Reference Guide

Option Number and Description

Signals the last option in the DHCP packet.

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.

Releasing and Renewing DHCP-based IP Addresses

On an Aggregator configured as a DHCP client, you can release a dynamically-assigned IP address

without removing the DHCP client operation on the interface.

To manually acquire a new IP address from the DHCP server, use the following command.

• Release a dynamically-acquired IP address while retaining the DHCP client configuration on the

interface.

EXEC Privilege mode

release dhcp interface type slot/port

Dynamic Host Configuration Protocol (DHCP)