During a regular upgrade process in a high availability setup for NetScaler, at some point, both nodes run different software builds. These two builds can have the same or different internal high availability version numbers.
If both the builds have different high availability version numbers, connection failover (even if it is enabled) for existing data connections is not supported. In other words, all existing data connections are lost, which leads to downtime.
To address this issue, in Service Software Upgrade (ISSU) can be used for high availability set-ups. ISSU introduces a migration functionality, which replaces the force failover operation step in the upgrade process. The migration functionality takes care of honoring the existing connections and includes the force failover operation.
After a migration operation is performed, the new primary node always receives traffic (request and response) related to the existing connections but steers them to the old primary node. The old primary node processes the data traffic and then sends them directly to the destination."
For more information:
https://docs.citrix.com/en-us/citrix-adc/current-release/upgrade-downgrade-citrix-adc-appliance/issu-high-availability.html
