Once a licensing model has been decided it is then possible to start designing and deploying an RDS infrastructure, which will include at least one RD Licensing Server. As these are typically lightweight roles it is not uncommon to virtualise the licensing server; though it is recommended to install this as a dedicated role. In-line with best practise guidelines, administrators should ensure at least two license servers are available to avoid single point of failure; though this is only truly impactful to end users in Per Device mode. (In Per User, you will see event log errors – but connections will still be permitted).
The type of licensing mode chosen will also have an impact on how best to install CALs across RD License Servers. For Per Device mode, it is recommended that CALs are split 50:50 across each License Server, whereas for Per User it may be beneficial for accurate CAL tracking to install a majority (if not all) licenses on the primary RD License Server. As Per User CALs are tracked but not strictly enforced, an RDSH server will not defer to a secondary RD License Server if the primary listed server has no remaining CALs. Instead, any reporting will show an over-allocation of licenses – and the customers need to ensure that they have enough CALs. As Per Device CALs are enforced however, it is recommended to split CALs 50:50 so that if a primary server does run out of CALs, the next specified server within RDSH Configuration Manager will be contacted for a valid CAL in order to allow a connection.
The list of RD License Servers to use is commonly configured within the RDSH Configuration Manager, whereby the servers are contacted top-down in the order specified. This is written to the registry of the RDS Host under HKLMSYSTEMCurrentControlSetServicesTermServiceParametersLicenseServers, though best practise guidelines recommend configuring all licensing settings including license servers and licensing mode via Group Policy to ensure complete consistency across the RDS infrastructure. For the perfectionists amongst you, allocating odd and even number servers to separate OUs, with reversed preferences set for the list of RD License Servers in each will ensure an even allocation of CALs from each server resulting in neater reports and easier tracking.
Whilst this behaviour may seem strange to some, it is the direct result of the deprecation of a feature known as CAL forwarding in previous versions of Windows Server Terminal Services that leveraged an auto-discovery mechanism to locate known license servers using an LDAP query in Active Directory. Over time, this auto-discovery mechanism often proved a high generator of support calls and a bug bare for many customers troubleshooting licensing issues and as such, has now been deprecated in favour of a specified list that an RDSH server pings directly.