Programmable Filtering
B-10 — Orbitor 3000 Reference Manual
DEC
DEC uses protocol types 6000 to 600F, and although some are undefined, a simple filter mask can be created to
filter all DEC traffic.
Filter all DEC
The mask to filter all DEC traffic would be:
12-600X
The X is a variable representing the last four bits (a nibble) of the type. This will effectively filter all Ethernet
frames that contain a protocol type of 6000 through to 600F. All 16 possible combinations are covered.
Bandwidth Conservation
Reducing traffic on each LAN segment is one benefit of the bridging functions of a Orbitor Ethernet bridge/router. There
are several simple methods that may be used to provide a further reduction of inter-LAN traffic. The examples that follow
present a few very simple methods to reduce inter-LAN traffic, without necessarily reducing resource capability.
Ethernet Broadcasting
On an Ethernet LAN, any station may broadcast information to all other stations by setting the Ethernet
Destination address to FF-FF-FF-FF-FF-FF. By configuring the destination address to this setting, it is telling all
other stations that this is a broadcast message.
In many situations, stations will abuse this broadcasting capability and send useless information to other stations
in the network. To prevent this information from being seen across the link on the other LAN segment, a filter
mask can be used.
To prevent broadcast information from being passed across the link, use the following filter mask:
0-FFFFFFFFFFFF
This prevents any frame with a destination address field set to the broadcast address from being passed to the
second LAN segment across the link.
Ethernet Multicasting
An Ethernet multicast is a frame of data where the destination address has the high-order bit set to a “one”
condition. It is similar to a broadcast, but is to be received by a “group” of stations that meet the remainder of the
address. In this manner, a broadcast is focused to a specific group of stations.
To filter multicast frames, the following mask could be used:
0-’1XXX’X
In this example the high-order bit by multi-cast definition must be set to a “one”. The single quotes around the
first four positions instructs that the four positions constitute 4 bits, or a nibble, of the entire expression; each
position representing a single bit. The “1” indicates that that bit position must be equal to a “1” before the
expression is true. The X’s that are included within the single quotes represent a single don’t care for those bit
positions in the first nibble. The X that is located outside of the single quotes represents a don’t care condition
for the later nibble. NOTE: With this mask, both broadcast frames and multicast frames will be filtered.
