Azure vNet system routes are automatically created and maintained by Azure to enable routing between subnets, on-premises networks, and the internet.
Azure vNet system routes are automatically associated via a default route table to the vNet.
System Routes are a collection of routing entries that define several destination networks and the next hop to send the traffic to. This is the path that the traffic should follow to get to the destination.
You can override the default system routes by creating User Defined Routes (UDRs) and associating them with subnets in your Azure Virtual Network (VNet) or by using BGP.
You can have up to 200 custom route tables per region per subscription.
A subnet can be associated with only one route table at a time.
Azure User Defined Routes (UDR) allow you to control the routing of traffic leaving a subnet in an Azure Virtual Network (VNet).
UDRs are used to override Azure’s default system routes, which control traffic between subnets, on-premises networks, and the internet.
UDRs can be used to direct traffic to specific next hops, such as virtual appliances, network virtual appliances (NVAs), or virtual machines (VMs).
UDRs are associated with subnets within a VNet and are evaluated in priority order to determine the routing of outbound traffic.
UDRs can be created, modified, and deleted using the Azure portal, Azure PowerShell, Azure CLI, or Azure Resource Manager (ARM) templates.
UDRs are commonly used in scenarios where you need to route traffic through specific network devices, apply network security policies, or optimize traffic flow within your Azure environment.
UDRs can be used in conjunction with Azure Virtual Network Gateways, Azure ExpressRoute, Azure VPN Gateway, and other networking services to control the flow of traffic in and out of your Azure Virtual Network.