ZyXEL Communications XGS-4526 User Manual

Chapter 26 IP Source Guard

XGS-4526/4528F/4728F User’s Guide

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.

Figure 116   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:

• It pretends to be computer A and responds to computer B.
• It pretends to be computer B and sends a message to computer A.

As a result, all the communication between computer A and computer B passes 
through computer X. Computer X can read and alter the information passed 
between them.

When the Switch identifies an unauthorized ARP packet, it automatically creates a 
MAC address filter to block traffic from the source MAC address and source VLAN 
ID of the unauthorized ARP packet. You can configure how long the MAC address 
filter remains in the Switch.

These MAC address filters are different than regular MAC address filters (

).

• They are stored only in volatile memory.
• They do not use the same space in memory that regular MAC address filters 

use.