You’ll want to use the same backup software which you would use to backup a standalone server. Most of the large backup software packages support clustered servers.
The method of backup for the cluster can vary widely depending on the software used. For instance Netbackup software for cluster backup is a simple process, however our organization uses IBM Tivoli Storage Manager (TSM) and in that case the cluster backup is quite a different and more complicated process altogether. So if you need specifics please provide the details of what you are asking. Thanks!