HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Services Conf - Page 18
Configuring gratuitous ARP, Overview, Gratuitous ARP packet learning
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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. 10