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Figure 1-6 Format of the value field of the ACS parameter sub-option 
 
 
z 
The value field of the service provider identifier sub-option contains the service provider identifier. 
z 
 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 filed contains the IP addresses of the 
PXE servers. 
Figure 1-7 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 before forwarding the message 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 is defined. Currently 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: 
z 
User-defined method: Manually specify the content of Option 82. 
z 
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: 
z 
Sub-option 1: Padded with the VLAN ID and interface number of the interface that received the 
client’s request. The following figure gives its format. The value of the sub-option type is 1, and that 
of the circuit ID type is 0.