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windows 2003 replication topology

Posted on 2004-09-22
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Last Modified: 2010-05-18
Hello,

I have been reading the Microsoft documentation about how the replication works in windows 2003.
I Know that in Windows 2003 replication topology KCC is able to select the bridgehead server from
a prefered list and so, when the ISTG creates the intersite replication conections, the bridgehead from
this list are ramdonly selected.
Microsoft talk about if the servers in the list are not available then the replication for the
directory parition will not occour.
I have a question. If the server in the list are not availble or KCC detects some problem with these
server:

is not the KCC able to select other Domain controller in the site which are not included in the
prefered bridgehead list ?
that involve,
Is dangerous to select only one domain controller in the prefered bridgehead list ?

This is because in my Datacenter (hub) are only two domain controllers and one of them is the PDC emulator that
at the same time Microsoft recomends to be not included in the prefered bridgehead list.
Then if I have only one prefered domain controller this is a risk.
Perhaps,
is it better does not select any prefered domain controller and so the KCC select one of them ?
or
is it better to select both domain controllers (also the pdc emulator) to be included in the prefered list
and in this way the initial load balancing configured by the KCC create the connections using both Bridgeheads ?

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Question by:intentalo69
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Expert Comment

by:JamesDS
ID: 12120954
intentalo69
In this case it would seem more sensible to NOT select any preferred bridgeheads and let the KCC decide for itself acording to availability.

Cheers

JamesDS
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Expert Comment

by:rj-smith
ID: 12120998
Yeah I agree with JamesDS, if you've no real reason for wanting a specific server to handle intersite replication then don't specify any preferred bridgehead servers.

Good luck.
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Author Comment

by:intentalo69
ID: 12121089
"is it better to select both domain controllers (also the pdc emulator) to be included in the prefered list
and in this way the initial load balancing configured by the KCC create the connections using both Bridgeheads ?"

Yes but it could be a difference I think
1.- If I select both hub domain controllers as prefered Bridgehead the Kcc could select
both servers to divide the conections between them
2.- If I don´t select any hub domain controllers as prefered bridgehead the kcc will
select only one domain controller to built the connection, is this true ?

Please, correct me if am wrong with any of these ideas.

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Accepted Solution

by:
rj-smith earned 250 total points
ID: 12121148
The KCC will still select one of your servers as a bridgehead server (it just prefers to route intersite replication through one server only). The difference is that in this scenario the KCC will have selected it automatically and if there is a problem will then select the remaining domain controller. In this scenario you can also add and remove domain controllers without having to worry about intersite replication.

Hope thats clear.
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Author Comment

by:intentalo69
ID: 12121290
rj-smith,
"it just prefers to route intersite replication through one server only",
I think with the new Windows 2003 KCC, the created connections are established randomly between all bridgeheads selected in the list not only with one bridgehead.
What do you think?

regards,
Richard


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LVL 6

Expert Comment

by:rj-smith
ID: 12121406
Hmm, I think we might be confusing some concepts here. Let me try to clarify my understanding of the situation.

Firstly, the term bridgehead infers only one server, i.e. one domain controller in every site will become the bridgehead server for that site. By default, the KCC will select that server (from it's own list of all domain controllers in the site) at random or via some algorithm (I'm not entirely sure which). This is unlike intrasite replication where all domain controllers are treated as equals.

If you select a preferred bridgehead server or servers then that server (or one from that list) will be selected as the bridgehead server.

Intersite replication will only occur via the bridgehead server.

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Expert Comment

by:JamesDS
ID: 12121801
intentalo69

A point worth noting is that under Windows 2003 the KCC and ISTG are far more robust than under Windows 2000. Unless you are building a very large AD with many hundreds of sites, you are better off leaving the KCC to do the job itself.

As a rule you should leave AD to it's own devices and only tune when addressing a specific issue. If sites, sitelinks and subnets are configured correctly and there is the proper distribution of GCs for your infrastructure then there is little need to get involved in the precise mechanics of replication.

Cheers

JamesDS
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Author Comment

by:intentalo69
ID: 12219363
Yes JamesdS, Our idea is to leave the KCC the automatically generated topology. The question is related to the
Bridgeheads prefered list (to specifiy more than one DC). And If the ISTG, when I select two different DC in the prefered list of bridgeheads, calculates the replication topology distributing the connections between these DCs. (I think in Windows 2003 does it)
What do you think?

thanks again
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Assisted Solution

by:JamesDS
JamesDS earned 250 total points
ID: 12224399
intentalo69

If you create more then one bridghead then Windows 2000 and 2003 will distribute connections between them.

If you create ANY bridgeheads, you are preventing the KCC from performing an automatic failover in the event your last bridgehead fails. Bridgehead functionality was envisaged to tackle specific performance issues with large numbers of sites (around 200+ under windows 2000 and many thousands under Windows 2003). So, unless you specifically have this issue then we are back to my earlier post - let the AD manage itself and if you have to use bridgeheads then make sure there at least 2 in EVERY site you use them in and that you moitor them for load and uptime.

Incidentally, when windows is picking a bridghead server for itself, it lists the eligible DCs in the local site and sorts them in descending order of GUID. Then it picks the lowest one.

Cheers

JamesDS
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