Some options, such as Option 43, have no unified definitions in RFC 2132.
Vendor-specific option (Option 43)
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:
• ACS parameters—Including the ACS URL, username, and password.
• Service provider—Identifier acquired by the CPE from the DHCP server and sent to the ACS for
selecting vender-specific configurations and parameters.
• PXE server address—For further obtaining the bootfile or other control information from the PXE
server.
1. Format of Option 43
Figure 19 Format of Option 43
Network configuration parameters are carried in different sub-options of Option 43, as shown in Figure 19. 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.
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,
username, and password separated by spaces (0x20).
Figure 20 Format of the value field of the ACS parameter sub-option
• 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.
Figure 21 Format of the value field of the PXE server address sub-option
Relay agent option (Option 82)
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. 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, 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. Figure 22 Sub-option 1 in normal padding format
Figure 23 Sub-option 2 in normal padding format
2. Verbose padding format
• 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 24.
Figure 24 Sub-option 1 in verbose padding format
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—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 23.
Option 184
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 contacts 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
SIP user uses to reach another SIP user when both the primary and backup calling processors are unreachable.