Azure UDR (User-Defined Routing) refers to the capability in Microsoft Azure to define and customize the routing of network traffic within a virtual network (VNet). With UDR, you can control how traffic is routed between subnets, to on-premises networks, and to the internet. Here are key aspects and features of Azure User-Defined Routing:

1. Custom Route Tables: UDR allows you to create custom route tables for your virtual network. Each route table contains a set of routing rules that define the next hop for traffic based on its destination.

2. Route Propagation: You can associate route tables with subnets within your virtual network. Subnets inherit the routes defined in their associated route table.

3. Route Prioritization: UDR allows you to define routes with different priorities, ensuring that the most specific route (based on destination prefix and subnet mask) is used when multiple routes apply.

4. Control over Traffic Flow: You can use UDR to control how traffic flows within your virtual network. For example, you can route traffic through network virtual appliances (NVAs) for network security and inspection.

5. Integration with VPN Gateway: UDR can be used in conjunction with Azure VPN Gateway to control the routing of traffic between your virtual network and on-premises networks over a VPN or ExpressRoute connection.

6. Route Override: UDR enables you to override the default system routes in Azure, giving you more control over how traffic is handled.

7. Internet Traffic Routing: You can configure UDR to direct internet-bound traffic through specific routes, such as a Network Virtual Appliance (NVA) or Azure Firewall, for inspection and security enforcement.

8. Route-Based Redundancy: UDR can be used to create route-based redundancy, ensuring that traffic is routed through multiple paths or appliances for high availability.

9. Integration with Azure Firewall: UDR can be used with Azure Firewall to control the routing of traffic through the firewall for inspection and filtering.

10. Security and Compliance: UDR allows you to implement routing policies that adhere to your organization’s security and compliance requirements.

11. Dynamic Routing: UDR supports dynamic routing protocols such as BGP (Border Gateway Protocol) for advanced routing scenarios.

12. Route Logging and Monitoring: Azure Monitor and Azure Network Watcher provide tools for monitoring and diagnosing routing issues within your virtual network.