Chapter 22 Configuring DHCP Features and IP Source Guard

Understanding DHCP Features

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

Catalyst 3750-E 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.

Figure 22-2shows 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.

Figure 22-2 Suboption Packet Formats

Circuit ID Suboption Frame Format

Suboption Circuit

type ID type

Length Length

1

6

0

4

VLAN

Module

Port

 

 

 

 

1 byte 1 byte 1 byte 1 byte

2 bytes

1 byte

1 byte

Remote ID Suboption Frame Format

Suboption Remote

type ID type

Length Length

2

8

0

6

MAC address

 

 

1 byte 1 byte 1 byte 1 byte

6 bytes

116300

Figure 22-3shows 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-idglobal configuration command and the ip dhcp snooping vlan information option format-typecircuit-id string interface configuration command are entered.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

 

OL-9775-02

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Cisco Systems 3750E manual Remote ID Suboption Frame Format, 22-5