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Chapter 24 IP Source Guard

GS3700/XGS3700 Series User’s Guide

215

24.1.1.3 DHCP Relay Option 82 Information

The Switch can add information to DHCP requests that it does not discard. This provides the DHCP
server more information about the source of the requests. The Switch can add the following
information:

• Slot ID (1 byte), port ID (1 byte), and source VLAN ID (2 bytes)
• System name (up to 32 bytes)

This information is stored in an Agent Information field in the option 82 field of the DHCP headers of
client DHCP request frames. See

Chapter 38 on page 313

for more information about DHCP relay

option 82.

When the DHCP server responds, the Switch removes the information in the Agent Information field
before forwarding the response to the original source.

You can configure this setting for each source VLAN. This setting is independent of the DHCP relay
settings (

Chapter 38 on page 313

24.1.1.4 Configuring DHCP Snooping

Follow these steps to configure DHCP snooping on the Switch.

Enable DHCP snooping on the Switch.

Enable DHCP snooping on each VLAN, and configure DHCP relay option 82.

Configure trusted and untrusted ports, and specify the maximum number of DHCP packets that
each port can receive per second.

Configure static bindings.

24.1.2 ARP Inspection Overview

Use ARP inspection to filter unauthorized ARP packets on the network. This can prevent many kinds
of man-in-the-middle attacks, such as the one in the following example.

Figure 135

Example: Man-in-the-middle Attack

In this example, computer B tries to establish a connection with computer A. Computer X is in the
same broadcast domain as computer A and intercepts the ARP request for computer A. Then,
computer X does the following things: