Self-defined options

Some options, such as Option 43, have no unified definitions in RFC 2132.

DHCP servers and clients use Option 43 to exchange vendor-specific configuration information. The client sends a request with Option 43, including a vendor string that identifies a vendor. Upon receiving the request, the DHCP server refers to the vendor-specific options table, and returns a response message with Option 43 to assign the appropriate vendor-specific information to the DHCP client.

The DHCP client can obtain the following information through Option 43:

• Auto-Configuration Server (ACS) parameters, including the ACS URL, username, and password.

• Service provider identifier acquired by the customer premises equipment (CPE) from the DHCP server and sent to the ACS for selecting vender-specific configurations and parameters.

• Preboot Execution Environment (PXE) server address for further obtaining the bootfile or other control information from the PXE server.

1. Format of Option 43

Network configuration parameters are carried in different sub-options of Option 43, as shown in Figure 10. The sub-option fields are described as follows:

• Sub-option type: Type of a sub-option. The field value can be 0x01, 0x02, or 0x80. 0x01 indicates an ACS parameter sub-option. 0x02 indicates a service provider identifier sub-option. 0x80 indicates a PXE server address sub-option.

• Sub-option length: Length of a sub-option excluding the sub-option type and sub-option length fields.

• Sub-option value: Value of a sub-option.

• As shown in Figure 11, the value field of the ACS parameter sub-option contains variable ACS URL, username, and password separated by spaces (0x20).

• The value field of the service provider identifier sub-option contains the service provider identifier.

• Figure 12 shows the format of the value field of the PXE server address sub-option. Currently, the value of the PXE server type can only be 0. The server number field indicates the number of PXE servers contained in the sub-option. The server IP addresses field contains the IP addresses of the PXE servers.

Option 82 is the relay agent option in the option field of the DHCP message. It records the location information of the DHCP client. When a DHCP relay agent or DHCP snooping device receives a client's request, it adds Option 82 to the request message and sends it to the server.

The administrator can locate the DHCP client to further implement security control and accounting. The Option 82 supporting server can also use such information to define individual assignment policies of IP address and other parameters for the clients.

Option 82 involves at most 255 sub-options. At least one sub-option must be defined. The DHCP relay agent supports two sub-options: sub-option 1 (Circuit ID) and sub-option 2 (Remote ID).

Option 82 has no unified definition. Its padding formats vary with vendors.

You can use the following methods to configure Option 82:

• User-defined method: Manually specify the content of Option 82.

• Non-user-defined method: Pad Option 82 in the default normal or verbose format.

If you choose the second method, you can specify the code type for the sub-options as ASCII or HEX.

1. Normal padding format

The padding contents for sub-options in the normal padding format are as follows:

• Sub-option 1: Padded with the VLAN ID and interface number of the interface that received the client's request. The value of the sub-option type is 1, and that of the circuit ID type is 0.

• Sub-option 2: Padded with the MAC address of the DHCP relay agent interface or the MAC address of the DHCP snooping device that received the client's request. The value of the sub-option type is 2, and that of the remote ID type is 0.

1. Verbose padding format

2. Sub-option 1: Padded with the user-specified access node identifier (ID of the switch that adds Option 82 in DHCP messages), and the type, number, and VLAN ID of the interface that received the client's request. Its format is shown in Figure 15.

	NOTE: The VLAN ID field has a fixed length of 2 bytes. All the other padding contents of sub-option 1 are length variable. See Figure 15.

• Sub-option 2: Padded with the MAC address of the DHCP relay agent interface or the MAC address of the DHCP snooping device that received the client's request. It has the same format as that in normal padding format. See Figure 14.

Option 184 is a reserved option, and parameters in the option can be defined as needed. The switch supports Option 184 carrying the voice related parameters, so a DHCP client with voice functions can get an IP address along with specified voice parameters from the DHCP server.

Option 184 involves the following sub-options:

• Sub-option 1: IP address of the primary network calling processor, which serves as the network calling control source and provides program downloads.

• Sub-option 2: IP address of the backup network calling processor that DHCP clients will contact when the primary one is unreachable.

• Sub-option 3: Voice VLAN ID and the result whether DHCP clients take this ID as the voice VLAN or not.

• Sub-option 4: Failover route that specifies the destination IP address and the called number that a Session Initiation Protocol (SIP) user uses to reach another SIP user when both the primary and backup calling processors are unreachable.

  	NOTE: You must define the sub-option 1 to make other sub-options effective.