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Cisco IE 3010 Switch Software Configuration Guide
OL-23145-01
Chapter 21 Configuring DHCP Features and IP Source Guard Features
Understanding DHCP Snooping
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 an d 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 in Figure 21-2 do not change:
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
eight 10/100 ports and small form-factor pluggable (SFP) module slots, port 3 is the Fast Ethernet 0/1
port, port 4 is the Fast Ethernet 0/2 port, and so forth. Port 11 is the SFP module slot 0/1, and so forth.
Figure 21-2 shows the packet formats for the remote-ID suboption and the circuit-ID suboption when
the default suboption configuration is used. The switch uses the packet formats whe n you globally enable
DHCP snooping and enter the ip dhcp snooping information option global configuration command.