Operation Manual – ARPH3C S7500 Series Ethernet Switches Chapter 1 ARP Configuration1-4Generally, a host will automatically trigger ARP procedure during IP addressing.1.1.5 Introduction to Gratuitous ARPGratuitous ARP packets have the following characteristics:z Both source and destination IP addresses of a gratuitous ARP packet are the localaddresses; the source MAC address of it is the local MAC address.z If a device finds that the IP address in an incoming gratuitous packet conflicts withits own IP address, it returns an ARP replay to the sending device to notify thesender of the IP address conflict.By sending gratuitous ARP packets, a network device can:z Determine whether or not IP address conflict exists between it and other networkdevices.z Trigger other network devices to update its hardware address stored in theircaches.With gratuitous ARP learning enabled on a device, each time the device receives agratuitous ARP packet, the device updates the ARP entry matching the packet in thecache (if exists) by using the hardware address of the sender carried in the gratuitousARP packet.I. Overview of gratuitous ARP update intervalWhen ARP aging timer expires, some hosts in the network directly delete the ARPentries learned dynamically, incapable of updating ARP entries actively. These hostshave to trigger a new ARP request packet with a new IP packet received to request forthe gateway address. As a host can buffer only one packet, when a ping is sent with along packet, multiple fragments will be lost, which interrupts the ping.When network load or the CPU occupancy of the receiving host is high, ARP packetsmay be lost or the host may be unable to process the ARP received timely. In such acase, after the dynamic ARP entries on the host age out, the traffic between the hostand the sending device will remain interrupted before the host learns the ARP entrieson the sending device again.To address this issue, you can configure the gratuitous ARP update interval on theS7500 series Ethernet switches. With gratuitous ARP packets sent periodically, thereceiving host can update the ARP entry for the gateway in its ARP table timely. In thisway, the ARP entry for the gateway has been updated before the host ages out theentry; therefore, this entry will not be deleted. This prevents traffic interruption asmentioned above.II. How gratuitous ARP update interval worksA switch periodically sends gratuitous ARP packets that carry the master IP addressand secondary IP address of VLAN interfaces and the IP addresses of all the VRRP
