Requirements and Principles

To ensure fault tolerance and high availability, create availability sets which contain several VMs, where one VM is backed up by one or several other VMs. When several VMs are included in an availability set, Azure places them into different fault domains and update domains (physical hardware) to provide a foundation for high availability.

Note: Azure does not provide high availability.
To ensure easier adding of redundancy for a VM, include the VM in an availability set. Even when an availability set contains a single VM, such a configuration will ensure easier adding of redundancy when needed, and will decrease downtime. If a VM is not included in an availability set but later must be included, such a VM must be re-deployed just to be included in an availability set.

Note: Azure includes OS disks in the same availability sets as the VMs to which the disks are assigned.
Hardware and software requirements
Network requirements
IP address requirements
Security requirements
AKS Cluster requirements
(optional) For more resource management convenience, your Azure cloud infrastructure should have a single VNet and all its resources should belong to a single resource group.

For more information, please go to https://azure.microsoft.com/en-us/.