3Com 8807 Configuration Manual page 703

DHCP Option 82 Configuration 703
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
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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.
Option 82 Structure
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.