I built my first two-node Exchange 2007 CCR last week and I have to say that I am impressed. In the past I have frowned upon Microsoft Clusters because there was only one data set that all nodes in the cluster must have access to. The MSCS cluster only provided HA in the situation the physical/Windows OS server failed. There was not mechanism for replicating the data, so if there was data corruption having a cluster didn’t buy you anything.
With an Exchange 2007 CCR (Continuous Cluster Replication) cluster in combination with the new MSCS quorum type (Majority Node Set, more on this later) shared storage is no longer required.
Majority Node Set quorum type – With an MNS quorum type each cluster node has its own, locally stored copy of the quorum DB. This type of quorum means that shared storage for the quorum is not a requirement. One downside to a typical MNS quorum is that at least 3 cluster nodes are required. 3 nodes are required in a typical MNS quorum is because there must be a majority of the nodes online before the cluster resources will come online. In order to have a majority there has to be 3, if there were only 2 then there would never be a majority and the cluster would not come online.
Windows 2003 Sp1 with this hotfix http://support.microsoft.com/kb/921181 or Sp2 there is new MNS configuration called “File Share Witness”.
The file share witness feature is an improvement to the current Majority Node Set (MNS) quorum model. This feature lets you use a file share that is external to the cluster as an additional “vote” to determine the status of the cluster in a two-node MNS quorum cluster deployment.
Microsoft best practice is to use the Exchange 2007 Hub Transport server for the “File Share Witness”. To do this simply create a new folder and share it. Make sure the Administrators group and the Cluster service account has full control permissions to this share/folder.
Below is a diagram of an Exchange 2007 CCR two-node cluster using the hub transport server as the file share witness.
Requirements
- Windows 2003 Sp2/Windows 2008 x64 Enterprise
- Exchange 2007 Enterprise
- Two servers with the same amount of RAM and disk space
- Two NICS, one for LAN one for cluster heartbeat
Steps
- Create MSCS choosing quorum type of Majority Node Set.
- Configure MNS to use file share witness on hub transport server by using this command
“cluster res “Majority Node Set” /priv MNSFileShare=\\servername\sharename. - Apply the MNS configuration by moving the cluster to the other node.
- Install Exchange 2007 Enterprise on the first node choosing Custom and then “Active Clustered Mailbox Role”
- When installing on the second node choose “Passive Clustered Mailbox Role”
- Choose Cluster Continuous Replication and file in the name and IP.
Jeremy Waldrop's Blog 
What happens if one of the servers goes down? As the majority of nodes drops below 3 is the exchange service lost?
No, that is what the file share witness is for. It acts as the quorum so that you do not have to have 3 nodes.
Why was a majority model selected? Meaning whats the purpose of having a majority?
I understand (thanks to you) that a majority is required to have access to the services provided but why not just use a standard clustering model and then add a file share witness?
Can CCR support say 4 nodes in a cluster?
Robert
So that you do not need shared storage for the quorum. In a standard cluster there is only one copy of the data that is shared by each node and controlled by SCSI reservations. The cool thing with CCR is that there are two copies of the data. Look at this for more info.
http://technet.microsoft.com/en-us/library/bb124521.aspx