Exchange 2019 Connection Problems with GCs

we got a very strange Problem and we can't find a solution.

We got 11 Exchange 2019 Server, on 11 bare metal MS Server 2019 maschines. They are all in a DAG and we have 3 DCs who are also all GCs.

Sometimes some of the Exchange Servers are losing the connection to the GCs but OLNY in the EMS. If we for example try a get-Mailbox we get the following message:

Microsoft.Exchange.Data.Directory.ADTransientException: Could not find any available Global Catalog in forest X

Where X is the name of the Forest.

It happens totaly random but the longer the Server are running the longer and more often this issue occurs.

The GCs and the Exchangerserver are in different Networks but the Firewall got an ALLOW ANY <-> ANY policy between them on every port.

We put another DAG in the same Network as the other Exchange Servers and they got the same Problem. We recreated the virtual directorys but the problem is still there. We cleaned the DNS and there are no old entrys left. We kicked some of the servers out of the AD and rejoined them, nothing changed.

All Servers got the newest CU and the Windows Server below got the newest Updates

Does anyone have a hint what could be the Problem? Regards

Solution 1:

Sorry for the late reply but we were busy celebrating.

The Problem has been the firewall... specifically the deep packet inspection.

Even with all ports open, the Firewall didn't like the MSRPC Requests and blocked them RANDOMLY from time to time... Maybe this information helps someone.

Thank you all very much for your replys :)