Dynamic Host Configuration Protocol (DHCP) | 323Configure the System for User Port StackingWhen you set the DHCP offer on the DHCP server, you can set the stacking-option variable to provide thestack-port detail so a stack can be formed when the units are connected.Configure Secure DHCPDHCP as defined by RFC 2131 provides no authentication or security mechanisms. Secure DHCP is asuite of features that protects networks that use dynamic address allocation from spoofing and attacks.• Option 82• DHCP Snooping• Dynamic ARP Inspection• Source Address ValidationOption 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.
