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Q216471: NetBT Improvements for Connections Between Multihomed Computers

Article: Q216471
Product(s): Microsoft Windows NT
Version(s): winnt:4.0
Operating System(s): 
Keyword(s): kbWinNT400sp5fix
Last Modified: 16-MAY-2002

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The information in this article applies to:

- Microsoft Windows NT Server version 4.0 
- Microsoft Windows NT Workstation version 4.0 
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SYMPTOMS
========

When connecting from a multihomed computer to another multihomed computer, where
both computers have two separate networks in common, the optimal route may not
be chosen. Additionally, the outbound path may differ from the inbound path.

CAUSE
=====

When a call is placed using NetBIOS over TCP/IP (NetBT), for example, "net use
\\mycomputer\myshare" on a multihomed computer, the outbound calls are placed on
both interfaces. Each interface should query WINS and then choose the best
target IP address for the called computer. Two improvements are now available in
this algorithm.

RESOLUTION
==========

The algorithm used in determining the best target IP address has been changed to
include two improvements, available in this new version of Netbt.sys.

Previously, a match was chosen in the order below. For additional information,
please see the following article(s) in the Microsoft Knowledge Base:

  Q161425 WinNT 4.0 SP2 Multihomed Computer Connection Enhancement

1. Check for a matching subnet address for the calling interface.

2. Check for a matching subnet address for any interface in the computer.

3. Pick a target address randomly.

The new version adds one more check, as shown here:

1. Check for a matching subnet address for the calling interface.

2. Check for a matching network address for the calling interface.

3. Check for a matching subnet address for any interface in the computer.

4. Pick a target address randomly.

Also added is support for a "per-interface" NetBIOS name cache. The per-interface
cache allows each adapter to choose target IP addresses for multihomed computers
independently, instead of picking up an IP address from the cache that was
chosen in the context of another adapter.

To illustrate:

                             ---------
          ------------------| Router1 |----------------
         | Net1SubnetA       ---------     Net1SubnetB |
         | NIC1                                   NIC1 |
   -----------                                     -----------
  | Computer1 |                                   | Computer2 |
   -----------                                     -----------
         | NIC2                                   NIC2 |
         | Net2SubnetA       ---------     Net2SubnetB |
          ------------------| Router2 |----------------
                             ---------

In the scenario shown above, if a NetBT call is placed from Computer1 to
Computer2, a call will go out each interface (NIC1 and NIC2). NIC1 will query
WINS, and, assuming that WINS returns an IP address for each interface on
Computer2, it will choose the IP address associated with Computer2NIC1 as the
best IP address to connect to, because both share the same network ID (even
though the subnet ID is different). This IP address will be cached for NIC1 in
the per-interface name cache. NIC2 will also query WINS, and assuming that WINS
returns an IP address for each interface on Computer2, it will choose the IP
address associated with NIC2, and this IP address will be cached in the
per-interface name cache for NIC2.

Calls are placed on both interfaces (and the WINS server for each interface is
queried separately) because the computer cannot assume that the target computer
is reachable on both networks in all cases.

This enhancement can improve the path used for connections from a multihomed
computer to another multihomed computer, when both have two networks in common.
It does not offer any changes for other scenarios.

To resolve this problem, obtain the latest service pack for Windows NT 4.0 or the
individual software update. For information on obtaining the latest service
pack, please go to:

- http://www.microsoft.com/Windows/ServicePacks/

-or-

- Q152734 How to Obtain the Latest Windows NT 4.0 Service Pack

For information on obtaining the individual software update, contact Microsoft
Product Support Services. For a complete list of Microsoft Product Support
Services phone numbers and information on support costs, please go to the
following address on the World Wide Web:

  http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS


STATUS
======

Microsoft has confirmed this to be a problem in Windows NT 4.0. This problem was
first corrected in Windows NT version 4.0 Service Pack 5.

Additional query words: 4.00 multi-homed multi homed

======================================================================
Keywords          : kbWinNT400sp5fix 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400
Version           : winnt:4.0
Hardware          : ALPHA x86
Issue type        : kbbug
Solution Type     : kbfix

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