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3.6.2. About reusing a VNet for your OpenShift Container Platform cluster

In OpenShift Container Platform 4.5, you can deploy a cluster into an existing Azure Virtual Network (VNet) in Microsoft Azure. If you do, you must also use existing subnets within the VNet and routing rules.

By deploying OpenShift Container Platform into an existing Azure VNet, you might be able to avoid service limit constraints in new accounts or more easily abide by the operational constraints that your company’s guidelines set. This is a good option to use if you cannot obtain the infrastructure creation permissions that are required to create the VNet.

重要

The use of an existing VNet requires the use of the updated Azure Private DNS (preview) feature. See Announcing Preview Refresh for Azure DNS Private Zones for more information about the limitations of this feature.

3.6.2.1. Requirements for using your VNet

When you deploy a cluster by using an existing VNet, you must perform additional network configuration before you install the cluster. In installer-provisioned infrastructure clusters, the installer usually creates the following components, but it does not create them when you install into an existing VNet:

  • Subnets
  • Route tables
  • VNets
  • Network Security Groups

If you use a custom VNet, you must correctly configure it and its subnets for the installation program and the cluster to use. The installation program cannot subdivide network ranges for the cluster to use, set route tables for the subnets, or set VNet options like DHCP, so you must do so before you install the cluster.

The cluster must be able to access the resource group that contains the existing VNet and subnets. While all of the resources that the cluster creates are placed in a separate resource group that it creates, some network resources are used from a separate group. Some cluster Operators must be able to access resources in both resource groups. For example, the Machine API controller attaches NICS for the virtual machines that it creates to subnets from the networking resource group.

Your VNet must meet the following characteristics:

  • The VNet’s CIDR block must contain the Networking.MachineCIDR range, which is the IP address pool for cluster machines.
  • The VNet and its subnets must belong to the same resource group, and the subnets must be configured to use Azure-assigned DHCP IP addresses instead of static IP addresses.

You must provide two subnets within your VNet, one for the control plane machines and one for the compute machines. Because Azure distributes machines in different availability zones within the region that you specify, your cluster will have high availability by default.

To ensure that the subnets that you provide are suitable, the installation program confirms the following data:

  • All the subnets that you specify exist.
  • You provide two private subnets, one for the control plane machines and one for the compute machines.
  • The subnet CIDRs belong to the machine CIDR that you specified. Machines are not provisioned in availability zones that you do not provide private subnets for. If required, the installation program creates public load balancers that manage the control plane and worker nodes, and Azure allocates a public IP address to them.

If you destroy a cluster that uses an existing VNet, the VNet is not deleted.

3.6.2.1.1. Network security group requirements

The network security groups for the subnets that host the compute and control plane machines require specific access to ensure that the cluster communication is correct. You must create rules to allow access to the required cluster communication ports.

重要

The network security group rules must be in place before you install the cluster. If you attempt to install a cluster without the required access, the installation program cannot reach the Azure APIs, and installation fails.

表3.10 Required ports

PortDescriptionControl planeCompute

80

Allows HTTP traffic

 

x

443

Allows HTTPS traffic

 

x

6443

Allows communication to the control plane machines

x

 

22623

Allows communication to the machine config server

x

 
注記

Since cluster components do not modify the user-provided network security groups, which the Kubernetes controllers update, a pseudo-network security group is created for the Kubernetes controller to modify without impacting the rest of the environment.