What is the most likely reason for an HA cluster failure if two devices do not form a cluster?

The most likely reason for an HA (High Availability) cluster failure when two devices do not form a cluster is due to incorrect HA mode configuration. For two devices to successfully establish an HA cluster, they must be configured with the same HA settings, including the HA mode (active-active or active-passive), group name, and other relevant parameters. If the configurations do not match, the devices will not be able to communicate effectively to synchronize their state and operate as a unified cluster.

In HA implementations, configurations must align precisely for the devices to recognize and authenticate each other. Thus, any incorrect setting in the HA mode or other critical configuration aspects can lead to failure in forming the cluster, making this reason the most plausible among the options provided.

Other potential issues, such as network latency or mismatched encryption protocols, while they can pose challenges, would typically not prevent an HA formation as outright as a configuration mismatch would. Similarly, while hardware version incompatibility may affect performance or features, it does not inherently prevent cluster formation as long as the devices are technically capable and configured correctly.