They give administrators the flexibility needed to direct traffic based on factors such as source address, port number, or protocol. This is particularly useful in scenarios where the default routing configuration does not meet the business requirements.
Let’s delve a bit deeper into this topic and define what we mean by user-defined routing.
What is User-Defined Routing?
User-Defined Routing or UDR is a feature in Azure that lets you create your own routing tables in your cloud environment, enabling you to dictate the flow of traffic in your network. By default, Azure uses system routes to direct traffic between resources. However, there may be situations where you need more control over network traffic, and configuring UDRs allows such flexibility.
For instance, suppose you want to route traffic through a Network Virtual Appliance (NVA), or maybe you want to implement a hub-and-spoke network topology with shared services in the hub and other resources in separate spokes. A UDR would help in such cases.
Creating A User-Defined Route
To configure a UDR, you would create a Route Table and add routes to it. Each route consists of an address prefix (CIDR notation), the type of next hop, and the specific next hop (if applicable). The following PowerShell command demonstrates how to create a Route Table and add a route to it:
$routeTable = New-AzRouteTable –
-ResourceGroupName “myResourceGroup” –
-Name “myRouteTable” –
-Location “East US”
$route = Add-AzRouteConfig –
-Name “myRoute” –
-AddressPrefix 0.0.0.0/0 –
-NextHopType “VirtualAppliance” –
-NextHopIpAddress “10.0.1.4” –
-RouteTable $routeTable
$routeTable | Set-AzRouteTable
In this example, we’re creating a route table called `myRouteTable` in the `myResourceGroup` resource group located in `East US`. We’re then adding a route named `myRoute` that forwards all traffic (0.0.0.0/0) to an NVA with IP address 10.0.1.4.
Applying The User-Defined Route
Once you’ve created your route table and added your routes, the final step is to apply this routing table to one or more subnet. The following PowerShell command shows how to apply the route table to a subnet:
$virtualNetwork = Get-AzVirtualNetwork -ResourceGroupName “myResourceGroup” -Name “myVnet”
$subnet = Get-AzVirtualNetworkSubnetConfig -Name “mySubnet” -VirtualNetwork $virtualNetwork
$subnet.RouteTable = $routeTable
$virtualNetwork | Set-AzVirtualNetwork
In this case, the route table `myRouteTable` is applied to a subnet named `mySubnet` in a virtual network `myVnet` located in the same resource group `myResourceGroup`.
Conclusion
When it comes to managing network traffic in Azure, User-Defined Routing provides the flexibility needed by many businesses. UDRs allow you to take control over the routing of your network traffic, directing it to specific resources based on your business needs. The ability to control routing can provide advantages in improved network performance, security, and management. Remember to plan your routes carefully to avoid unnecessary complications or performance issues.
Practice Test
True or False: Azure routes traffic between subnets, virtual networks, and on-premise networks by default.
- Answer: True.
Explanation: Azure automatically routes traffic between Azure subnets, Azure virtual networks, and on-premises networks by default using built-in system routes.
What are User-Defined Routes (UDRs) in Azure?
- a) Routes created by users to overwrite system routes.
- b) Network gateways defined by the user.
- c) Virtual machines defined for a network.
- d) Private IP addresses assigned by users.
Answer: a) Routes created by users to overwrite system routes.
Explanation: User-Defined Routes (UDR) are custom routes in Azure that are defined by users to override Azure’s default system routes.
True or False: User-Defined Routes can be associated with multiple subnets.
- Answer: True.
Explanation: A User-Defined Route can be associated with multiple subnets, which allows traffic flowing to those subnets to be controlled using the route.
How many User-Defined Routes can be created in an Azure Route Table?
- a) 50
- b) 100
- c) 400
- d) Unlimited
Answer: c)
Explanation: The maximum limit for User-Defined Routes in an Azure Route Table is
Does Azure allows reserved IP addresses to be used in User-Defined Routes?
- Answer: No.
Explanation: The Azure platform reserves some IP addresses, and these cannot be used in User-Defined Routes.
True or False: Azure automatically assigns system routes in each subnet to ensure that traffic can flow between subnets, Internet and on-premise locations.
- Answer: True.
Explanation: By default, Azure assigns a set of system routes to each subnet in a virtual network to ensure network connectivity.
What is the lowest possible value for route priority in Azure User-Defined Routes?
- a) 1
- b) 0
- c) 100
- d) 10
Answer: b) 0
Explanation: The lowest possible value for route priority in Azure UDRs is Lower values take precedence over higher values.
True or False: Azure allows the creation of both IPv4 and IPv6 User-Defined Routes.
- Answer: True.
Explanation: Azure supports both IPv4 and IPv6 User-Defined Routes helping in managing dual-stack virtual network.
User-Defined Routes can be used to route traffic:
- a) To a Network Virtual Appliance.
- b) Through a desired virtual network gateway.
- c) To a specific network interface.
- d) All of the above.
Answer: d) All of the above.
Explanation: User-Defined Routes in Azure can be used to direct traffic to a Network Virtual Appliance, through a desired virtual network gateway, or to a specific network interface.
Can User-Defined Routes utilize Application Security Groups as source or destination in a rule?
- Answer: No.
Explanation: Currently, Azure User-Defined Routes do not support the use of Application Security Groups as sources or destinations in a rule.
True or false: Default routes (route via 0/0) in Azure always include the Internet.
- Answer: False.
Explanation: Although many default routes access the Internet, not all of them do. For example, on-premises default routes do not include the Internet.
Which Azure feature can let a Network Virtual Appliance function as a router?
- a) User-Defined Routes
- b) Virtual Network Gateways
- c) ExpressRoute
- d) VPN Gateway
Answer: a) User-Defined Routes
Explanation: User-Defined Routes in Azure can be utilized to make a Network Virtual Appliance function like a router by directing all or part of the network traffic to it.
What command is used to create a User-Defined Route in Azure CLI?
- a) az network route-table route create
- b) az network vnet create-route
- c) az create-route network
- d) az network vnet create
Answer: a) az network route-table route create
Explanation: To create a User-Defined Route in Azure CLI, ‘az network route-table route create’ command is used.
True or false: Azure system routes can’t be deleted or deprovisioned.
- Answer: True.
Explanation: Azure system routes are automatically created and can’t be deleted or deprovisioned.
Azure user-defined routes require what mandatory properties?
- a) Name and Route table name
- b) Name, Address prefix, and Next hop type
- c) Name, Address prefix, and Resource group name
- d) Address prefix and Next hop type
Answer: b) Name, Address prefix, and Next hop type
Explanation: Name, Address prefix, and Next hop type are mandatory properties for Azure User-Defined Routes.
Interview Questions
What is the purpose of user-defined routes in Azure?
User-defined routes in Azure are custom routes that network administrators can configure to override Azure’s default system routes, or to create additional routes for traffic between subnets.
How do you create a route table in Azure?
You can create a route table in Azure using the Azure portal, Azure CLI, Azure PowerShell, or an Azure Resource Manager template. In the portal, browse to the “Route tables” service, click “Add,” and fill in the required information.
What is the next hop in Azure user-defined route?
The next hop in Azure user-defined route is the next point to which the network traffic is forwarded. Some possible next hop types are Virtual Network Gateway, Virtual Appliance, and Internet.
How do you associate a subnet with a route table?
You associate a subnet with a route table from the Azure portal. After selecting a route table, go to the “Subnets” tab and click “Associate”, then select the appropriate subnet.
How can one verify that a route is working as expected in Azure?
One can verify that a route is working as expected by using the “Effective Routes” capability in Azure, which allows you to see all routes that apply to a network interface attached to a virtual machine.
Can you delete a route table associated with a subnet in Azure?
Yes, but you have to disassociate the route table from the subnet first.
Can Azure route tables and network security groups be associated with the same subnet?
Yes, both Azure route tables and network security groups can be associated with the same subnet.
Which Azure CLI command helps you to create a route?
The command “az network route-table route create” helps you to create a route.
What is the default priority assigned to User-defined routes in Azure?
User-defined routes in Azure have a default priority of 0, which means that they take precedence over Azure’s default system routes.
Can user-defined routes be used for traffic between two subnets within the same virtual network?
Yes, user-defined routes can be used for traffic between two subnets within the same virtual network.
What type of next hop is required to send network traffic to the internet in a user-defined route?
The type of next hop required to send network traffic to the internet in a user-defined route is typically “Internet”.
Is it possible to change the priority of a user-defined route after it has been created?
Yes, you can adjust the priority of a user-defined route after it has been created by updating the properties of the route.
How can you configure a user-defined route to route traffic through a virtual appliance?
You can configure a user-defined route to route traffic through a virtual appliance by creating a route with the next hop type set to “Virtual Appliance” and then specifying the IP address of the virtual appliance as the next hop address.
Can a single route table be associated with multiple subnets within an Azure virtual network?
Yes, a single route table can be associated with multiple subnets within an Azure virtual network.
In an Azure user-defined route, what does the address prefix represent?
In an Azure user-defined route, the address prefix represents the CIDR notation for the subnet or range of IP addresses to which the route applies.
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