Topic Last Modified: 2006-05-17

The Microsoft® Exchange Server Analyzer Tool queries the Win32_PingStatus Microsoft Windows® Management Instrumentation (WMI) class using various, specific BufferSize and NoFragmentation setting values to determine whether black hole routers might be present in the WAN environment.

BufferSize Setting	NoFragmentation Setting
548	True
1472	True
1472	False
4096	True
4096	False

The Exchange Server Analyzer displays an error, if the following conditions are true:

• PING with a smaller BufferSize and NoFragmentation = True is successful
  
  
• PING with a larger BufferSize and NoFragmentation = True times out
  
  
• PING with a larger BufferSize and NoFragmentation = False is successful but the reply BufferSize size differs from the sent BufferSize.
  
  

This error indicates the possible presence of one or more black hole routers on the wide area network (WAN).

On a TCP/IP WAN, communication over some routes may fail if the following conditions are true:

• Intermediate network segments have packet sizes smaller than the communicating hosts
  
  
• Routers do not send appropriate ICMP responses to this condition.
  
  

Alternatively, the firewall on the path may drop such responses. A router that causes this condition is sometimes known as a black hole router.

The presence of a black hole router can cause a variety of errors that do not occur if a program connects to a computer on a local subnet. The behavior may seem intermittent, but you will find that the behavior can be reproduced, for example, by having a client read a large file that is sent from a remote host.

For more information, follow the guidance in Microsoft Knowledge Base article 314825, "How to Troubleshoot Black Hole Router Issues" (http://go.microsoft.com/fwlink/?linkid=3052&kbid=314825).

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