322 | Dynamic Host Configuration Protocolw w w . d e l l . c o m | s u p p o r t . d e l l . c o m • DHCP Snooping on page 322• Dynamic ARP Inspection on page 325• Source Address Validation on page 327Option 82RFC 3046 (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 IDand Remote ID.• Circuit ID is the interface on which the client-originated message is received.• Remote ID identifies the host from which the message is received. The value of this sub-option is theMAC 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 canuse this information to:• track the number of address requests per relay agent; restricting the number of addresses available perrelay 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 spoofingthe same MAC address on a different relay agent.• assign IP addresses according to the relay agent. This prevents generating DHCP offers in response torequests from an unauthorized relay agent.The server echoes the option back to the relay agent in its response, and the relay agent can use theinformation in the option to forward a reply out the interface on which the request was received rather thanflooding it on the entire VLAN.The relay agent strips Option 82 from DHCP responses before forwarding them to the client.DHCP SnoopingDHCP Snooping protects networks from spoofing. In the context of DHCP Snooping, all ports are eithertrusted or untrusted. By default, all ports are untrusted. Trusted ports are ports through which attackerscannot connect. Manually configure ports connected to legitimate servers and relay agents as trusted.Task Command Syntax Command ModeInsert Option 82 into DHCP packets.For routers between the relay agentand the DHCP server, enter thetrust-downstream option.ip dhcp relay information-option[trust-downstream]CONFIGURATION
