ZyXEL Communications MES3500-24 User Manual

Chapter 26 IP Source Guard

MES3500-24/24F User’s Guide

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 

 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 (

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.

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.

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: