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Cisco ASA Series CLI Configuration Guide
Chapter 1 Configuring DHCP
Configuring a DHCP Server
Note The ASA does not verify that the option type and value that you provide match the expected type and
value for the option code as defined in RFC 2132. For example, you can enter the dhcpd option 46 ascii
hello command, and the ASA accepts the configuration, although option 46 is defined in RFC 2132 to
expect a single-digit, hexadecimal value. For more information about option codes and their associated
types and expected values, see RFC 2132.
Table 1-2 shows the DHCP options that are not supported by the dhcpd option command.
DHCP options 3, 66, and 150 are used to configure Cisco IP phones. For more information about
configuring these options, see the “Using Cisco IP Phones with a DHCP Server” section on page 1-6.
Using Cisco IP Phones with a DHCP Server
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 can 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.
Table 1-2 Unsupported DHCP Options
Option Code Description
0 DHCPOPT_PAD
1 HCPOPT_SUBNET_MASK
12 DHCPOPT_HOST_NAME
50 DHCPOPT_REQUESTED_ADDRESS
51 DHCPOPT_LEASE_TIME
52 DHCPOPT_OPTION_OVERLOAD
53 DHCPOPT_MESSAGE_TYPE
54 DHCPOPT_SERVER_IDENTIFIER
58 DHCPOPT_RENEWAL_TIME
59 DHCPOPT_REBINDING_TIME
61 DHCPOPT_CLIENT_IDENTIFIER
67 DHCPOPT_BOOT_FILE_NAME
82 DHCPOPT_RELAY_INFORMATION
255 DHCPOPT_END
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