This is the use case for the alternate witness server. From: [email protected] [mailto:[email protected]] On Behalf Of ccollins9 Sent: Sunday, October 26, 2014 5:15 AM To: exchange Subject: Re: [Exchange] Isolated DAG member still allow local email???
You are correct in terms of quorum. With your scenario of 4 servers in a single DAG, a witness server is required. Even if you place the witness in Site B, and Site B gets isolated from the rest of the sites, Site B would only have 2 votes out of the necessary 3 needed to establish quorum. This doesn't even take into account DAC mode, which you most definitely should be running when DAGs span multiple physical datacenters. DAC mode prevents split-brain syndrome and if you haven't read up on it, you should. The most efficient way to resolve this would be with a 2 DAG scenario like you already may have. If your DR site is exactly that, DR, then you should have: DAG1, member servers: Site A, DR site. This has an odd numbered DAG and needs no witness server DAG2, member servers: Site B, DR site. The is an even numbered DAG and the witness server should be placed in Site B. That way if site B is isolated it has the witness server there to establish quorum Even with 2 DAGs, the DR site would not have quorum if Site A and Site B went offline. Microsoft resolves this by the use of DAC mode. I think last time I tested this scenario, I had to establish a witness server in the DR site before I could bring the DAG online there. On Fri, Oct 24, 2014 at 2:35 PM, Maglinger, Paul <[email protected]<mailto:[email protected]>> wrote: Because of various problems - up to and including the issue I'm working on with the certificate pop-up - it has been proposed to simplify the Exchange 2010 environment. Currently consisting of 2 DAGs - we're looking to consolidate into one. In the attached drawing MB1 through MB4 would be one DAG. Site 1 and 2 have users, Site 3 is DR only. All 3 sites connected by MPLS. We would like, in worst case scenario, if Site 2 was completely disconnected from the others, that the users there would be able to send local email to each other. We were told by a vendor that would happen as laid out in this diagram and, that once connectivity was restored the email would sync back up to the other servers. I always understood that in DAGs with even-numbered members that we would need a witness server. And, if a DAG member couldn't see the witness or form a quorum, that the Information Store service wouldn't even come up on the server. I'm I right? This wouldn't work as stated? Looking for a checksum... -Paul
