When an aggregation switch can be connected to an edge switch through an untrusted interface and you enter the ip dhcp snooping information option allow-untrusted global configuration command, the aggregation switch accepts packets with option-82 information from the edge switch. The aggregation switch learns the bindings for hosts connected through an untrusted switch interface. The DHCP security features, such as dynamic ARP inspection or IP source guard, can still be enabled on the aggregation switch while the switch receives packets with option-82 information on untrusted input interfaces to which hosts are connected.
The port on the edge switch that connects to the aggregation switch must be configured as a trusted interface. Option-82 Data Insertion In residential, metropolitan Ethernet-access environments, DHCP can centrally manage the IP address assignments for a large number of subscribers. When the DHCP option-82 feature is enabled on the switch, a subscriber device is identified by the switch port through which it connects to the network (in addition to its MAC address). Multiple hosts on the subscriber LAN can be connected to the same port on the access switch and are uniquely identified.
The DHCP option-82 feature is supported only when DHCP snooping is globally enabled on the VLANs to which subscriber devices using option-82 are assigned. Note The following illustration shows a metropolitan Ethernet network in which a centralized DHCP server assigns IP addresses to subscribers connected to the switch at the access layer. Because the DHCP clients and their associated DHCP server do not reside on the same IP network or subnet, a DHCP relay agent (the Catalyst switch) is configured with a helper address to enable broadcast forwarding and to transfer DHCP messages between the clients and the server. Figure 1:
DHCP Relay Agent in a Metropolitan Ethernet Network When you enable the DHCP snooping information option 82 on the switch, the following sequence of events occurs: • The host (DHCP client) generates a DHCP request and broadcasts it on the network. • When the switch receives the DHCP request, it adds the option-82 information in the packet. By default, the remote-ID suboption is the switch MAC address, and the circuit-ID suboption is the port identifier, vlan-mod-port , from which the packet is received. You can configure the remote ID and circuit ID. • If the IP address of the relay agent is configured, the switch adds this IP address in the DHCP packet.
• The switch forwards the DHCP request that includes the option-82 field to the DHCP server. Configuring DHCP Option-82 Data Insertion Configuring DHCP 4