AD type repository gives a unexpected errors for GC search

I came across a very strange issue few days ago with a customer. They deployed IAG appliances at a site with identical configuration but one of the appliance started showing weird issues. Few users were able to logon to the portal and once number of session reached a certain level then they started seeing the following symptoms:

1- IAG portal login works fine for few users and when the number of sessions increases, they do see the error message from /InternalSite/ “Web Site too busy”. this error was presneted to the end user.

2- Then you will start getting all kind of weird unrelated error messages from Trunk, authentication, sessions, Global catalogue errors etc.

3- Network stack will show too much traffic back and forth from AD.

4- You could see GC is found correctly but tracings will show various failures to identify GC on the network.

5- It has nothing to do with permissions as I have found.

6- The major indication for the problem is when you open your AD type repository and click OK to accept the configuration you have done for it, you should see the following error:

So what we found is if NetBIOS over TCP/IP is enabled on the Internal NIC of IAG is then you will start seeing all of the above.

Disable the NetBIOS over TCP/IP from the Internal NIC of IAG and things will get back to normal.