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AD Replication - SOmetimes getting "RPC server is unavailable" error

Hi there,

I have 2 Windows 2008 DCs in 2 different locations and I am often getting Replication errors from the AD replication, usually the typical "RPC server is unavailable".

I know this is easy to troubleshoot when it NEVER works, but here like 90% of the time it works and then again i sometimes have a situation where it can't replicate for a few minutes up to an hour. The weird thing is that even though I am getting the AD replication errors (RPC unavailable) i can PING, access shares, open RPD sessions and such between the 2 servers, so it's not like connectivity on that route is down or so. I'd also rule out the typical suggestions (Firewall blocks something, DNS problem, time synchronization) as it works like 90% of the time just fine. Any ideas what else i could try?

Neither DCDIAG nor REPADMIN have given any useful information. When the error occurs i can see the "RPC server not available" messages in REPADMIN but what puzzles me is that if the RPC service is really "dead" then i also shouldn't be able to get onto that machine via RPD or CIFS, but that always works fine, it's really just the AD replication that is having issues. DFS replication is also working fine, files in the DC SYSVOL get replicated instantly.


Any ideas or comments welcome!
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kyodai
Asked:
kyodai
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5 Solutions
 
XaelianCommented:
Do you have an "Intrusion Prevention System" on one of the servers? This sometimes can give you this error. Because it can prevent a partial packet to get to the other server.
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kyodaiAuthor Commented:
No we don't use any intrusion prevention system.
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XaelianCommented:
Hi,

Can you use follow microsoft page to troubleshoot the problem?

It can be due to a whole bunch of reasons. Probably, the destination server can't connect the source server for a few moment. That's why you get this error. In following document, there are steps that you can take to resolve the problem.

If this fails can you post the total log of the error? Maybe there is something strange in it. Because like you said, normally you can't RDP, ... when you get this error. With the information you provided, this MS page, is the best step.

http://support.microsoft.com/kb/2102154
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jmanishbabuCommented:
This error can occur because of connectivity issues.
Unable to connect the source server using the RPC protocol, The "RPC server unavailable" error can occur for the following reasons

1. source domain controller down
2. Network connectivity down between source and destination domain controller
3. Due to network latency
4. Intermediate network issue
5. Not enough network bandwidth to establish the connection
6. All bandwidth used by some other traffic .

Check this blog for replication related errors

http://www.windowstricks.in/2009/11/troubleshoot-active-directory-server.html
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jmanishbabuCommented:
Wait for some time .. Replication will work fine ..
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kyodaiAuthor Commented:
OK, what i found out so far is i can only replicate in one direction, from Server Berlin to Server Cologne. If i try to replicate the same context from Cologne to Berlin i get the "RPC not reachable" error. So far i have refrained from posting any error logs as we unfortunately have German versions of windows installed, so all errors are in german.

But on an attempt to replicate from Cologne to Berlin i get the notorious 1722 - The RPC Server is unavailable.

I have gone through that microsoft article and no relevant errors so far.

I tested the ports with PortqueryUI and the strange thing is that in both directions PortQueryUI gets the AD info fine via TCP 389 and 3268 and shows the information correctly.

What is a notable failure though is that UDP on port 389 fails from cologne to Berlin - the direction in which replication is not working. So this might be the cause of the error i think, at least thats the only thing i observed. Our network admin says all ports are open and i already rebooted the machine. Is AD replication "sometimes" using UDP and "Mostly" TCP? That would explain why it "sometimes" doesn't work...
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XaelianCommented:
This will normally be a firewall issue. Below you find a link about what ports need to be open for AD replication to work. TCP and UDP ports.

http://social.technet.microsoft.com/wiki/contents/articles/584.active-directory-replication-over-firewalls-en-us.aspx
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kyodaiAuthor Commented:
It looks like this is not really a windows issue but rather a network issue. I will still give points to all of you for your most helpful suggestions.
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