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Exchange 2010 DAG x2 only with DNS

Just want to know if this will work. going to set up a DAG on 2nd exchange server so databases are available but did a full install on 2nd exchange with client access roles.

So i grasp the fact that i have backup of the database and thats good but if 1st exchange server goes down would clients be able to access exchange2 ? with a reboot or something ?

anyone explain the client access on exchange 2010

2 Solutions

For the 2nd Exchange server to be part of DAG, you should install Mailbox Server role. DAG is used for Database Availability where you will be creating multiple copies of a Database in different Exchange 2010 Mailbox Servers. One of the copies will be active at any instant of time and other copies will be passive. If there is any error on the server where active copy is there and database is inaccessible, then passive copy will be active automatically. DAG uses Fail Over Clustering in the background. After the DB Failover, client machines will get the updated location of mailbox thru AutoDiscover service.

Following is a good statup document for DAG.


All client accesses (Outlook, OWA, mobile/Activesync, Pop, Imap) require a Client Access Server (CAS) Role. If on your 2 exchange servers, you only have one CAS role (and 2 mbx in a DAG) then if the machine with the CAS role crashes then nobody could access to the data in their mailbox in the DAG.
DAG is a clustering technology only for the data storage in Exchange, and not a fault tolerant technology to clients access.
You have to make a "cluster" of CAS role in your Exchange infra to achieve global high avalaibility. Clustering for CAS role relies on another technology than Mailbox/DAG, you need "Load balancing" techno on the network to do  this.
Bruno PACIIT ConsultantCommented:

I just wanted to give a precision about what have been said here: the Autodiscover process is not involved in the high availability of the CAS role.
Autodiscover process won't help clients to failover on another CAS server in case of failure of the first one !

Clients only use Autodiscover to locate a CAS server at startup, and this is just to interrogate the CAS about which database contains their mailbox.
After that, the database contains an attribute (RpcClientAccessServer) that contains the name of the CAS server or CAS array that must be reached to conenct to the mailbox. The client read this attribute and tries to reach this CAS server name or CAS Array name.
If the server under the RpcClientAccessServer is not alive the client will fail to reach the mailbox. Autodiscover won't help about that.

As said before, the CAS role should be High Available, and that can be done by the way of Hardware Load Balancer, or by using Windows NLB but is this last case the CAS role must be installed on dedicated servers (not on the mailbox servers).
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elite74Author Commented:
Thanks  PaciB

I Found this link im going to try walk through. CentOS NLB with Exchange 2010

Im hopeing its going to work. anyone had much experience using Linux for Network load balancing ?

you definetly need to have 2 exchange servers to form a CAS array and + 2 for DAG, you cant do 2 2008 R2 WNLB and 2 exchange CAS + DAG Member servers ?
DAG and WNLB will not co-exist in the same server. DAG uses Windows Failover clustering and WNLB is also a form of clustering, so both cannot co-exist on the same server.

You can create a CAS Array with two servers running Client Access Server Role in Exchange 2010. Same servers you can configure WNLB for CAS Load Balancing.

For DAG you will require two servers running Mailbox Server Role in Exchange 2010.

So in your case, if you are running mailbox role and cas role on the both the servers, it will not be possible to use WNLB. You may try to use another load balanacing solution and configure its IP Address as CAS Array IP Address.
Simon Butler (Sembee)ConsultantCommented:
If you want to do this with just two servers, you can do it with DNS only. You will just need to update the CAS Array DNS entry manually. That may well be enough for you.
The key thing is having the CAS Array in place before you start and that all clients are using it.
There is no need for a load balancer unless you want automatic failover of the clients.

If you want a load balancing solution you could also look at Zen, which is open source and available as an appliance.

Bruno PACIIT ConsultantCommented:

WNLB can only load balance local IP services. It can not be used to load balance services on remote servers.
That's why WNLB and CAS role must be installed on the same serve, and that's why CAS and MBX roles need to be separated if you wnat to make a full Microsoft HA solution.

If you use external load balancing solution, like hardware appliances or any load balancing software that is able to load balance remote services you can use only 2 Exchange servers that host all roles and are member of a DAG. The external load balancer will load balance the CAS role.

As Sembee2 said, if you don't need immediate and full automatic failover, if you accept to have an administrative action to do in case of failure before the clients can reach their mailbox, you can avoid to install Load Balancing.
You can create a CAS Array object in your Exchange organization and create a DNS alias for the CAS Array FQDN that points to the IP of one CAS. All clients will use this CAS to reach their mailbox. In case of CAS failure you'll have to modify the DNS alias so it points to the surviving CAS.

Have a good day

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