Chapter 24 IP Source Guard
GS3700/XGS3700 Series User’s Guide
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• 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.
24.1.2.1 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.
24.1.2.2 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/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.
24.1.2.3 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.
24.1.2.4 Configuring ARP Inspection
Follow these steps to configure ARP inspection on the Switch.
1
Configure DHCP snooping. See
.
Note: It is recommended you enable DHCP snooping at least one day before you enable
ARP inspection so that the Switch has enough time to build the binding table.
2
Enable ARP inspection on each VLAN.
3
Configure trusted and untrusted ports, and specify the maximum number of ARP packets that each
port can receive per second.
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