Troubleshoot Azure Load Balancer
This page provides troubleshooting information for Basic and Standard common Azure Load Balancer questions. For more information about Standard Load Balancer, see Standard Load Balancer overview.
When the Load Balancer connectivity is unavailable, the most common symptoms are as follows:
- VMs behind the Load Balancer aren't responding to health probes
- VMs behind the Load Balancer aren't responding to the traffic on the configured port
When the external clients to the backend VMs go through the load balancer, the IP address of the clients will be used for the communication. Make sure the IP address of the clients are added into the NSG allow list.
No outbound connectivity from Standard internal Load Balancers (ILB)
Validation and resolution
Standard ILBs are secure by default. Basic ILBs allowed connecting to the internet via a hidden Public IP address called the default outbound access IP. This isn't recommended for production workloads as the IP address is neither static nor locked down via NSGs that you own. If you recently moved from a Basic ILB to a Standard ILB, you should create a Public IP explicitly via Outbound only configuration, which locks down the IP via NSGs. You can also use a NAT Gateway on your subnet. NAT Gateway is the recommended solution for outbound.
Can't change backend port for existing LB rule of a load balancer that has virtual machine scale set deployed in the backend pool.
Cause: The backend port cannot be modified for a load balancing rule that's used by a health probe for load balancer referenced by virtual machine scale set
Resolution In order to change the port, you can remove the health probe by updating the virtual machine scale set, update the port and then configure the health probe again.
Small traffic is still going through load balancer after removing VMs from backend pool of the load balancer.
Cause: VMs removed from backend pool should no longer receive traffic. The small amount of network traffic could be related to storage, DNS, and other functions within Azure.
To verify, you can conduct a network trace. The FQDN used for your blob storage accounts are listed within the properties of each storage account. From a virtual machine within your Azure subscription, you can perform nslookup to determine the Azure IP assigned to that storage account.
Additional network captures
If you decide to open a support case, collect the following information for a quicker resolution. Choose a single backend VM to perform the following tests:
- Use ps ping from one of the backend VMs within the VNet to test the probe port response (example: ps ping 10.0.0.4:3389) and record results.
- If no response is received in these ping tests, run a simultaneous Netsh trace on the backend VM and the VNet test VM while you run PsPing then stop the Netsh trace.
Load Balancer in failed state
- Once you identify the resource that is in a failed state, go to Azure Resource Explorer and identify the resource in this state.
- Update the toggle on the right-hand top corner to Read/Write.
- Click on Edit for the resource in failed state.
- Click on PUT followed by GET to ensure the provisioning state was updated to Succeeded.
- You can then proceed with other actions as the resource is out of failed state.
If the preceding steps don't resolve the issue, open a support ticket.
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