Are multiple sites needed for a single Active DIrectory domain and single forest?

Hello Experts,
I'm wondering if there are any advantages or disadvantages for creating multiple sites for an AD enviornment with one single forest and one single domain.  Our domain is expected to grow to about 10,000 users, and our network enviornment consists of a city-wide campus between several different buildings, all of which are on the same network but different subnets.

We have setup multiple domain controllers within the various subnets to balance load, but would you recommend creating multiple sites and multiple global catalog servers as well?  What's the best architecture for a seemingly flat, but large AD enviornment?

Thanks
esckeyrwmAsked:
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QBRadCommented:
I would set them up in one forest / domain but make them different sites so that you can control replication between the different sites.  I would make each building a separate subnet / site.  Then i would put a DC at each site and make it a global catalog for redundancy.  If the links between sites go down you will be able to authenticate your users locally from the local dc / global cat server.  Make sure the users in each building are set to authenticate with the dc at that site.  I've seen setups where the users are all going to another site to login and authenticate, completely bypassing their local dc.
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Toni UranjekConsultant/TrainerCommented:
Hi!

Site and site links design depends on  physical locations and WAN links between locations.Main benefeit of using sites in configurable replication traffic between and localizatoin of authentication traffic. For second one you will have to create IP subnet objects and associate themwith site objects. Each site has to contain at least one DC fora each domain and one global caatalog. In single domain forest each domain controller should also be global catalog. If you will have five sites, you will need also five domain controllers.

HTH

Toni
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esckeyrwmAuthor Commented:
QBRad & Toni,

Thank you both for your advice.  Seems like setting up multiple sites is the way to go for a large geographic location.  Part of the reason why we've kept it a single-site enviornment is because we weren't sure what kind of impact a multi-site enviornment would have on replication.  We offer a lot of "on-demand" services (from user account creation, to GPO updates, to password resets) and we can't afford replication to take longer then a matter of minutes.  By creating a mulit-site enviornment, am I running the risk of a 15-minute minimum on replication?  I understand there is a bandwidth cost of replicating at a more frequent interval.  

Thanks again.
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Toni UranjekConsultant/TrainerCommented:
When you set replication frequency to once in 15 minutes, this is maximum interval. AD will be replicated at least once in 15 minutes.
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