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Global Catalog Replication Paths

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by kals235

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In a Win2000 tree structure, (Example One parent and 2 child domains with sites internal to the domain) do the Global Catalogs replicate between each other in a domain and or between each domain?s Global Catalog or do they rely on active directories?
If the Global Catalogs replicate between each other how do they replicate and what are the replication paths?
If the Global Catalogs replicate between each other, what method/s do they use.
I.E. UDP/TCP port (what port #), or through multicast or what?

Note: A Basic diagram of replication paths would be helpful.

6 replies

by bigfon

GC is the master DB that runs AD. Every forest has at least one GC. If you have more than one GC they are simply clones. Microsoft reccommends that you have a GC per site as a minimum and at least two per site for redundancy. The GC is replicated as part of the AD replication which occurs periodically based on how you have your sites and replication schedules set up. AD uses the GC to authenicate users and many other functions.

by kals235

I know all that stuff and some of what's in your anwser is wrong. GC is created from the AD and the GC is a subset of the AD. So how could the GC be the master? Also the replication that you mention is inside a domain not between parent and child domains. Which is the meat of the question. AD contains detail info of it's domain. GC contains subset info from each domain in the tree/forest. I will up the ante to 500 to get an exact anwser. Microsoft has had this question for over a month and has not anwser it yet.

by tony.colgrove

You are right the GC is only a partition of AD. This partition is identical on all GC's (assuming replication is up to date).
The Knowledge Consistancy Checker (KCC) determines how to build replication paths. By default a PDC emulator and GC willhave 6 replication partners (if you have 6 or more DC's in the domain (PDC Em) or forest. A normal DC will only have 3 replication partners. These general guide lines fall into a single site.
Install the resource kit to get access to replmon. This will allow you to get a better grasp on the replication pattern on the forest.
Now once you start to add sites, and bridgehead servers, these rules can change. All replication traffic is done through RPC's, unless you are using SMTP links acrossdomains.
Also through the KCC, each GC which other DC's are GC's. GC replication follows normal AD repl rules, except when there is a schema change or an object is selected to be published to the GC. This causes a full replication of the catalog.

by kals235

Below is the response from Microsoft.
[JX] The global catalog is built automatically by the Active Directory replication system and its replication topology is generated automatically. The Knowledge Consistency Checker (KCC) is responsible for calculating a replication topology in the following order: per-domain directory partitions, Schema and Configuration directory partition, and then GC. Depending upon the replication topology, you may end up with additional ntdsConnection objects specific to GC replication. The replication logic is smart enough such that GCs can source their read-only directory partitions from other GCs. For example, if you have one site with two domains and each domain have one DC with the global catalog, you can setup a replication schema so that the global catalog informtion will only be replicated once over a slow WAN link to the site.

Active Directory (and Global Catalog) supports two replication mechanisms, depending on whether the domain controllers involved are located in the same site or they are in different sites. "Intra-Site Replication" (where the DCs are in the same site) is RPC-based, meaning that each DC is responsible for notifying its replication partners of any new changes it has written to the database. Once informed by the originating DC, replicating partners negotiate a convenient time to pull down the changes. "Inter-Site Replication? (where the DCs are in separate sites) is SMTP based on pre-defined scheduled intervals which are controllable by administrators.

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