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This chapter is from the book

This chapter is from the book

Understanding the Phone Bootup Process

After completing the configuration of the switch and activating DHCP, the Cisco IP phone proceeds through the standard bootup cycle.

After an IP phone receives power, the following happens:

  1. Power-on self-test (POST): The phone performs a set of tests to ensure basic functionality.
  2. The phone begins the boot process.
  3. The phone uses Cisco Discovery Protocol to learn the voice VLAN.
  4. The phone initializes a basic IP stack.
  5. The IP phone (DHCP client) sends a DHCPDISCOVER request to the 255.255.255.255 broadcast address.
  6. A DHCP server returns a DHCPOFFER message and assigns the following for the requested scope: a free IP address, the subnet mask, the default gateway, the DNS server (optional), and the TFTP server (option 150) for the scope. This information is sent to the DHCP client (the IP phone) using the broadcast address 255.255.255.255 (the router uses the IP phone MAC address at Layer 2).
  7. The IP phone takes the values received from the DHCP response and applies them to the IP stack of the IP phone.
  8. The IP phone uses the value received in option 150 to attempt to retrieve a configuration file from the TFTP server.

The IP phone is prepared and ready to operate on the network, but it needs its unique identity and operating parameters. When the TFTP request for configuration files is issued, there is a sequence of requests issued in an attempt to register with a call agent and operate. The following sections cover the configuration files.

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