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25-4
Cisco IE 2000 Switch Software Configuration Guide
OL-25866-01
Chapter 25 Configuring DHCP
Information About Configuring DHCP
When you enable the DHCP snooping information option-82 on the switch, this 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.
• 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.
• 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. The DHCP server then
repeats 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 in
Figure 25-2 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
1/1
port, port 4 is the Fast Ethernet 1/2 port, and so forth. Port 11 is the SFP module slot 1/1, and so
forth.
Figure 25-2 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 when you globally enable
DHCP snooping and enter the ip dhcp snooping information option global configuration command.