User-defined Routing

By default, Azure automatically creates system routes and assigns them to the different subnets within a VNet. The system routes help guide the network traffic to reach the intended destination, such as another VM in the same subnet or VNet, or even to reach other resources outside of the existing VNet you are in. These routes cannot be removed but can be overridden by custom routes known as User-Defined Routes (UDRs) that will enable you to have more control over the next hop of your traffic. These are commonly used in environments that have a hub-and-spoke topology for the networking layer. The next hop for known resources may be to a Network Virtual Appliance (NVA) such as a third-party firewall solution that also controls routing between networks. This can force all traffic to be interrogated by the firewall before being directed to the approach location. UDR rules are based on the most granular (i.e., most specific) rules being applied first, so in the event...