General question about AD Sites and Site Links

[mspain]

Hi all,

I'm reading about setting up Active Directory Sites and need a little help visualizing something. I understand that to schedule replication between 2 sites, we want to create a site link that contains the two sites. Also we will want to assign a cost to these links.

Now lets say I have Site A and Site B connected via a T1, with a dial-up connection for backup. I would want to create two site links right? One for the T1 and one for the dial-up connection and assign the dial-up connection a higher cost right? All good so far?

Now, I get confused here. Lets say the primary T1 link is down and we are using our dial-up backup connection. How does Active Directory know to use the link we created for the dial-up connection?

I understand the gateway routers at each site would be responsible for the actual failing over to a backup link. But still, if I had two site links defined in AD, say one for the T1 that is set to replicate every hour vs. a site link for the backup connection that is set to replicate every say 4 hours, how does AD know it is now time to use the other link with a longer replication time?

Probably a dumb question, but I have seen this scenario presented when trying to explain sites and site links and haven't seen anyone explain exactly how Active Directory know what site link to use.

Thanks for any help!

 

[58sniper]

No - I think you're a bit confused. Site costs and such come into play if there are different routes to take from one site to another. Let's say you have three sites, A, B, and C. Picture them like a triangle. A can connect to B and C directly, but can also connect to B via C, and to C via B. If you assign each link a cost of 10, traffic should take the direct route since the total cost is 10. But if that direct link goes down, it would use the indirect link instead.

Pat Richard MVP
Plan for performance, and capacity takes care of itself. Plan for capacity, and suffer poor performance.

http://www.ucblogs.net/blogs/exchange/

 

[mspain]

Hey thanks sniper! I think that makes a little more sense.

One more question maybe you can help with or point me in the right direction. Replication schedules? In Sites and Services, there are 3 different places to set schedules. On the Site NTDS settings, on the connection objects under the Server objects, and on the Site Links themselves.

I'm having a hard time finding clear documentation on the difference between them. I feel that the schedule for the Site NTDS settings relates to INTRAsite replication? Once every 15 minutes by default. This is purely for failure detection, ie if a domain controller has not received a notification in 15 minutes it will try to replicate. Am I close or is this totally off base?

The Site Link schedules I would assume are for INTERsite replication. IE, replication can take place across that link based on the availability and frequency defined on the properties of the link.

And the schedule on the actual connection objects under the server objects? I am assuming is a way to have finer control over replication between specific DCs?

If you have any input I'd greatly appreciate it!!