Which DHCP option do Cisco phones use to obtain their configuration?

Cisco phones utilize DHCP option 150 to obtain their configuration. This option specifically provides the IP address of the TFTP (Trivial File Transfer Protocol) server, which is essential for the phone to download its configuration files and firmware. Once the phone receives this option from the DHCP server, it can then contact the specified TFTP server to retrieve necessary configuration information, such as settings for registration with a call manager or other operational parameters.

Option 43 can also provide information related to the location of services but is generally not used specifically for Cisco phones and their configuration needs. Option 60 is used for vendor identification, allowing the DHCP client to inform the server of its type or capabilities, which is not specifically geared towards configuration file retrieval. Option 55 allows a client to request specific options from the server, but it does not relay configuration data on its own. Therefore, option 150 is the primary option for Cisco phones to access their configuration files directly.