Configuring Gratuitous Arp; Overview; Gratuitous Arp Packet Learning; Periodic Sending Of Gratuitous Arp Packets - HP 6125G Configuration Manual

Configuring gratuitous ARP

Overview

In a gratuitous ARP packet, the sender IP address and the target IP address are the IP address of the
sending device.
A device sends a gratuitous ARP packet for either of the following purposes:
• Determine whether its IP address is already used by another device. If the IP address is already used,
the device is informed of the conflict by an ARP reply.
Inform other devices of a change of its MAC address.
•

Gratuitous ARP packet learning

This feature enables a device to create or update ARP entries by using the sender IP and MAC addresses
in received gratuitous ARP packets.
With this feature disabled, the device uses received gratuitous ARP packets to update existing ARP entries
only.

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.
When 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 gateway spoofing attacks, enable the gateway to send gratuitous ARP packets
containing its primary IP address and manually configured secondary IP addresses at a specific
interval, so hosts can learn correct gateway address information.
Prevent ARP entries from aging out.
•
If network traffic is heavy or if a host's CPU usage is high on a host, received ARP packets may be
discarded or not be processed in time. Eventually, the dynamic ARP entries on the receiving host
age out, and the traffic between the host and the corresponding devices is interrupted until the host
re-creates the ARP entries.
To prevent this problem, 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, so the receiving host can update ARP entries in time, ensuring traffic
continuity.
Prevent the virtual IP address of a VRRP group from being used by a host.
•
The master router of a VRRP group can periodically send gratuitous ARP packets to the hosts on the
local network, so that the hosts can update local ARP entries and avoid using the virtual IP address
of the VRRP group.
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