Examples Of Creating Filters - 3Com LANPLEX 6000 User Manual

4 Apply a logic operation to the values in steps 2 and 3. The operator you use
Examples of
Creating Filters
depends on what comparison you want to make.
Variations to these four basic steps of writing packet filters include:
Using for each additional comparison you intend to make with
pushTop
the pushField value. This makes a duplicate of the pushField value and
places it on top of the original pushField on the stack. The pushtop
instruction makes a copy of the field more efficiently than if you just
used a second pushfield instruction.
Using or
accept reject
sequential tests, and you would like the filter to accept or reject a packet
before the entire expression has been evaluated. Using accept and reject
can significantly improve the performance of certain types of filters. See the
section "Implementing Sequential Tests in a Packet Filter" on page 14-9 for
more information.
Using ,
pushSAGM pushDAGM
address or port groups. See Chapter 14: Configuring Address and Port Groups
to Use in Packet Filters for more information.
The following is an example of a complex packet filter built from three
simple packet filters. Each of the shorter, simpler packet filters can be used
on its own to accomplish its own task. Combined, these filters create a
solution for a larger filtering problem.
Filtering Problem
Your network contains market data feed servers that receive time-critical
financial data needed for trading floor applications. At the center of the
trading floor networks is a LANplex system used to switch Ethernet traffic
and to concentrate the market data feed servers onto the FDDI departmen-
tal backbone.
The difficulty is that the market data feed servers transmit data to users
with broadcast packets that are forwarded to all stations on all segments
attached to the LANplex system. Not all of the segments attached to the
LANplex system have stations that require these broadcast updates. In order
to optimize the performance of these Ethernet segments, it is necessary to
filter the broadcasts.
Creating Packet Filters
with and operators when you have
and or
, , or
pushSPGM pushDPGM
14-11
for filtering by