Windows Server 2012 introduced a new quorum model called Failover Clustering Dynamic Quorum, which we can use with Exchange. When using Dynamic Quorum, the cluster dynamically manages the vote assignment to nodes based on the state of each node. When a node shuts down or crashes, it loses its quorum vote. When a node successfully re-joins the cluster, it regains its quorum vote. By dynamically adjusting the assignment of quorum votes, the cluster can increase or decrease the number of quorum votes that are required to keep it running. This enables the cluster to maintain availability during sequential node failures or shutdowns.
With a dynamic quorum, the cluster quorum majority is determined by the set of nodes that are active members of the cluster at any time. This is an important distinction from the cluster quorum in Windows Server 2008 R2 where the quorum majority is fixed, based on the initial cluster configuration.
The advantage this brings, is that it is now possible for a cluster to run even if the number of nodes remaining in the cluster is less than 50%! By dynamically adjusting the quorum majority requirement, the cluster can sustain sequential node shutdowns down to a single node and still keep running. It does not allow the cluster to sustain a simultaneous failure of a majority of voting members though. To continue running, the cluster must always have a quorum majority at the time of a node shutdown or failure.
The following picture shows a DAG still operational even though two out of three servers are offline:
To read more about this feature, including tests with an Exchange 2013 DAG, please check my Exchange 2013 DAGwith Dynamic Quorum article on MSExchange.org.