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Q105282: Incorrect Source Routing Bit Causes NetBEUI Network Storm

Article: Q105282
Product(s): Microsoft LAN Manager
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 26-FEB-2002

SYMPTOMS
========

Receiving a frame on an Ethernet with the source routing bit enabled can cause a
machine running NetBEUI to propagate a network storm: entering a loop of
transmitting invalid frames to other machines on the net. This seems to occur
mostly with 3Com Etherlink3 network interface cards (NICs).

CAUSE
=====

NOTE: SABME and UA are defined below.

When a link is established and the SABME frame has the source routing bit set
erroneously, the routing length field is accounted for in the link table entry
as well as in the original routing header info.

This causes all subsequent frames on this link (including the following UA) to
contain the original source routing info, but the source routing bit is ignored
on the receive end, causing the frame to be misinterpreted. When the original
frame contains a SABME with the source routing bit set incorrectly, the two
machines enter a loop broadcasting SABME frames back and forth.

STATUS
======

A fix has been made to NetBEUI, causing it to refuse to accept frames containing
source routing information when operating on an Ethernet network.


DEFINITIONS
-----------

- SABME stands for "set asynchronous balanced mode extended." It is a frame
  sent from one logical link control (LLC) layer to another requesting
  establishment of a link connection between them.

- UA stands for "unnumbered acknowledgment." It is a frame sent in response to
  a SABME that accepts the request and establishes the link connection.

Additional query words: 2.20 2.2 multicast

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Keywords          :  

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