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45-5
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 45 Configuring DHCP Snooping, IP Source Guard, and IPSG for Static Hosts
About DHCP Snooping
• 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 45-2 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
Figure 45-2 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 module number. The switch uses the packet formats when you globally enable DHCP
snooping and enter the ip dhcp snooping information option global configuration command.
Figure 45-2 Suboption Packet Formats
Length Length
Circuit
ID type
Suboption
type
Circuit ID Suboption Frame Format
Remote ID Suboption Frame Format
6 bytes
MAC address
1 byte 1 byte 1 byte
Suboption
type
1 byte
Length Length
Remote
ID type
1 byte 1 byte 1 byte1 byte
116300
4061
6082
Module Port
1 byte 1 byte2 bytes
VLAN