Which RDS CAL Is Compatible with Your Windows Server?

Which RDS CAL Do You Need? Compatibility Guide for Windows Server

Navigating Microsoft licensing can be tricky. Understanding Windows Server Remote Desktop Services Client Access Licenses (RDS CALs) helps ensure compliance and avoid disruption.

Remote Desktop Services (RDS) allow users or devices to remotely connect to a Windows Server to access desktops or applications. To do this legally and without disruption, organizations must license each user or device with a Remote Desktop Services Client Access License (RDS CAL).

Common customer questions:

• Can I use an RDS 2016 CAL with Windows Server 2022?
• Why are my licenses not being issued even though they’re installed?

This article explains how RDS CAL version compatibility works, why your setup might encounter Windows Server CAL errors, and provides CAL troubleshooting steps.

Understanding RDS CAL Version Compatibility

Microsoft enforces strict Windows Server licensing rules about which RDS CAL versions can be used with specific Windows Server versions.

Core Compatibility Rules

• RDS CALs are NOT forward-compatible: You cannot use an older RDS CAL (e.g., 2016) to access a newer Windows Server (e.g., 2019 or 2022).
• RDS CALs are backward-compatible: You can use newer RDS CALs (e.g., 2022 or 2025) with older Windows Server versions.

For example, to connect to a Windows Server 2022 RD session host, you need an RDS 2022 CAL or newer. That same RDS 2022 CAL can also be used to access Windows Server 2016 or 2019 session hosts.

The following table shows which RDS CAL and RD session host versions are compatible with each other.

RD Session Host Versions Compatibility Table

RD Session Host Version | RDS 2016 CAL | RDS 2019 CAL | RDS 2022 CAL | RDS 2025 CAL
------------------------|--------------|--------------|--------------|--------------
Windows Server 2016     | ✅           | ✅           | ✅           | ✅
Windows Server 2019     | ❌           | ✅           | ✅           | ✅
Windows Server 2022     | ❌           | ❌           | ✅           | ✅
Windows Server 2025     | ❌           | ❌           | ❌           | ✅
    

You must install your RDS CALs on a compatible RD license server. An RD license server can host licenses for its version and all earlier versions of Remote Desktop Services—but not newer ones.

For example, a Windows Server 2022 RD license server can manage RDS CALs for 2022 and all prior versions, while a Windows Server 2016 license server can only manage CALs up to 2016.

The following table shows which RDS CAL and license server versions are compatible with each other.

RD License Server Versions Compatibility Table

RD License Server Version | RDS 2016 CAL | RDS 2019 CAL | RDS 2022 CAL | RDS 2025 CAL
--------------------------|--------------|--------------|--------------|--------------
Windows Server 2016       | ✅           | ❌           | ❌           | ❌
Windows Server 2019       | ✅           | ✅           | ❌           | ❌
Windows Server 2022       | ✅           | ✅           | ✅           | ❌
Windows Server 2025       | ✅           | ✅           | ✅           | ✅
    

What is an RDS CAL?

RDS CALs are licenses that grant users or devices the legal right to connect to an RDS deployment (a server hosting remote desktop sessions). There are two types:

• Per User CALs – tied to a specific individual.
• Per Device CALs – tied to a single device, regardless of the user.

These CALs must be installed on an RD Licensing Server within your environment.

Read more about 'What is RDS - Everything you need to know about Remote Desktop Services'

Why Microsoft Enforces This

Microsoft’s policy ensures that CALs align with server feature sets, security models, and APIs introduced in each Windows Server version. Older CALs aren’t built to handle newer session hosts, leading to:

• License denial
• Session failures
• Legal non-compliance

What Is the RD Licensing Server?

The RD Licensing Server is the component that:

• Holds installed Remote Desktop Services CALs
• Issues them during the first RDP connection
• Tracks usage and license consumption

You can install the RD Licensing Server separately from the RD Session Host.

Real-World Scenario

Scenario: Licenses installed, but users are denied access
Likely Cause: Installed CALs are incompatible with the session host.
Fix: Check the RDS license compatibility chart. Upgrade CALs as necessary.

How to Troubleshoot Common RDS CAL Issues

If you’ve installed CALs but face licensing issues:

1. Validate CAL Version – Open the RD Licensing Manager and confirm that the installed CAL version matches your Windows Server version.
2. Check RD License Server Configuration:
   • Open Server Manager → Remote Desktop Services → Overview
   • Edit Deployment Properties
   • Ensure the correct license server is listed
   • PowerShell: Get-WmiObject -Namespace "Root\\CIMV2\\TerminalServices" -Class Win32_TSLicenseServer
3. Configure Licensing Mode in Group Policy:
   • Run gpedit.msc
   • Navigate to: Computer Configuration → Administrative Templates → Windows Components → Remote Desktop Services → RD Session Host → Licensing
   • Enable:
     • “Use specified Remote Desktop license servers” → Enter your license server’s FQDN or IP
     • “Set the Remote Desktop licensing mode” → Choose either “Per User” or “Per Device”
   • Run gpupdate /force and restart the server
4. Check Firewall & Port Access – Ensure TCP ports 135, 445, 3389 are open for RPC communication.
5. Clear License Cache on Client – Delete: HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\MSLicensing and restart.
6. Use RD Licensing Diagnoser from Server Manager for detailed license issue information.

If You Bought the Wrong CALs

Purchased an incompatible RDS CAL version? At Brytesoft, we’ve got your back:

• Purchase the correct version from our site
• Email our support team with:
  • Your new order number
  • Your old (mistaken) order number
• We’ll process a refund for the wrong purchase after verifying

Final Recommendations

• Verify your Windows Server licensing requirements before buying RDS CALs
• Prefer newer CAL versions for maximum compatibility
• Document your CAL purchases clearly
• Clarify with your vendor before installing

Need Help?

Facing ongoing RDS CAL setup or licensing errors? Contact us with:

• Your Windows Server version
• Type and version of CAL purchased
• Error messages and screenshots (if applicable)