Gratuitous ARP

Definition

Gratuitous ARP allows a device to send an ARP Request packet that carries its own IP address as the destination IP address.

Purpose

Gratuitous ARP has the following functions:

• Checks for IP address conflicts.

  When the protocol status of a device interface changes to Up, the device broadcasts gratuitous ARP packets. If the device receives an ARP reply, another device is using the same IP address. When detecting an IP address conflict, the device periodically broadcasts gratuitous ARP Reply packets until the conflict is removed.

• Advertises a new MAC address.

  If the MAC address of a device is changed because its network adapter is replaced, the device sends a gratuitous ARP packet to notify all devices of the change before the ARP entry is aged out.

• Notifies other devices of a master/backup switchover in a VRRP group.

  After a master/backup switchover, the master device broadcasts a gratuitous ARP packet in the VRRP group to notify other devices of the switchover.

Action Upon Receipt of Gratuitous ARP Packets

With strict ARP learning disabled, a device receiving a gratuitous ARP packet will update the ARP entry in its table.

With strict ARP learning enabled, a device receiving a gratuitous ARP packet will check whether the source IP address of the packet is the same as the local IP address.

• If the IP addresses are the same, the device periodically broadcasts a gratuitous ARP Reply packet to announce the address conflict until the conflict is removed.
• If the IP addresses are different, the device updates the ARP entry according to the received gratuitous ARP packet only if it receives the packet on its VLANIF interface and has the dynamic ARP entry mapping the source IP address of the packet.

By default, strict ARP learning is disabled.

For the application of strict ARP learning, see Configuring Strict ARP Learning in "ARP Security Configuration" in the S2720, S5700, and S6700 V200R019C10 Configuration Guide - Security.

For the application of gratuitous ARP in the security feature, see Gratuitous ARP Packet Sending in "ARP Security Configuration" in the S2720, S5700, and S6700 V200R019C10 Configuration Guide - Security.