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I know when you manually kick off Repadmin and specify source/destination DC it's going to ignore any site, site link and site link bridge settings as long as the DCs are reachable over RPC to each other.

The question is when KCC(ISTG) calculate replication connections, will it always strictly respect the site link settings, even though some DC is down or not all sites are fully routed? Consider scenarios below

Three sites A,B,C. Auto site link bridging is disabled. Site link A-B, B-C are enabled

1) Domain X has DC1,2,3 in site A,B,C respectively. Assume DC2 is down, will DC1 and DC3 tries to replicate with each other? There is no route avaialble given the site settings but it's the only possible way to replicate which KCC can figure out.

2) Domain X has DC1,3 in site A,C respectively. Another domain Y in the same forest place DC2 in site B. In this case KCC will not find any link can connect DC1 and DC3, will DC2 act as "store-and-forward" for the replication of domain X partition? Or DC1&DC3 replicate directly? Or will replicatoin never occur? (though DC2 itself doens't hold a replica of that)

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Yes.

This is one more reason you should always have more than one DC in a "normal" site.

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  1. No, AD will not auto-generate new site-links when a DC is down. You should always create backup links so that this will not be an issue. Remember that AD is a mulit-master database so changes made to any DC will eventually replicate out across all.

  2. Domain Y DCs will not "store-and-forward" replication for Domain X.

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    Re: 1: What? From technet.microsoft.com/en-us/library/cc756899%28WS.10%29.aspx -- "The KCC also dynamically adjusts the topology to accommodate new domain controllers, domain controllers moved to and from sites, changing costs and schedules, and domain controllers that are temporarily unavailable." So unless you explicitly disable the KCC (it defaults to on), no, the KCC will rebuild connections from site to site in an effort to get connectivity re-established. Wait an hour (or so), and you will have a link from A to C, thanks to the KCC. – Kyle Brantley Apr 15 '11 at 3:54
  • Thanks both! Actually for condition 2, I did a test (which is not exactly same but can prove the point) – user78486 Apr 15 '11 at 6:50
  • Sorry i was typing too fast, Please ignore my previous comment. I did a test which is similar to case 1: I've two sites A,B. Each have one DC for domain X. If I remove all links(direct/indirect) between them, and run "repadmin /kcc" on both DC it's very interesting the existing connection object is still there. But if I delete existing connection , it won't recreate next time when i force KCC to run again. (On the contrast, if there is a link A-B exist, the deleted connection will regenerate next time KCC runs. – user78486 Apr 15 '11 at 7:01

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