• The DHCP server receives the packet. If the server is option-82-capable, it can use the remote ID, the circuit ID, or both to assign IP addresses and implement policies, such as restricting the number of IP addresses that can be assigned to a single remote ID or circuit ID. Then the DHCP server echoes the option-82 field in the DHCP reply. • The DHCP server unicasts the reply to the switch if the request was relayed to the server by the switch. The switch verifies that it originally inserted the option-82 data by inspecting the remote ID and possibly the circuit ID fields.
The switch removes the option-82 field and forwards the packet to the switch port that connects to the DHCP client that sent the DHCP request. In the default suboption configuration, when the described sequence of events occurs, the values in these fields do not change (see the illustration, Suboption Packet Formats ): • Circuit-ID suboption fields • Suboption type • Length of the suboption type • Circuit-ID type • Length of the circuit-ID type • Remote-ID suboption fields • Suboption type • Length of the suboption type • Remote-ID type • Length of the remote-ID type In the port field of the circuit ID suboption, the port numbers start at 3.
For example, on a switch with 24 10/100/1000 ports and four small form-factor pluggable (SFP) module slots, port 3 is the Gigabit Ethernet 1/0/1 port, port 4 is the Gigabit Ethernet 1/0/2 port, and so forth. Port 27 is the SFP module slot Gigabit Ethernet1/0/25, and so forth. The illustration, Suboption Packet Formats . shows the packet formats for the remote-ID suboption and the circuit-ID suboption when the default suboption configuration is used. For the circuit-ID suboption, the module number corresponds to the switch number in the stack.
The switch uses the packet formats when you globally enable DHCP snooping and enter the ip dhcp snooping information option global configuration command. Configuring DHCP Option-82 Data Insertion Configuring DHCP 5