Exchange DAG / What happen if network switch goes down?


I have a question regarding an exchange 2010 or exchange 2013 dag.
For easier understand I will describe the scenario in easy way.
We have two hardware nodes which containg two vms.
We have two switches (one switch for internal network, and one switch for external network)

Hardware-Node 1:
BackendServer 1 (Part of DAG)
FrontEndServer 1

Hardware Node 2:
BackendServer 2 (Part of DAG)
FrontEndServer 2

Both hardware-nodes have same internal network switch.

What happens if internal switch goes down?
In this szenario both Server think they are master, because they can not communicate to second server.
Both servers will still providing fully exchange functionality to the clients.

But with this we have two activate databases?

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Naushad shaikhIT Consultant - Messaging SupportCommented:
i have same Q

1.what about your witness server where you placed it? both DAG server have  two NIC , is one NIC  dedicated  for DAG replication and heart bit check . another for serving exchange services.
3. IS both server are in same  subnet .
uhscaleAuthor Commented:

thanks for your reply.
1. We have no witness server.
2. both DAG servers have two nics, but both nics are on the same switch
3. yes both servers are in the same subnet
uhscaleAuthor Commented:

I have to correct my answer.
Our witness server is on FrondEnd 1, so it is on the same switch.
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Naushad shaikhIT Consultant - Messaging SupportCommented:
first if you lost swtich for internal communication Exchange will down as there is no contact with DC and GC

 Second  let think it is not releated with AD and GC and we going to talk about only exchange

Two member DAG will lost quorum when one of the mailbox servers goes offline so a File Share Witness need to be  deploy in exchange 2010 as it not support dynamic Quoram.

let there is not Quoram issue than also  Split Brain condition will get created if both server loss conecction
uhscaleAuthor Commented:

I want to deploy a 2 server dag.
So we have

Hardware Node 1:

Hardware Node 2:

So if for example Hardware-Node 1 goes down, then hole exchange server is down, because witness server is installed on Frontend-VM-1

I read something about alternate-witness-server. How this can help us here?

Jamie McKillopIT ManagerCommented:

You should have your witness server on a third piece of hardware. it just needs to be a file share so you can put it on any server.

The alternate witness server is for data center switch overs and is not meant as HA for the witness role.

Configuring Exchange for high availability requires consideration of the cluster networking setup, ideally with two independent network paths between the member nodes. In your configuration, if the switch goes down, only the DAG member node which has access to both a DC and the witness server can continue to function. In your configuration, the switch becomes a single point of failure and you would need to have a path though another switch to give redundancy.

Agree that alternate witness server does not help with your hypothetical example.

Witness server can be any server except it's something of a pain to set up on a DC. For it to be an independent witness, you need to have it on a different hardware node than the DAG nodes and the concerns about redundant network paths apply to it as well.

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uhscaleAuthor Commented:

thanks for your replies. It is clear now. I know what I have to do.
Thank you very much.
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