Ed Brill

SearchExchange: Exchange Admin 101: An introduction to Exchange clustering

Why Lotus Domino's application layer shared-nothing active/active clustering model is such a huge competitive differentiator...

Microsoft advises staying away from active/active clusters though, and imposes some serious limitations on them in order to avoid potential disaster -- and with good reason.

For starters, an active/active cluster is limited to two nodes. If you want more than two nodes, you will have to use the active/passive configuration. If you are running an active/active cluster, you must never have more than 1,900 simultaneous connections to a node, and the individual CPUs must never exceed 40% capacity for more than 10 minutes at a time. Otherwise, bad things can happen.

In contrast, let's review:
Lotus Domino can have a six-way active-active cluster, across operating systems, versions of Domino, physical locations, and without a single point of failure.  
Link: SearchExchange: Exchange Admin 101: An introduction to Exchange clustering >