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Sub-option 2 also belongs to Option 82 and defines the Remote ID. Usually 
configured on the DHCP relay devices, it indicates that the forwarded packets will 
carry the MAC address of the relay device. Generally, sub-option 1 and sub-option 
2 are used together to identify a DHCP client.

■

Sub-option 5

Sub-option 5 also belongs to Option 82 and defines the link selection. It contains 
an IP address which is added by the DHCP relay, so that the DHCP server can 
assign to the DHCP client an IP address that is in the same network segment with 
this address.

There is a field named options in the DHCP packets. It can be null or contains at 
least one feature-specific option, such as Option 82 which may comprise multiple 
sub-options. Figure 164 illustrates the structure of Option 82.

Figure 164   Option 82 structure

Code: Identifies the number of the relay agent information option. It is 82 in the 
packet, which represents Option 82. Option 82 is before Option 255 and is behind 
other options in RFC 3046.

Len: Indicates the length of the Agent Information Field.

Agent Information Field: Specifies the available sub-options.

Sub-option packet structure: Figure 165 shows the sub-option structure.