Assigning Sites in Active Directory

Based on Microsoft's recommendations, the best practice in this scenario would be to place the Los Angeles subnet in a separate site to cut down on replication traffic over the 64K WAN link.
The Dallas and Austin offices are connected by a relatively high bandwidth link (a T-1 line at 1.5 MBps), and so could be configured as two IP subnets combined into one Active Directory site. However, because this would make for a relatively large number of computers (550) at this site, all of which might be requesting logon authentication at the same time, for optimum performance, Microsoft recommends creating three separate sites for the three geographic locations.

1. You probably agree that LA should be a separate site due to its slow link, but given the number of computers and the speed of the link, should you make Dallas and Austin separate sites or part of one site?
2. What are the potential challenges to this approach?

Introduce your own variables, and think about how they might change your answer. We encourage you to join the discussion by posting a message on the discussion board.
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An Active Directory site is a physical grouping of computers. A site, by definition, encompasses a certain geographic location in which all computers reside on one or more well-connected subnets. In reality, a site can be one specific geographic place, or it can span several geographic locations. The key to understanding sites in the Active Directory is to see sites in the same way the Active Directory sees them in terms of connectivity. The Active Directory expects your sites to be based on well connected TCP/IP subnets.
The term well-connected is a soft term in that it does not mean one specific thing. Typically, a well-connected subnet has fast, reliable, inexpensive, and abundant bandwidth, such as in typical LAN connectivity. How you define well-connected will vary from one Active Directory planner to the next, but the key point one is to understand that the Active Directory always assumes a site has fast, reliable, inexpensive, and abundant bandwidth.
In short, if your view of site definition is different from how the Active Directory sees sites, be sure to get on board with the Active Directory's definition before planning your own sites. When a site is established, the Active Directory assumes that bandwidth is readily available, reliable, inexpensive, and fast. Because the Active Directory assumes these qualities, it plans replication traffic within the site based on these well-connected features. So, it is important that you build your sites based on the Active Directory's definition of a site.