In Azure, UDR stands for “User-Defined Route” or “User-Defined Routing.” UDRs are used to customize and control the routing of network traffic within a virtual network (VNet) or between virtual networks in a hub-and-spoke architecture. User-Defined Routes allow you to specify the path that network traffic should take, providing flexibility and control over your Azure network topology.

Here are key points about UDRs in Azure:

1. Custom Routing: UDRs enable you to define custom routing rules for network traffic. By default, Azure uses system routes to route traffic. UDRs allow you to override these default routes.

2. Scenario: UDRs are particularly useful in scenarios where you want to route traffic through specific virtual appliances or route traffic between VNets in a hub-and-spoke architecture.

3. Effective Routes: Each subnet within a VNet can have an associated UDR. When you associate a UDR with a subnet, it becomes the “effective route table” for that subnet. This means that all traffic leaving that subnet will be subject to the routing rules defined in the associated UDR.

4. Routing Priority: UDRs have a priority or “route table order” that determines the order in which routes are evaluated. Routes with lower priorities are evaluated before routes with higher priorities.

5. Route Propagation: UDRs can be used to control the propagation of BGP (Border Gateway Protocol) routes in Azure virtual network gateways. This is important for scenarios involving VPN or ExpressRoute connections.

6. Next Hop Types: UDRs can specify different types of next hops, including virtual network interfaces, virtual appliances (such as virtual firewalls or load balancers), VPN gateways, and ExpressRoute circuits.

7. Use Cases: Common use cases for UDRs include routing traffic through network virtual appliances (NVAs) for security inspection, routing traffic to specific virtual machines or services, and enabling complex network topologies.

8. Integration: UDRs can be created and managed using the Azure portal, Azure PowerShell, Azure CLI, or Azure Resource Manager (ARM) templates.