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21-5
Catalyst 3560 Switch Software Configuration Guide
OL-8553-06
Chapter 21 Configuring DHCP Features and IP Source Guard
Understanding DHCP Snooping
• 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 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 27 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 when you globally enable
DHCP snooping and enter the ip dhcp snooping information option global configuration command.
Figure 21-2 Suboption Packet Formats
Figure 21-3 shows the packet formats for user-configured remote-ID and circuit-ID suboptions The
switch uses these packet formats when DHCP snooping is globally enabled and when the ip dhcp
snooping information option format remote-id global configuration command and the ip dhcp
snooping vlan information option format-type circuit-id string interface configuration command are
entered.
The values for these fields in the packets change from the default values when you configure the
remote-ID and circuit-ID suboptions:
• Circuit-ID suboption fields
–
The circuit-ID type is 1.
–
The length values are variable, depending on the length of the string that you configure.
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