server, it contacts the TFTP server and downloads its configuration file. Included in the configuration file is a list of valid call processing agents ( CUCM or CME).

--> If the IP Phone needed to upgrade its software then it downloads the firmware from TFTP Server.

--> The Cisco IP phone then attempts to contact the first call processing server (the primary server) listed in its configuration file to register.

--> If this fails, the IP phone moves to the next server in the configuration file. This process continues until the IP phone registers successfully or the list of call processing agents is exhausted.

--> If the IP Phone finds active call processing server in the list, it registers to the server by using SIP ( Open Standard) or SCCP protocol.

--> Call Processing Server ( CUCM or CME) checks the database for the MAC address of the requested IP Phone to send the operating configuration if the MAC address is available.

--> IP Phone downloads softkey template from CUCM.

Reference: Cisco Press

Md.Kareemoddin

CCIE # 54759