Big-IP DNS Exchange 2010 load balancing

I have big-ip dns in place, WIP for Exchange ActiveSync, with Global Availability as the load balancing method. I would like to change it to round robin, maybe do geo load balancing later. I'm confident Big-ip is configured but would like to hear, from the F5 view, how InternalURL/ExternalURL should be set. It seems there's conflict.

 

Setting aside any load balancing, Exchange uses the internalurl and externalurl settings for proxying CAS servers from site to site. If my mailbox is in site A, but the traffic hits a CAS in site B, the externalurl value is used to direct the traffic to a CAS in site A.

 

For big-ip dns, what is the recommended setting for the externalURL? Should they be all the same? Should the externalURL setting be blank?