While on Horizon or on the CLI it's not mandatory to pass availability zone when creating a host aggregate:
$ nova help aggregate-create usage: nova aggregate-create <name> [<availability-zone>] Create a new aggregate with the specified details. Positional arguments: <name> Name of aggregate. <availability-zone> The availability zone of the aggregate (optional).
While on the tripleO templates, it's mandatory:
OS::Nova::HostAggregate [...] Required Properties availability_zone Name for the availability zone. String value expected. Can be updated without replacement. name Name for the aggregate. String value expected. Can be updated without replacement. [...]
- Red Hat OpenStack Platform 13
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