Knowing how something works is always beneficial. Having said that, I believe network engineers should know the boot process of the Cisco IP phones so they can assist with troubleshooting.
- The Cisco IP phone connects to an Ethernet switchport. If the IP phone and switch support PoE, the IP phone receives power through either Cisco-proprietary PoE or 802.3af PoE.
- As the Cisco IP phone powers on, the Cisco switch delivers voice VLAN information to the IP phone using CDP as a delivery mechanism. The Cisco IP phone now knows what VLAN it should use.
- The Cisco IP phone sends a DHCP request asking for an IP address on its voice VLAN. The router connecting to the voice VLAN receives this DHCP request and, through the ip helper-address command, forwards the request directly to the DHCP server.
- The DHCP server responds with an IP address offer. When the Cisco IP phone accepts the offer, it 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 unique DHCP option is included, known as Option 150. This option directs the IP phone to a TFTP server (you learn more about this in the upcoming section, “Configuring a Router-Based DHCP Server”).
- Once 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 (such as Cisco Unified Communications Manager or CME agents).
- The Cisco IP phone 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.
I hope this has been helpful and I thank you for reading!
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