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If I am going to set an Alwayson Availability group with 3 nodes, with read-only routing. Do I need to configure a Quorum ?

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You don't have a choice as to whether or not to have quorum: Always On Availability Groups are built on top of Windows Server Failover Clustering, which requires quorum voting to understand who the primary is for a given AG at a given time.

I think what you might be asking is whether or not you need an old-school quorum drive (like a Q drive on a SAN). These days, Windows Server Failover Clustering (WSFC) can either use a quorum drive witness, OR a file share witness.

Windows Server 2012 R2 includes dynamic witness capabilities: if you have an even number of voters (like if 1 of your 3 cluster nodes goes down), then Windows will automatically add in your witness in order to be a tiebreaker. That's why you want to configure the witness (either a disk or a file share) ahead of time - Windows will automatically manage its voting rights whenever you have node failures or network failures.

I hate telling people to read the manual, but in this case, Books Online's quorum section is actually fantastic. Just make sure to read the appropriate one for your version of Windows, because Win2008R2, 2012, and 2012R2 all have different quorum options.

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You need to make certain that the quorum is set up according to your needs. In most cases when you have an odd number of nodes node majority is the best way to go so make certain that your quorum is set up accordingly.

In a 3 node cluster you can setup node majority as your quorum mode and then you can survive the failure of a single node within the cluster . That is in most cases sufficient. If your cluster spans multiple sites you might want to fiddle with the quorum-vote weight depending on your failover plans.

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