Creating a Cluster Subnet Group is necessary for creating redshift cluster. The steps for creating a redshift cluster are as follows:-
Step 1: Navigate to Redshift and security and Subnet Groups.Click on Create Cluster Subnet Group:
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Step 2: Provide relevant information, Select the VPC and click on Create.
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Step 3: You can see that your Subnet is created.
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This way you can create a Cluster Subnet.