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Consider Exchange Server High Availability

Why would you consider implementing Exchange Server High Availability (HA)? One of the answers is pretty simple, the Exchange Server needs to stay up and running. In this article, you will have an understanding of the Exchange Server HA. Did you know that High Availability is one of the most used terms in the IT world? It’s all about staying online.

Why choose for High Availability in Exchange?

There are two High Availability configurations in Exchange Server. One of them is to have the Exchange Server in HA. The other is having the mailbox databases in HA. I recommend configuring both.

Note: You can’t configure HA when you don’t have more than one Exchange Server.

For example, you have an Exchange Server running. The organization needs to send and receive emails 24/7. Ask yourself the following questions:

  • What if the Exchange Server will crash?
  • What if you are planning maintenance on the Exchange Server?

If you don’t have HA configured and the Exchange Server crashes, the email service will stop working. It means that the organization can’t send or receive emails. That’s when High Availability comes into play. When a server goes offline, there will be no downtime. Another Exchange Server will take it all from there.

Exchange Servers High Availability

You can configure High Availability with round-robin DNS or with a load balancer.

Exchange Database Availability Group (DAG)

You can have the mailbox databases in High Availability. You will make use of the Database Availability Group. Read more on how to create a DAG in Exchange Server.

Percentage High Availability

What does it mean when a company grants you a 99,99% percent uptime guarantee? It means that the downtime is 52.60 minutes a year and 4.38 minutes a month.

It will help if you have a good understanding of the HA in percentages. A great list that you can use for uptime guarantees.

Availability %Downtime per yearDowntime per monthDowntime per weekDowntime per day
90% (“one nine”)36.53 days73.05 hours16.80 hours2.40 hours
95% (“one and a half nines”)18.26 days36.53 hours8.40 hours1.20 hours
97%10.96 days21.92 hours5.04 hours43.20 minutes
98%7.31 days14.61 hours3.36 hours28.80 minutes
99% (“two nines”)3.65 days7.31 hours1.68 hours14.40 minutes
99.5% (“two and a half nines”)1.83 days3.65 hours50.40 minutes7.20 minutes
99.8%17.53 hours87.66 minutes20.16 minutes2.88 minutes
99.9% (“three nines”)8.77 hours43.83 minutes10.08 minutes1.44 minutes
99.95% (“three and a half nines”)4.38 hours21.92 minutes5.04 minutes43.20 seconds
99.99% (“four nines”)52.60 minutes4.38 minutes1.01 minutes8.64 seconds
99.995% (“four and a half nines”)26.30 minutes2.19 minutes30.24 seconds4.32 seconds
99.999% (“five nines”)5.26 minutes26.30 seconds6.05 seconds864.00 milliseconds
99.9999% (“six nines”)31.56 seconds2.63 seconds604.80 milliseconds86.40 milliseconds
99.99999% (“seven nines”)3.16 seconds262.98 milliseconds60.48 milliseconds8.64 milliseconds
99.999999% (“eight nines”)315.58 milliseconds26.30 milliseconds6.05 milliseconds864.00 microseconds
99.9999999% (“nine nines”)31.56 milliseconds2.63 milliseconds604.80 microseconds86.40 microseconds

Read more: Course Exchange Server high availability »


You learned about Exchange Server High Availability. HA is not something that you only configure on the Exchange Server level. You can apply it to other IT-related hardware and software configurations.

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ALI TAJRAN is a passionate IT Architect, IT Consultant, and Microsoft Certified Trainer. He started Information Technology at a very young age, and his goal is to teach and inspire others. Read more »

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