H3C Technologies H3C SecPath F1000-E - Manual (Page 10)

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Gratuitous ARP Configuration
This chapter includes these sections:
• Introduction to Gratuitous ARP
• Configuring Gratuitous ARP
Introduction to Gratuitous ARP
In a gratuitous ARP packet, the sender IP address and the target IP address are both the IP address of the
device issuing the packet, the sender MAC address is the MAC address of the device, and the target
MAC address is the broadcast address ff:ff:ff:ff:ff:ff.
A device implements the following functions by sending gratuitous ARP packets:
• Determining whether its IP address is already used by another device. If the IP address is already
used, the device issuing the gratuitous ARP packet will be informed by an ARP reply of the conflict.
• Informing other devices about the change of its MAC address so that they can update their ARP
entries.
Enabling learning of gratuitous ARP packets
With this feature enabled, a device receiving a gratuitous ARP packet adds the sender IP and MAC
addresses carried in the packet to its ARP table if no corresponding ARP entry exists. If a corresponding
ARP entry is found, the device updates the ARP entry.
After this feature is disabled, the device will use the address information in the received gratuitous ARP
packets to update the existing ARP entries only, but not to create new ARP entries.
Configuring periodic sending of gratuitous ARP packets
Enabling a device to periodically send gratuitous ARP packets helps downstream devices update their
corresponding ARP entries or MAC entries in time. This feature can be used to:
• Prevent gateway spoofing
If an attacker sends forged gratuitous ARP packets to the hosts on a network, the traffic destined for the
gateway from the hosts is sent to the attacker instead. As a result, the hosts cannot access the external
network.
To prevent such gateway spoofing attacks, you can enable the gateway to send gratuitous ARP packets
containing its primary IP address and manually configured secondary IP addresses at a specific interval.
In this way, each host can learn correct gateway address information.
• Prevent ARP entries from being aged out
If network traffic is heavy or the CPU utility is high on a host, ARP packets received may be discarded or
cannot be processed in time. Eventually, the dynamic ARP entries on the receiving host will be aged out,
and the traffic between the host and the corresponding devices will be interrupted until the host creates
the ARP entries again.
To prevent such a problem, you can enable the gateway to send gratuitous ARP packets periodically. The
gratuitous ARP packets contain the gateway's primary IP address or one of its manually configured
secondary IP addresses. In this way, the receiving host can update ARP entries in time and thus ensure
traffic continuity.