ZyXEL Communications XS3800-28, User Manual, Page: 640 / 789

Chapter 84 ARP Inspection
XS3800-28 User’s Guide
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Figure 509 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.
84.15.1.1 ARP Inspection and MAC Address Filters
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.
• They appear only in the
ARP Inspection screens and commands, not in the MAC Address Filter screens
and commands.
84.15.1.2 Trusted vs. Untrusted Ports
Every port is either a trusted port or an untrusted port for ARP inspection. This setting is independent of
the trusted or untrusted setting for DHCP snooping. You can also specify the maximum rate at which the
Switch receives ARP packets on untrusted ports.
The Switch does not discard ARP packets on trusted ports for any reason.
The Switch discards ARP packets on untrusted ports in the following situations:
• The sender’s information in the ARP packet does not match any of the current bindings.
• The rate at which ARP packets arrive is too high.
84.15.1.3 Syslog
The Switch can send syslog messages to the specified syslog server when it forwards or discards ARP
packets. The Switch can consolidate log messages and send log messages in batches to make this
mechanism more efficient.