Clustering has three types :
1- Local continous Replication.
you can say that you keep another copy of your Mailboxes Databases on another Partition on
Same Hard Disk.
2- Cluster Continous Replication.
You have another Exchange Server and you can use it for clustering like the Primary Dc and
Additional Dc, But here one of them is Active the other is Passive.
3- Single Copy Cluster.
like the previous case but you store the MailBoxes Database on an intermediate Storage, like San Storage.
And one of the 2 Exchangers have the full privilege on the Database and they send Heart beats to each
Other to ensure that they are working well.