As of Houdini 19.0, redundant license server is now supported.
For more information on how to configure a redundant server, please click here

In Houdini versions prior to 19.0, there is no built-in redundancy configuration.
If you have a virtual machine (vm) for the license server, you can create a clone. The clone can be then taken offline and spun up in the event of the main server failure.