title: WIndows 2008 R1 suffers recurring 'Error 81' when running diagnostics. Unable to connect to DC
body: <p>I've a recurring issue on a number of 2008 R1 servers. Replication stops completely, and any attempt to run any AD-related snap-ins or diagnostics fail.</p>
  
  <p>For example:</p>
  
  <pre><code>C:\Windows\system32&gt;dcdiag
  
  Directory Server Diagnosis
  
  Performing initial setup:
     Trying to find home server...
     Home Server = MYDC
     Ldap search capabality attribute search failed on server MYDC, return
     value = 81
  </code></pre>
  
  <p>Other tools also fail in the same manner:</p>
  
  <pre><code>C:\Windows\system32&gt;repadmin /showreps
  [d:\rtm\ds\ds\src\util\repadmin\repbind.c, 444] LDAP error 81 (Server Down) Win32 Err 58.
  </code></pre>
  
  <p>LDP can't connect to the server either:</p>
  
  <pre><code>ld = ldap_open("mydc", 389);
  Error &lt;0x51&gt;: Fail to connect to mydc.
  </code></pre>
  
  <p>There are a couple of event log entries, but frankly I'm not sure if they are a cause or a symptom.</p>
  
  <p>Event 1311:
      The Knowledge Consistency Checker (KCC) has detected problems with the following directory partition. </p>
  
  <pre><code>Directory partition:
  CN=Configuration,DC=ad,DC=mydomain,DC=co,DC=uk 
  </code></pre>
  
  <p>Event 1566:
      All directory servers in the following site that can replicate the directory partition over this transport are currently unavailable. </p>
  
  <pre><code>Site:
  CN=MYSITE,CN=Sites,CN=Configuration,DC=ad,DC=mydomain,DC=co,DC=uk 
  Directory partition:
  DC=ad,DC=mydomain,DC=co,DC=uk 
  Transport:
  CN=IP,CN=Inter-Site Transports,CN=Sites,CN=Configuration,DC=ad,DC=mydomain,DC=co,DC=uk
  </code></pre>
  
  <p>A reboot always clears this up, and always after a month or two the same problem will crop up again on a different server. I've seen it on 5 different servers in different domains so far. Reboots are fine, but it always feels like a bit of a blunt instrument. I was hoping someone might have come across this before and be able to offer a fix.</p>
  
  <p>thanks...</p>