In an Active-Passive High Availability (HA) cluster, only one FortiWeb unit (the primary) actively processes traffic, while the other (the secondary) remains in a standby state. When a failover is triggered, the standby unit assumes the primary role. To redirect traffic to itself, the newly promoted primary unit broadcasts gratuitous Address Resolution Protocol (ARP) packets. These packets advertise that the cluster's virtual IP (VIP) address is now associated with its own MAC address, prompting neighboring switches and routers to update their ARP tables and forward traffic to the new active unit.

A. Passive-Passive: This is not a standard or valid high availability mode, as it implies no device is actively processing traffic, which defeats the purpose of HA.

C. Active-Active: While an Active-Active cluster also uses gratuitous ARP during a failover (e.g., if the primary load-balancing unit fails), its primary purpose is load distribution across all nodes, not just failover. Active-Passive is the model most defined by this specific failover mechanism.

D. Passive-Active: This is not a distinct HA mode; it is simply another way of describing the Active-Passive configuration.

1. FortiWeb 7.0 Administration Guide

Page 478

Section: "How HA works".

The document states: "When a failover occurs

the new primary unit (the former standby) notifies the network that it is now the owner of the virtual MAC address and the IP addresses of the HA cluster. It sends gratuitous ARP packets that contain the virtual MAC address and the IP addresses of the cluster. Network devices (routers and switches) update their forwarding tables

and traffic begins to flow to the new primary unit." This description is the fundamental mechanism of an Active-Passive failover.