Scenario: A Citrix Administrator manages an environment that has a Citrix ADC high availability (HA) pair running on two MPX appliances. The administrator notices that the state of the secondary Citrix ADC is 'Unknown'.
What is causing the secondary state to be 'Unknown'?
In a Citrix ADC high availability setup, the communication between the primary and secondary ADCs is crucial for synchronizing their statuses. TCP port 22 is used for Secure Shell (SSH) which is essential for HA sync operations and state sharing. If TCP port 22 is disabled between the primary and secondary ADCs, the secondary ADC cannot properly communicate its status to the primary, leading to the state being 'Unknown'.
https://docs.citrix.com/en-us/citrix-adc/current-release/system/high-availability-introduction/troubleshooting-high-availability.html