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Using DHCP to obtain an IP address and other configuration
information
Address acquisition process
As mentioned before, a device sets the first up interface as the DHCP client during startup. The DHCP
client broadcasts a DHCP request, where the Option 55 field specifies the information that the client
wants to obtain from the DHCP server such as the configuration file name, domain name and IP address
of the TFTP server, and DNS server IP address.
After receiving the DHCP response from the DHCP server, the device obtains the IP address and resolves
the following fields in the DHCP response:
•
Option 67 or the file field
—
Obtains the configuration file name. The device resolves Option 67 first.
If Option 67 contains the configuration file name, the device does not resolve the file field. If not, the
device resolves the file field.
•
Option 66
—
Obtains the TFTP server domain name
•
Option 150
—
Obtains the TFTP server IP address
•
Option 6
—
Obtains the DNS server IP address.
If no response is received from the DHCP server, the device removes the temporary configuration and
starts up with factory defaults.
NOTE:
•
The temporary configuration contains two parts: the configuration made on the interface through which
automatic configuration is performed, and the
ip host
command in the host name file (For more
information about the
ip host
command, see
Layer 3—IP Services Command Reference.). The
temporary configuration is removed by executing the corresponding
undo
commands.
•
For more information about DHCP, see
Layer
3—IP S
ervices Configuration Guide.
Principles for selecting an address pool on the DHCP server
The DHCP server selects IP addresses and other network configuration parameters from an address pool
for clients. DHCP supports the following types of address pools.
•
Dynamic address pool
—
A dynamic address pool contains a range of IP addresses and other
parameters that the DHCP server dynamically assigns to clients.
•
Static address pool
—
A static address pool contains the binding of an IP address and a MAC
address (or a client ID). The DHCP server assigns the IP address of the binding and specific
configuration parameters to a requesting client whose MAC address or ID is contained in the
binding. In this way, the client can get a fixed IP address.
Select address pools by using one of the following methods:
•
If devices use the same configuration file, you can configure a dynamic address pool on the DHCP
server to assign IP addresses and the same configuration parameters (for example, configuration
file name) to the devices. In this case, the configuration file can only contain common configurations
of the devices, and the specific configurations of each device need to be performed in other ways.
For example, the configuration file can enable Telnet and create a local user on devices so that the
administrator can telnet to each device to perform specific configurations (for example, configure
the IP address of each interface).
