Azure Route Tables

Understanding Azure Route Tables

Azure route tables are a core component of network routing within Azure Virtual Networks (VNet). They enable you to define custom routes that control how network traffic is directed between subnets, to the internet, or to your on-premises networks.

What are Route Tables?

A route table contains a set of rules, called routes, that are used to classify packets and direct them to a specific destination. Each subnet within an Azure VNet is associated with at most one route table. If no route table is explicitly associated with a subnet, it uses the System Route Table, which provides default routing behavior.

Key Concepts

• Routes: Each route consists of a destination prefix (CIDR block) and a next hop type.
• Destination Prefix: The IP address range in CIDR notation for which the route applies.
• Next Hop Type: Specifies where to send the traffic that matches the destination prefix. Common types include:
  • VirtualAppliance: Traffic is sent to a network virtual appliance (NVA) such as a firewall or WAN optimizer.
  • VirtualNetworkGateway: Traffic is sent to a VPN gateway or ExpressRoute gateway for connectivity to on-premises networks.
  • Internet: Traffic is sent directly to the internet.
  • None: Traffic is dropped.
  • VirtualNetworkSink: Traffic is dropped (less common for general routing).
• Associated Subnet: The subnet to which the route table's rules are applied.
• Propagated Routes: Routes learned from other sources, such as VPN gateways or ExpressRoute circuits, that are automatically added to the route table.

Use Cases for Route Tables

Route tables are essential for advanced network configurations, including:

• Network Virtual Appliances (NVAs): Directing all traffic from subnets through a firewall, intrusion detection system (IDS), or other security appliances.
• Forced Tunneling: Forcing all internet-bound traffic from subnets through an on-premises network for inspection and logging before it reaches the internet.
• Custom Routing: Defining specific paths for traffic between different parts of your VNet or to external destinations.
• Inter-VNet Communication: Controlling traffic flow between connected VNets.

Creating and Managing Route Tables

You can create and manage route tables using the Azure portal, Azure PowerShell, or Azure CLI. When creating a route table, you can define custom routes immediately or add them later.

Example: Routing traffic through a firewall (Conceptual Azure CLI)

# Create a route table
az network route-table create --resource-group MyResourceGroup --name MyRouteTable

# Add a custom route to send all traffic to a firewall appliance
az network route-table route create --resource-group MyResourceGroup --route-table-name MyRouteTable --name ToFirewall --address-prefix 0.0.0.0/0 --next-hop-type VirtualAppliance --next-hop-ip-address 10.0.1.4

# Associate the route table with a subnet
az network vnet subnet update --resource-group MyResourceGroup --vnet-name MyVNet --name MySubnet --route-table MyRouteTable
            

Route Precedence

Azure determines the route to use based on the following precedence:

1. User-defined routes (from route tables)
2. System routes (default routes provided by Azure)
3. BGP routes (propagated from on-premises devices via VPN/ExpressRoute)

If multiple user-defined routes match the destination prefix, the route with the most specific prefix (longest prefix match) is chosen.

Best Practices

• Plan your routing strategy carefully before implementing route tables.
• Use descriptive names for your route tables and routes.
• Avoid broad destination prefixes (like 0.0.0.0/0) unless you have a clear understanding of the traffic flow and security implications.
• Regularly review and audit your route tables to ensure they align with your network security policies.
• Consider using Network Watcher's IP flow verify feature to troubleshoot routing issues.