Consider the following when configuring IPv6 HA Monitoring:
In the Interface Settings dialog, enable Physical/Link Monitoring and Override Virtual MAC are dimmed because they are layer 2 properties. That is, the properties are used by both IPv4 and IPv6, so you configure them in the IPv4 monitoring page.
The primary/backup IPv6 address must be in the same subnet of the interface, and it can not be same as the global IP and Link-Local-IP of the primary/backup Security Appliance.
If the primary/backup monitoring IP is set to (not ::), then they cannot be the same.
If Allow Management on Primary/Secondary IPv6 Address is enabled, then primary/backup monitoring IPv6 addresses cannot be unspecified (that is, ::).
If Logical/Probe IPv6 Address is enabled, then the probe IP cannot be unspecified.