Learning Gateway Transit and Connectivity in Azure VPN Gateway

Azure VPN Gateway is a service that provides secure, cross-premises connectivity between an Azure Virtual Network (VNet) and on-premises or other VNets through IPsec/IKE VPN tunnels.

Gateway transit is a feature of Azure Virtual Network Peering that allows one VNet (spoke) to utilize the gateway in another peered VNet (hub) for external connectivity.

Here’s an in-depth guide to understanding and configuring gateway transit and connectivity with Azure VPN Gateway.

Key Concepts

Gateway Transit

• Definition: Allows a VNet to share the VPN or ExpressRoute gateway of a peered VNet.

• Use Case: Centralizes gateway management in a hub VNet, reducing costs and simplifying architecture in hub-and-spoke network topologies.

VPN Gateway

• A VPN Gateway is a logical gateway on Azure that allows encrypted traffic between VNets and/or on-premises locations.

Prerequisites for Gateway Transit

1. Peered VNets

• VNets must be peered using Azure VNet Peering.

• IP address spaces must not overlap.

2. Gateway Deployment

• A VPN Gateway must be deployed in the hub VNet.

3. Permissions

• Proper Azure RBAC permissions are required (e.g., Network Contributor role).

4. Subscription and Tenant

• The VNets can be in different subscriptions but must belong to the same Azure AD tenant.

Gateway Transit in Hub-and-Spoke Topology

In a hub-and-spoke topology:

1. Hub VNet: Contains the VPN Gateway and provides external connectivity.

2. Spoke VNets: Peer with the hub VNet and utilize the gateway transit.

Configuring Gateway Transit

Step 1: Deploy the VPN Gateway

1. Navigate to the Azure Portal and create a new VPN Gateway in the hub VNet.

   • Example:

     • VNet: 10.0.0.0/16

     • Subnet: Create a GatewaySubnet (e.g., 10.0.255.0/27).

2. Configure the VPN Gateway with the required SKU (e.g., VpnGw1).

Step 2: Configure VNet Peering

1. In the hub VNet:

   • Navigate to the Peering settings and create a new peering connection to the spoke VNet.

   • Enable Allow gateway transit.

2. In the spoke VNet:

   • Navigate to the Peering settings and create a new peering connection to the hub VNet.

   • Enable Use remote gateways.

Step 3: Test Connectivity

• Verify that the spoke VNet's traffic is routed through the VPN Gateway in the hub VNet.

Connectivity with Azure VPN Gateway

Types of Connectivity Supported

1. VNet-to-VNet

• Connect multiple Azure VNets using VPN gateways.

2. Site-to-Site (S2S)

• Connect Azure VNets to on-premises networks.

3. Point-to-Site (P2S)

• Enable secure access for individual devices (e.g., laptops) to connect to Azure.

4. Multi-Site

• Connect a single VPN Gateway to multiple on-premises sites.

Configuring Connections

1. Site-to-Site Connection:

1. Configure a local network gateway with the on-premises VPN details.

2. Establish a S2S connection between the Azure VPN Gateway and the on-premises VPN device.

2. VNet-to-VNet Connection:

1. Deploy VPN Gateways in both VNets.

2. Establish a connection between the gateways.

3. Point-to-Site Connection:

1. Enable P2S VPN on the Azure VPN Gateway.

2. Provide client configuration files to users for secure access.

6. Routing in Gateway Transit

Routing Overview

• Traffic from the spoke VNet is routed through the hub VNet's gateway and then to:

  • On-premises networks.

  • Other Azure VNets connected to the hub.

  • External networks.

Routing Table Considerations

• Use User-Defined Routes (UDRs) to control routing behavior in hub-and-spoke designs.

• Example UDR in the spoke VNet:

  • Destination: On-premises network (e.g., 192.168.1.0/24)

  • Next Hop: Virtual network gateway.

7. Monitoring and Troubleshooting Gateway Transit

Monitoring Tools

1. Azure Network Watcher

Diagnose connectivity issues and view effective routes.

2. Azure Monitor

Monitor VPN Gateway performance and connection metrics.

Troubleshooting Tips

1. Connectivity Issues

• Verify peering settings and ensure Allow gateway transit and Use remote gateways are configured correctly.

• Check if the GatewaySubnet exists and has sufficient IP addresses.

2. Routing Issues

• Confirm route table configurations in both hub and spoke VNets.

3. On-premises VPN Device Issues

• Ensure compatibility with Azure VPN Gateway settings (e.g., IPsec/IKE configurations).

8. Cost Considerations

1. Gateway SKU

Costs depend on the selected gateway SKU (e.g., VpnGw1, VpnGw2).

2. Data Transfer

Inbound data is free, but outbound data incurs charges.

3. Peering Costs

VNet peering incurs additional data transfer charges.

9. Best Practices

1. Centralized Gateways

Use hub-and-spoke architecture to reduce costs and simplify management.

2. Resilience

Use high-availability configurations for VPN Gateways.

3. Monitoring

Regularly monitor gateway performance and traffic.

Let me know if you need step-by-step instructions for a specific setup or scenario.