| 12.2 |  | Multimedia Support |
| 12.2.4 |  | IP phones and the PIX Security Appliance DHCP server |

| Enterprises with small branch offices implementing a Cisco IP Telephony VoIP solution typically implement Cisco Call Manager at a central office to control IP phones at small branch offices. This implementation enables centralized call processing, reduces the equipment required, and eliminates the administration of additional Cisco Call Managers and other servers at branch offices.

Cisco IP phones download their configurations from a trivial file transfer protocol (TFTP) server. When a Cisco IP phone starts, if it does not have an IP address or a TFTP server IP address pre-configured, it sends a request to the dynamic host control protocol (DHCP) server to obtain this information. The PIX Security Appliance DHCP server can only be enabled on the inside interface. Therefore, the server can only respond to DHCP requests from Cisco IP phones on the internal network.

The PIX Security Appliance supports DHCP option 150, which enables it to provide a list of TFTP server addresses, as well as option 66, which enables it to provide the IP address or hostname of a single TFTP server. The PIX Security Appliance also supports DHCP option 3, which enables it to distribute a default gateway address.