We currently have 2 Exchange 2013 servers with all 3 roles installed. We have four dags with 2 live databases on one server and two on the other. We also have a hardware network load balancer which our external dns and mx records point too. We now want to add a third Exchange 2013 server to the organisation for DR purposes. The server will be located in a different site and have a fast, low latency link. We want the third server to provide business continuity should we ever lose the 2 Exchange servers in our main office. We would like to automate this dr process where possible. As the nlb device is located in our primary office (which would probably be lost in the event of a disaster) we would have to manually change dns records to point to the third server.

- There is around 1tb of mailbox databases on the 2 servers. Should we setup the 3rd servers in our main office so the initial dag replication occurs over the lan rather than across the wan? if so what changes would we need to make to the 3rd server/exchange once its moved to the other site (obviously its ip settings as it will be moved to another subnet).

- We need to schedule or at least throttle the dag replication traffic to the 3rd server during working hours. what is the best way to do this?

- Any tips or advise on implementing the plan above.

Thanks in advance.

I think you mean you have 1 DAG with 4 databases yes?

With a fast, low latency link, why the need to setup the 3rd server in the main office? 1TB is really not that much.  There is no way to throttle the replication on the Exchange side and if you have to do that, then I would recommend against putting a DR server in a remote location. 

Thanks for your reply and correct there is 1 dag with 4 databases (2 active on each server).

Is there anyway to monitor the bandwidth used by Exchange replication between the two existing servers? We need an idea of how much Exchange data is going to be sent down the link. It's a 50mbit private/point to point connection. While we can make use of the link for exchange replication we need to ensure it doesn't consume too much bandwidth and impact existing services/applications which also use the link.

Replication is on port 64327 by default, so you can monitor with whatever network tools are avail.

That port can be changed in the properties of the DAG itself with Set-DatabaseAvailabilityGroup.

You can use the Exchange Mailbox Calculator to assist on the min bandwidth requirements:

https://gallery.technet.microsoft.com/office/Exchange-2013-Server-Role-f8a61780

There is a tab called replication requirements and the recommended network link