MsExchange Blog Spot Telnet25

October 13, 2010

DAG with two Servers will databases fail over automatically if one server dies?

Filed under: General — telnet25 @ 9:22 pm

This seems to be one of the most asked question and answer will be yes and also no it depends the given setup scenario. I think the main point to understand is the new attribute called “RpcClientAccessServer” and the term Middle tier speaking of MAPI locating user mailbox.

let me try to elaborate in a simple way , now in your scenario you have two server member of let’s say DAG1

  • Ex1= DB1, DB2, DB3, DB4
  • EX1= DB1, DB2, DB3, DB4


Now let’s take a look at your MAPI clients (they are the outlook clients) if I am user A and my mailbox is located on DB1 , how will I get to my mailbox?

In a simple way, I open outlook , my outlook finds out the DNS server configured in the TCP/IP Stack and DNS server finds out what DC (Domain controller to talk too) , DC has bunch of information in regards to my E210 Servers and their configurations , the DAG configuration is also is located in the ADDS database.

My outlook simply is trying to find out which mail server to connect too to get the mails. So outlook says, hey I need to know the mailbox box for user A , which mail server I need to connect too???? The DC will check ADDS database and look for attribute


  • This is where the magic happens , this attribute tells the MAPI client & clients which DB to connect too.

Great, if you have not configured your CAS array yet, the attribute is set to first CAS server introduced within the SMTP domain, so most likely EX1 in this example ( Assuming we installed EX1 first and have not configured CAS Array yet)

Now let’s make assumption of this to connect this information to your question , you have two servers called EX1 and EX2 and you have 4 DB on these servers ( each)

Ex1= DB1, DB2 (Mounted, ACTIVE) ——–DB3,DB4 (Healthy, WAITING TO BE ACTIVE IF NEEDED)



  • If user A mailbox is located on DB3 meaning my outlook needs to connect to DB4 on EX2 to get my mail.
  • Now, I open my outlook
  • Outlook asked the question-What is the value of the attribute
  • RpcClientAccessServer
  • Outlook Gets the Answer  by finding out the value in the attribute called RpcClientAccessServer=EX1
  • EX1 says hey the DB is mounted-ACTIVE on EX2 this time
  • DNS give the IP Address to MAPI to connect too
  • I get connected to EX2

This is the story about “Middle Tier” has been removed from MAILBOX to CAS servers, you no longer connect to your mailbox server directly.

  • Now lets come back to your question, EX1 is hard DOWN !!!!!! crashed what happens ????
  • You go through same process and get same answer which is RpcClientAccessServer=EX1
  • Your Outlook tries to connect to EX1 ( Which has blue screen) won’t happen , you see the Christmas balloon “Outlook trying to retrieve blah blah and desktop start difficult to manage (-: , uppsssss

**** IN reality EX2 is perfectly up and running you cannot get to it**** Because the ADDS is telling you to connect to RpcClientAccessServer=EX1

  • To remedy this now we configured this attribute

I made this up “” call it anything you like this is DNS A record

  • ( IP Address)


  • EX1=
  • EX2= ,  Are you seeing what is going on here??????

  • If EX1 is death, all I need to do is to do to DNS and update the IP Address of existing A record
  • NOW THIS IS EX2 which will make your outlook happy (-:

To make this automated you need load balancer or use poor man method to update DNS (-: your own

Create a Database Availability Group

Oz Casey, Dedeal
MCSE 2003, M+, S+, MCDST
Security+, Project +, Server + (Blog (Blog) (Blog)

Leave a Comment »

No comments yet.

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Create a free website or blog at

%d bloggers like this: