How Does a Private Endpoint Impact Redis Cache and Subnet Configuration in Azure?

Introduction

In Azure environments, using a Private Endpoint for services like Redis Cache ensures secure, private connections between resources. However, when configuring these endpoints, questions often arise regarding subnet usage, IP consumption, and overall networking best practices. This article addresses some common concerns and provides clarity on how to manage your subnets when working with Private Endpoints.

1. Where Can I See in the Azure Portal That My Private Endpoint Is Connected to a Subnet?

To verify which subnet your Private Endpoint is connected to, you can follow these steps in the Azure portal:

1. Navigate to the Azure portal (https://portal.azure.com).
2. Search for your Redis Cache: In the search bar, type "Redis" and select your Redis Cache resource.
3. Access Private Endpoints: Under the "Settings" section, select Private Endpoint Connections to view the associated endpoints.
4. Check Subnet Details: Select the Private Endpoint and review the Network Configuration section to identify the subnet.

This straightforward process will give you insight into which subnet your Redis Cache's private endpoint is connected to.

2. Is It Fine to Have Other Components in the Same Subnet as My Redis Cache’s Private Endpoint?

Yes, it is typically fine to have multiple components in the same subnet as long as your network is properly configured. However, there are a few factors to consider:

• Subnet IP Address Range: Ensure that there are enough available IP addresses to accommodate all resources within the subnet, including the Redis Cache's private endpoint.
• Network Security Group (NSG): Double-check that your NSG rules do not block necessary traffic to and from the Redis Cache.
• Private DNS Zone: Make sure that the private DNS zone resolves the Redis Cache’s private endpoint correctly to ensure seamless internal communication.

Testing Redis Cache Connectivity

To ensure everything is working smoothly, you can test the Redis Cache using:

• Redis CLI: Try connecting to the Redis Cache using a client or the Redis CLI from a VM in the same virtual network and run commands like PING, SET, or GET.
• Azure Network Watcher: Use Connection Troubleshoot to test connectivity between the source (e.g., VM) and the Redis Cache private endpoint.
• Logs: Review Redis Cache logs to detect any potential connectivity issues.

3. How Many IPs Does Redis Cache Using a Private Endpoint Use in a Subnet?

When Redis Cache is connected via a Private Endpoint, it consumes one IP address in the subnet for each private endpoint. This IP address is assigned to the private link interface that connects the Redis Cache to the virtual network.

It's important to note that the Redis Cache service itself doesn't consume additional IPs directly from the subnet—only the Private Endpoint requires one. If you have multiple private endpoints for the same Redis Cache across different subnets or regions, each of those endpoints will also consume one IP address per subnet.

4. What Size Should My Subnet Be for a Redis Cache Private Endpoint?

Determining the correct subnet size is crucial for both current operations and future scalability. Here’s how you can plan:

IP Address Consumption

• Private Endpoint: Each private endpoint consumes 1 IP address.
• Azure Reserves: Azure reserves the first 4 and last 1 IP addresses of every subnet for internal use, leaving the rest for your resources.
• Other Resources: Any other VMs, application gateways, or services in the same subnet will consume additional IP addresses.

Subnet Size Calculation

Consider using the following CIDR blocks based on your needs:

• /28 gives 16 IP addresses (11 usable).
• /27 gives 32 IP addresses (27 usable).
• /26 gives 64 IP addresses (59 usable).
• /25 gives 128 IP addresses (123 usable).

For a Redis Cache private endpoint with minimal additional resources, a /28 subnet might suffice. However, for future scalability and more components, a /26 subnet is recommended.

5. Why Does Azure Reserve 5 IP Addresses in a Subnet?

Azure reserves 5 IP addresses in every subnet, regardless of size, for the following reasons:

1. First IP (Network address): This IP defines the subnet and is reserved for the network identifier.
2. Second IP: Reserved for Azure services.
3. Third IP: Reserved for future use by Azure.
4. Fourth IP (Default gateway): This IP is used as the default gateway for routing traffic outside the subnet.
5. Last IP (Broadcast address): Reserved as the broadcast address, though broadcast traffic isn't typically used in Azure.

For example, in a 10.0.0.0/24 subnet:

• 10.0.0.0 is the network identifier.
• 10.0.0.1 is reserved for Azure.
• 10.0.0.2 is reserved for future use.
• 10.0.0.3 is the default gateway.
• 10.0.0.255 is the broadcast address.

Conclusion

Configuring Redis Cache with Private Endpoints in Azure requires a solid understanding of how subnets, IP addresses, and networking rules work together. By following these guidelines, ensuring adequate subnet sizing, and testing your configuration, you can optimize performance and maintain secure connections.

Always monitor your subnet usage, plan for scalability, and review security configurations regularly to ensure smooth operations within your Azure environment. 

About Me

I’m Sujith Sagar Gandi, an IT professional with extensive experience in cloud architecture, networking, and infrastructure solutions. I specialize in configuring and managing Azure environments, with a focus on optimizing infrastructure for businesses and individuals alike. Sharing insights about cloud technologies is something I’m passionate about, and I aim to simplify complex concepts for others.

Feel free to connect with me on LinkedIn to follow my work and get in touch!