Custom options

Some options, such as Option 43, Option 82, and Option 184, have no unified definitions in RFC 2132.

DHCP servers and clients use Option 43 to exchange vendor-specific configuration information.

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, which is 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, which is used to obtain 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 19.

   Figure 19: Option 43 format

   

   • Sub-option type—Type of a sub-option. The field value can be 0x01 (an ACS parameter sub-option), 0x02 (a service provider identifier sub-option), or 0x80 (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. The value format varies with sub-options.

2. Format of the sub-option value field of Option 43

   • As shown in Figure 20, the value field of the ACS parameter sub-option contains variable ACS URL, ACS username, and ACS password separated by spaces (0x20):

   • Figure 21 shows the format of the value field of the PXE server address sub-option. 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.

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

• Figure 21 shows the format of the value field of the PXE server address sub-option. 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 about 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 can contain up to 255 sub-options and must have one sub-option at least. Option 82 supports two sub-options: sub-option 1 (Circuit ID) and sub-option 2 (Remote ID). DHCP snooping device supports three sub-options: sub-option 1 (Circuit ID), sub-option 2 (Remote ID), and sub-option 9.

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

There are two methods for configuring Option 82:

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

• Non-user-defined method—Pad Option 82 in the default normal format, verbose format, private format, or standard format.

  	NOTE: Only the DHCP snooping device supports sub-option 9, padded in either private or standard format.

If you choose normal format and verbose format, you can specify the code type for the sub-options as ASCII or HEX.

• Normal padding format

  • Sub-option 1—Contains 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—Contains 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.

• Verbose padding format

  • Sub-option 1—Contains the user-specified access node identifier (ID of the device that adds Option 82 in DHCP messages), and the type, number, and VLAN ID of the interface that received the client's request. 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 24.

• Sub-option 2—Contains 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 23.

• Private padding format

  • Sub-option 1—Contains the VLAN ID of the interface that received the client's request, module (subcard number of the receiving port) and port (port number of the receiving port). The value of the sub-option type is 1.

• Sub-option 2—Contains the MAC address of the DHCP snooping device that received the client's request. The value of the sub-option type is 2.

• Sub-option 9—Contains the Sysname and the primary IP address of the Loopback0 interface. The value of the sub-option type is 9.

• Standard padding format

  • Sub-option 1—Contains the VLAN ID of the interface that received the client's request, module (subcard number of the receiving port) and port (port number of the receiving port). The value of the sub-option type is 1, and the value of the circuit ID type is 0.

• Sub-option 2—Contains 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 23.

Option 184 is a reserved option, and parameters in the option can be defined as needed. The device supports Option 184 carrying 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. DHCP clients contact the backup when the primary is unreachable.

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

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

You must define sub-option 1 to make other sub-options take effect.