Azure Routing Simplified – UDR, Route Tables & BGP

calendar Jul 2, 2025 · 2 min read · Azure Networking Route Table BGP UDR Routing  ·
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Azure Routing – UDR, Route Tables, and BGP

Ever wonder how traffic knows where to go inside your VNet? That magic is thanks to Routing – and in Azure, it’s handled by:

  • System Routes
  • User Defined Routes (UDR)
  • Border Gateway Protocol (BGP)

System Routes (Default)

Azure automatically adds system routes to all subnets.

Examples:

Destination Next Hop
0.0.0.0/0 Internet
Virtual Network
168.63.129.16 Virtual Network

These work fine in most cases — until you need customization.

User Defined Routes (UDR)

UDRs are custom routes you create and assign to subnets via Route Tables.

Use Cases:

  • Force traffic through a firewall
  • Send all outbound traffic to a NAT gateway
  • Isolate traffic between subnets

Example:

Destination CIDR Next Hop Type Next Hop IP
0.0.0.0/0 Virtual Appliance 10.0.1.4
10.1.0.0/16 Virtual Network —

“Virtual Appliance” = firewall, NVA, etc.

Route Table Basics

  • You can create a Route Table in Azure
  • Add UDRs (custom rules)
  • Associate the table with one or more subnets

Changes apply immediately once associated.

What is BGP?

BGP = Border Gateway Protocol

Used mostly with ExpressRoute and VPN Gateway to dynamically share routes between:

  • On-premises
  • Azure
  • Peer networks

Benefits:

  • Avoids manual route entries
  • Learns new prefixes automatically
  • Supports active-active gateways

For most standard Azure VNets, BGP is optional — used in hybrid/enterprise setups.

Use Effective Routes

Want to see what routes apply to a VM?

  1. Go to VM > Networking > NIC > Effective Routes
  2. Azure shows a full list of all inherited + UDRs

Summary

Route Type Use For
System Routes Default routing in VNet
UDR Custom control (firewalls, NAT, etc)
BGP Hybrid/ExpressRoute dynamic routing

Learn More

Next: NSG, ASG, and network security in Azure

– Kasi @ KasdevTech or LinkedIn