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 that specifies the configuration file name. 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 that specifies the TFTP server domain name

•Option 150 that specifies the TFTP server IP address

•Option 6 that specifies 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 configuration file name is saved in the Option 67 or file field of the DHCP response. The device first resolves the Option 67 field. If this field contains the configuration file name, the device does not resolve the file field. If not, it resolves the file field.

•The temporary configuration contains two parts: the configuration made on the interface through which automatic configuration is performed, and the configuration made by executing the ip host commands in the host name file (For more information about the ip host command, see the Layer 3—IP Services Command Reference.). The temporary configuration is removed by executing the undo commands.

•For more information about DHCP, see the Layer 3—IP Services 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. 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

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