The correct upgrade method for upgrading from R80.40 to R81.20 without any downtime is the Multi-Version Cluster Upgrade (MVC). MVC is a new feature in R80.40 that replaces the deprecated Connectivity Upgrade (CU). MVC allows you to upgrade cluster members to a newer version without losing connectivity and test the new version on some of the cluster members before you decide to upgrade the rest of the cluster members. MVC synchronizes connections between cluster members that run different versions and ensures that the cluster remains operational during the upgrade process. MVC is intended only to test the current configuration in the newer version and not to change the security policy and install it on cluster members with different software versions. MVC is disabled by default and can be enabled on each cluster member individually. MVC has some limitations, such as not supporting VSX clusters, IPS blade, or SecureXL acceleration.

Multi-Version Cluster (MVC) replaces Connectivity Upgrade (CU) in R80.40

Multi-Version Cluster (MVC) Upgrade

Configuring the Multi-Version Cluster Mechanism