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Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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Option 82

RFC 3046 (the relay agent information option, or Option 82) is used for class-based IP address assignment.

The code for the relay agent information option is 82, and is comprised of two sub-options, circuit ID and remote ID.
Circuit ID This is the interface on which the client-originated message is received.
Remote ID This identifies the host from which the message is received. The value of this sub-option is the MAC address of the relay agent that adds Option 82.
The DHCP relay agent inserts Option 82 before forwarding DHCP packets to the server. The server can use this information to:
  • track the number of address requests per relay agent. Restricting the number of addresses available per relay agent can harden a server against address exhaustion attacks.
  • associate client MAC addresses with a relay agent to prevent offering an IP address to a client spoofing the same MAC address on a different relay agent.
  • assign IP addresses according to the relay agent. This prevents generating DHCP offers in response to requests from an unauthorized relay agent.

The server echoes the option back to the relay agent in its response, and the relay agent can use the information in the option to forward a reply out the interface on which the request was received rather than flooding it on the entire VLAN.

The relay agent strips Option 82 from DHCP responses before forwarding them to the client.

To insert Option 82 into DHCP packets, follow this step.

Insert Option 82 into DHCP packets. CONFIGURATION mode 
int ma 0/0
                                 ip add dhcp relay information-option remote-id
For routers between the relay agent and the DHCP server, enter the trust-downstream option.

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