Ref: 13680010
Title: Filters for 3Com's NetBios Protocol (NBP) on IBs or Brouters
Date: 3/7/90

Copyright 3Com Corporation, 1991.  All rights reserved.

The NBP protocol described in this article is the version introduced with
3+Open 1.1.  Later releases of NBP may be subject to change.

Currently, IBs do not filter NBP packets.  For local bridges (IB/1, IB/2
and IB/2000), NBP packets do not cause any problems, but they will cause
high path utilization for the serial links and will bother remote networks
via remote bridges (IB/3).  Therefore, you may want to block NBP traffic
over serial links.

An NBP packet starts with a MAC header for the medium being used--currently
either Ethernet (DIX) or Token Ring.  This article discusses the Ethernet
MAC header only.  It looks like this:

    Offset
    0                 6                 C

    DD DD DD DD DD DD SS SS SS SS SS SS TY PE

"DD DD DD DD DD DD" is the destination address.  "SS SS SS SS SS SS"
is the source address (both 6-byte IEEE addresses).  "TY PE" is Ethertype.
The destination and source addresses start with "02 60 8C," 3Com's assigned
prefix.

The Ethertype field identifies a packet as belonging to NBP.  NBP uses
Ethertypes whose first byte is 3C (for 3Com).  The current range is from
"3C 00" to "3C 0D," each assigned to a different NBP packet type:

    3C 00     Virtual Circuit (VC) Data
    3C 01     System
    3C 02     Connect Request
    3C 03     Connect Response
    3C 04     Connect Complete
    3C 05     Close Request
    3C 06     Close Response
    3C 07     Datagram Data
    3C 08     Broadcast Datagram Data
    3C 09     Claim Name
    3C 0A     Delete Name
    3C 0B     Remote Adapter Status Request
    3C 0C     Remote Adapter Status Response
    3C 0D     Reset

Note:  These Ethertypes are not registered with the IEEE.


To block all NBP packets across an IB, use the "%C:%3C" filter.

    %C        offset C (Ethertype field)
    %3C       will block all packet types starting with 3C (from
              3C 00 to 3C FF)

IB software 2.x     :  Filter x = %C:%3C

IB software 3.x or
Brouter software 1.x:  ADD FIlters !x %C:%3C
                       ADD FilterQualify !x N-N


Note:  With new IB software 3.x and Brouter software 1.x, there is no
equal sign between Filterid and the list of patterns.  Also, an
exclamation mark (!) is required for the Filterids and Filterqualifiers.
You also need to set the FilterQualifier to "N-N" (both directions) because
the default set is "disabled".


These filters may not work if and when 3Com incorporates SNAP (IEEE 802.3)
headers in NBP; IEEE headers are not covered in this article.