DHCP option 82

The DHCP option 82 is the DHCP Relay Agent Information option. The DHCP relay agent inserts option 82 when it forwards the client-originated DHCP packets to a DHCP server. The Relay Agent Information option is organized as a single DHCP option that contains one or more sub-options that convey information known by the relay agent. The DHCP server echoes the option back to the relay agent in server-to-client replies, and the relay agent removes the option before forwarding the reply to the client.

The DHCP option 82 is added at the DHCP relay level as shown in the following image.

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DHCP Client-Relay-Server Architecture

The Relay Agent Information option (code 82) is a container for specific agent-supplied suboptions; Agent Circuit ID (code 1) and Agent Remote ID (code 2). The suboptions can represent different information relevant for the relay. The fields are encoded in the following manner, where N or n is the total number of octets in the Agent Information Field (all bytes of the suboptions):

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Format of the Relay Agent Information

Because at least one of the sub-options must be defined, the minimum Relay Agent Information length is two (2), and the length n of the suboption can be zero (0). The sub-options do not have to display in any particular order. No pad suboption is defined and the Information field is not terminated with 255 suboption.