1-4mode, all hosts on this subnet can receive the request, but only the requested host (namely, Host B)will process the request.3) Host B compares its own IP address with the destination IP address in the ARP request. If they arethe same, Host B saves the source IP address and source MAC address into its ARP mappingtable, encapsulates its MAC address into an ARP reply, and unicasts the reply to Host A.4) After receiving the ARP reply, Host A adds the MAC address of Host B into its ARP mapping tablefor subsequent packet forwarding. Meanwhile, Host A encapsulates the IP packet and sends it out.Usually ARP dynamically implements and automatically seeks mappings from IP addresses to MACaddresses, without manual intervention.Introduction to Gratuitous ARPThe following are the characteristics of gratuitous ARP packets:z Both source and destination IP addresses carried in a gratuitous ARP packet are the localaddresses, and the source MAC address carried in it is the local MAC addresses.z If a device finds that the IP addresses carried in a received gratuitous packet conflict with those ofits own, it returns an ARP response to the sending device to notify of the IP address conflict.By sending gratuitous ARP packets, a network device can:z Determine whether or not IP address conflicts exist between it and other network devices.z Trigger other network devices to update its hardware address stored in their caches.With the gratuitous ARP packet learning function enabled:A device receiving a gratuitous ARP packet adds the information carried in the packet to its owndynamic ARP table if it finds no corresponding ARP entry for the ARP packet exists in the cache.Periodical sending of gratuitous ARP packetsIn an actual network, when the network load or the CPU occupancy of the receiving host is high, ARPpackets may be lost or the host may be unable to timely process the ARP packets received. In such acase, the dynamic ARP entries on the receiving host may age out, and the traffic between the host andthe sending device will get interrupted before the host learns the MAC address of the sending deviceagain and installs a corresponding entry in the ARP table.To address this issue, by default, the S5500-EI series allow VLAN interfaces to send gratuitous ARPpackets periodically. That is, as long as a VLAN interface is in the Up state, it sends gratuitous ARPpackets at an interval of 30 seconds so that the receiving host can refresh the MAC address of theswitch in the ARP table timely, thereby preventing traffic interruption mentioned above.Periodical sending of ARP packets in a VRRP backup groupIf a VRRP backup group exists on a network, the master switch sends gratuitous ARP packetsperiodically to hosts on the network, which then update their local ARP tables, ensuring that no deviceon this network uses the same IP address with the VRRP virtual router.As you can create mappings between the IP address and MAC address of the VRRP virtual router,there are two cases:z If the IP address of the virtual router corresponds to a virtual MAC address, the source MACaddress in the gratuitous ARP packet will be the virtual MAC address.z If the IP address of the virtual router corresponds to an actual MAC address, the source MACaddress in the gratuitous ARP packet will be the VLAN interface’s MAC address of the masterswitch in the VRRP backup group.
