Ref: 13680009
Title: Reverse Filtering on IBs
Date: 1/31/90

Copyright 3Com Corporation, 1991.  All rights reserved.

Reverse filtering is a new feature on IB software 2.0 or later.
To make filters work more efficiently, you can define reverse filters,
which are also easier to understand.  If reverse filters were not
available, you would have to define several filters describing the
types of packets that should be discarded.

For example, if you have XNS, IP and LAT packets, and want to allow only
the XNS packets to be forwarded and block all others, you might use the
following filters:

     Filter 1  = %C:%0800   - IP
     Filter 2  = %C:%0806   - ARP (IP Address Resolution Protocol)
     Filter 3  = %C:%6004   - LAT

With a reverse filter, you need only one filter:

     Filter 1 != %C:%0600   - XNS

This reverse filter will block all packet types except XNS.

When displaying filters, the system displays number of FORWARDED packets
that meet the conditions with a reverse filter:

     SHOW FILTERS <CR>
     Filter 1 != %C:%0600    (123, 6356)

Alternately, you may wish to use a filter such as the one below.  This
works well as long as the filter has only one condition:

     Filter 1 = %C:!%0600

This filter will block all packets except XNS, and the system will
display the number of DISCARDED packets that meet the conditions:

     SHOW FILTERS <CR>
     Filter 1  = %C:!%0600   (3256, 184567)

CAUTION:  Defining a reverse filter improperly may cause the
bridge to discard some packets that you actually wanted to
forward.  Also, for other filters to work properly, reverse
filters must be defined last.

