Configuring an Exchange 2007 CCR Based Mailbox Server
This has been forwarded to me by Steve, it is an excellent resource if you need to set up a CCR based mailbox server
In part two it discusses majority node set (MNS) and File share witness
More on this is here but in brief the following taken from the article sums it up quite nicely
"Consider a two-node MNS quorum cluster. Because a MNS quorum cluster can only run the majority of the cluster nodes are available, a two-node MNS quorum cluster is unable to sustain the failure of any cluster node. This is because the majority of a two-node cluster is two. To sustain the failure of any one node in an MNS quorum cluster, you must have at least three devices that can be considered as available. The file share witness feature enables you to use an external file share as a witness. This witness acts as the third available device in a two-node MNS quorum cluster. Therefore, with this feature enabled, a two-node MNS quorum cluster can sustain the failure of a single cluster node. Additionally, the file share witness feature provides the following two functionalities:
• It helps protect the cluster against a problem that is known as a split brain. This problem occurs if the two nodes in a MNS quorum cluster cannot communicate with each other. In this situation, each cluster node is unable to determine whether the loss of communication occurred because the other cluster node failed, or whether the loss of communication occurred because of a problem with the network. The file share witness can designate one of the cluster nodes as the surviving cluster node. That cluster node can then determine that it should continue to run the cluster. In this scenario, the surviving cluster node can determine that the other cluster node failed, or that the other cluster node was not sanctioned by the file share witness.
•It helps protect the cluster against a problem that is known as a partition in time. This problem occurs if the following conditions are true:
•Cluster node A is running, but cluster node B is not running.
•Cluster node A stops running.
•Cluster node B tries to run the cluster.
In this situation, cluster node B may not have cluster state information that was updated on cluster node A. Therefore, cluster node B may run the cluster by using incorrect state information. The file share witness feature helps prevent this problem by detecting that the cluster state has changed. The file share witness feature prevents the cluster node that contains outdated cluster state information from running the cluster