Those of you with any type of security certification (or basic knowledge for that matter), understand the principle...
of avoiding single points of failure. A single point of failure is an Achilles heel that you should attempt to design out of your IT infrastructure (not to mention every other aspect of your organization). If there is a single element, component, system, device, or person that your company cannot live without, you have a single point of failure.
All too often, companies rely upon convenience and low cost to guide network design rather than security. The aspect of security concerned with avoiding single points of failure is known as availability. If you are unable to get to your online assets or communicate with others on other networks, a violation of availability has occurred. Any risk to the timeliness and bandwidth of your communications should be thoroughly considered.
So, how does avoiding single points of failure apply to Active Directory? Well, it is mostly a matter of understanding how AD functions and how your organization's IT infrastructure operates.
First and foremost, you should always have a minimum of 2 domain controllers for every domain. If you can afford it, I would make the minimum 3 to give yourself that one extra level of protection.
Second, you should not place all of your domain controllers in the same physical location. Have two or more server vaults. Thus, if one of the server vaults is compromised by intrusion, fire, etc. the other vault may remain viable.
Third, deploy redundant copies of all the systems that AD depends upon. For the most part this means your DNS system. So, be sure to have backup or secondary DNS servers in each domain.
Fourth, consider your cabling. Is there only one cable link between the network and each domain controller? Why not deploy two or more NICs in each domain controller so they can be accessed through multiple pathways.
Obviously, these recommendations will increase the cost of deploying and maintaining your AD infrastructure. But when problems occur, the penny of prevention is often worth more than the dollar of cure (i.e. incident response).
James Michael Stewart is a partner and researcher for ITinfopros, a technology-focused writing and training organization.