Cisco ASA 5505 - Manual (Page 352)

11-6
Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter 11 Configuring DHCP
Configuring a DHCP Server
Using Cisco IP Phones with a DHCP Server
Enterprises with small branch offices that implement a Cisco IP Telephony Voice over IP solution
typically implement Cisco CallManager at a central office to control Cisco IP Phones at small branch
offices. This implementation allows centralized call processing, reduces the equipment required, and
eliminates the administration of additional Cisco CallManager and other servers at branch offices.
Cisco IP Phones download their configuration from a TFTP server. When a Cisco IP Phone starts, if it
does not have both the IP address and TFTP server IP address preconfigured, it sends a request with
option 150 or 66 to the DHCP server to obtain this information.
• DHCP option 150 provides the IP addresses of a list of TFTP servers.
• DHCP option 66 gives the IP address or the hostname of a single TFTP server.
Note Cisco IP Phones might also include DHCP option 3 in their requests, which sets the default route.
A single request might include both options 150 and 66. In this case, the ASA DHCP server provides
values for both options in the response if they are already configured on the ASA.
You can configure the ASA to send information for most options listed in RFC 2132. The following
examples show the syntax for any option number, as well as the syntax for options 3, 66, and 150:
Command Purpose
dhcpd option number value
Example:
hostname(config)# dhcpd option 2
Provides information for DHCP requests that include an option number as
specified in RFC-2132.
Command Purpose
dhcpd option 66 ascii server_name
Example:
hostname(config)# dhcpd option 66 ascii
exampleserver
Provides the IP address or name of a TFTP server for option 66.
Command Purpose
dhcpd option 150 ip server_ip1
[server_ip2]
Example:
hostname(config)# dhcpd option 150 ip
10.10.1.1
Provides the IP address or names of one or two TFTP servers for option
150. The server_ip1 is the IP address or name of the primary TFTP server
while server_ip2 is the IP address or name of the secondary TFTP server.
A maximum of two TFTP servers can be identified using option 150.