Diagnose a virtual machine network routing problem using the Azure CLI

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10/29/2024

In this article, you learn how to use Azure Network Watcher next hop tool to troubleshoot and diagnose a VM routing problem that's preventing it from correctly communicating with other resources.

Prerequisites

An Azure account with an active subscription. Create an account for free.
Azure Cloud Shell or Azure CLI.

The steps in this article run the Azure CLI commands interactively in Azure Cloud Shell. To run the commands in the Cloud Shell, select Open Cloud Shell at the upper-right corner of a code block. Select Copy to copy the code, and paste it into Cloud Shell to run it. You can also run the Cloud Shell from within the Azure portal.

You can also install Azure CLI locally to run the commands. This article requires the Azure CLI version 2.0 or later. Run az --version command to find the installed version. If you run Azure CLI locally, sign in to Azure using the az login command.

Create a virtual machine

Before you can create a VM, you must create a resource group to contain the VM. Create a resource group with az group create. The following example creates a resource group named myResourceGroup in the eastus location:

az group create --name myResourceGroup --location eastus

Create a VM with az vm create. If SSH keys do not already exist in a default key location, the command creates them. To use a specific set of keys, use the --ssh-key-value option. The following example creates a VM named myVm:

az vm create \
  --resource-group myResourceGroup \
  --name myVm \
  --image Ubuntu2204 \
  --generate-ssh-keys

The VM takes a few minutes to create. Don't continue with remaining steps until the VM is created and the Azure CLI returns output.

Test network communication

To test network communication with Network Watcher, you must first enable a network watcher in the region the VM that you want to test is in, and then use Network Watcher's next hop capability to test communication.

Enable network watcher

If you already have a network watcher enabled in the East US region, skip to Use next hop. Use the az network watcher configure command to create a network watcher in the East US region:

az network watcher configure \
  --resource-group NetworkWatcherRG \
  --locations eastus \
  --enabled

Use next hop

Azure automatically creates routes to default destinations. You may create custom routes that override the default routes. Sometimes, custom routes can cause communication to fail. To test routing from a VM, use az network watcher show-next-hop to determine the next routing hop when traffic is destined for a specific address.

Test outbound communication from the VM to one of the IP addresses for www.bing.com:

az network watcher show-next-hop \
  --dest-ip 13.107.21.200 \
  --resource-group myResourceGroup \
  --source-ip 10.0.0.4 \
  --vm myVm \
  --nic myVmVMNic \
  --out table

After a few seconds, the output informs you that the nextHopType is Internet, and that the routeTableId is System Route. This result lets you know that there is a valid route to the destination.

Test outbound communication from the VM to 172.31.0.100:

az network watcher show-next-hop \
  --dest-ip 172.31.0.100 \
  --resource-group myResourceGroup \
  --source-ip 10.0.0.4 \
  --vm myVm \
  --nic myVmVMNic \
  --out table

The output returned informs you that None is the nextHopType, and that the routeTableId is also System Route. This result lets you know that, while there is a valid system route to the destination, there is no next hop to route the traffic to the destination.

View details of a route

To analyze routing further, review the effective routes for the network interface with the az network nic show-effective-route-table command:

az network nic show-effective-route-table \
  --resource-group myResourceGroup \
  --name myVmVMNic

The following text is included in the returned output:

{
  "additionalProperties": {
    "disableBgpRoutePropagation": false
  },
  "addressPrefix": [
    "0.0.0.0/0"
  ],
  "name": null,
  "nextHopIpAddress": [],
  "nextHopType": "Internet",
  "source": "Default",
  "state": "Active"
},

When you used the az network watcher show-next-hop command to test outbound communication to 13.107.21.200 in Use next hop, the route with the addressPrefix 0.0.0.0/0** was used to route traffic to the address, since no other route in the output includes the address. By default, all addresses not specified within the address prefix of another route are routed to the internet.

When you used the az network watcher show-next-hop command to test outbound communication to 172.31.0.100 however, the result informed you that there was no next hop type. In the returned output you also see the following text:

{
  "additionalProperties": {
    "disableBgpRoutePropagation": false
      },
  "addressPrefix": [
    "172.16.0.0/12"
  ],
  "name": null,
  "nextHopIpAddress": [],
  "nextHopType": "None",
  "source": "Default",
  "state": "Active"
},

As you can see in the output from the az network watcher nic show-effective-route-table command, though there is a default route to the 172.16.0.0/12 prefix, which includes the 172.31.0.100 address, the nextHopType is None. Azure creates a default route to 172.16.0.0/12, but doesn't specify a next hop type until there is a reason to. If, for example, you added the 172.16.0.0/12 address range to the address space of the virtual network, Azure changes the nextHopType to Virtual network for the route. A check would then show Virtual network as the nextHopType.

Clean up resources

When no longer needed, you can use az group delete to remove the resource group and all of the resources it contains:

az group delete --name myResourceGroup --yes

Next steps

In this article, you created a VM and diagnosed network routing from the VM. You learned that Azure creates several default routes and tested routing to two different destinations. Learn more about routing in Azure and how to create custom routes.

For outbound VM connections, you can also determine the latency and allowed and denied network traffic between the VM and an endpoint using Network Watcher's connection troubleshoot capability. You can monitor communication between a VM and an endpoint, such as an IP address or URL over time using the Network Watcher connection monitor capability. For more information, see Monitor a network connection.

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