Cisco 500 Series, Configuration Manual

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10-4
Cisco Security Appliance Command Line Configuration Guide
OL-12172-03
Chapter 10 Configuring DHCP, DDNS, and WCCP Services
Configuring a DHCP Server
Table 10-1 shows the DHCP options that are not supported by the dhcpd option command.
Specific options, DHCP option 3, 66, and 150, are used to configure Cisco IP Phones. See the “Using
Cisco IP Phones with a DHCP Server” section on page 10-4 topic for more information about
configuring those options.
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.
Cisco IP Phones might also include DHCP option 3 in their requests, which sets the default route.
Cisco IP Phones might include both option 150 and 66 in a single request. In this case, the security
appliance DHCP server provides values for both options in the response if they are configured on the
security appliance.
You can configure the security appliance to send information for most options listed in RFC 2132. The
following example shows the syntax for any option number, as well as the syntax for commonly-used
options 66, 150, and 3:
Table 10-1 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