In a Microsoft Lync Server 2010 deployment, it was almost a foregone conclusion to use two front-end servers. With the introduction of User Groups and the Brick Model architecture in Lync Server 2013, you'll often hear that the number of front-end servers to deploy is three. To some degree, this number is related to the Brick Model architecture, in which user groups are assigned to primary, secondary, and tertiary servers in a Lync 2013 pool for an enterprise deployment of front-end servers. However, it's not necessarily a good number. Let's dig a little deeper to see why three front-end servers might not be adequate and how to determine the correct number in a Lync Server 2013 Enterprise Edition pool deployment.
In a Lync 2013 front-end server pool, the maximum number of supported users is 80,000, with a limit of 10 front-end servers in the pool. Lync 2013 supports 6,660 users per front-end server, which is lower than the 10,000 supported users per front-end server in Lync 2010. In Lync 2013, if a front-end server in a pool goes down, the users homed on the downed server would be transferred to the remaining active front-end servers in the pool.
Let's say that your organization has 25,000 users and all are located in a single Lync 2013 pool. Because Lync 2013 supports 6,660 users per front-end server, you deploy four of them, which means you have 6,250 (25,000/4) users per server. Suppose that one of the four front-end servers goes down and is unable to be restored for specified period of time. The 6,250 users who were homed on the failed server are now spread across the remaining active front-end servers in the same pool. That comes to approximately 2,083 users added to each of the three remaining front-end servers. Now each active server has 8,333 users, which is well over the supported limit of 6,660 users.
So, how do you avoid this problem? You simply add an additional server to the pool, which would bring the count to five front-end servers for the 25,000 users in the organization. With five front-end servers servicing 25,000 users in the single pool, each server would handle 5,000 users. If one of the front-end servers goes down, 1,250 users would be reallocated to each of the remaining four servers. This would bring the total to 6,250 users per active front-end server, which is below the supported limit of 6,660 users.
Take a What-If Approach and Do the Math
When determining the number of Lync 2013 front-end servers to use, you shouldn't go by what you hear. Instead, you should take a what-if approach and do the math. That way, if a front-end server goes down, the remaining front-end servers will be able to handle the user load adequately and not impact the performance of the IM, conferencing, and Enterprise Voice features.