F3215-HP Load Balancing Module Network Management Configuration Guide-6PW101

Configuring gratuitous ARP

Gratuitous ARP can be configured only at the CLI.

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.

Enabling learning of gratuitous ARP packets

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 the received gratuitous ARP packets to update only existing

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,

prevent ARP entries from aging out, and prevent the virtual IP address of a VRRP group from being used

by a host.

• Prevent gateway spoofing.

An attacker can use the gateway address to send gratuitous ARP packets to the hosts on a network

so that 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, 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, received ARP packets may be discarded

or may 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, 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, so the receiving hosts can update ARP entries in time.

• Prevent the virtual IP address of a VRRP group from being used by a host.