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Configuring gratuitous ARP
About gratuitous ARP
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 MAC address change.
IP conflict detection
When an interface obtains an IP address, the device broadcasts gratuitous ARP packets in the LAN
where the interface resides. If the device receives an ARP reply, its IP address conflicts with the IP
address of another device in the LAN. The device displays a log message about the conflict and
informs the administrator to change the IP address. The device will not use the conflicting IP address.
If no ARP reply is received, the device uses the IP 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.
When this feature is disabled, the device uses received gratuitous ARP packets to update existing
ARP entries only. ARP entries are not created based on the received gratuitous ARP packets, which
saves ARP table space.
Periodic sending of gratuitous ARP packets
Periodic sending of gratuitous ARP packets helps downstream devices update ARP entries or MAC
entries in a timely manner.
This feature can implement the following functions:
• Prevent gateway spoofing.
Gateway spoofing occurs when an attacker uses the gateway address to send 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 at intervals. Gratuitous ARP packets contain the primary IP address and manually
configured secondary IP addresses of the gateway, so hosts can learn correct gateway
information.
• Prevent ARP entries from aging out.
If network traffic is heavy or if the host CPU usage is high, received ARP packets can be
discarded or are not promptly processed. Eventually, the dynamic ARP entries on the receiving
host age out. 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. Gratuitous ARP packets contain the primary IP address and manually configured
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