Filtering
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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 P840 router. There are several simple
methods that may be used to provide a furthe r 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 i nformation 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 l ink on the other LAN segment, a filter mask
can be used.
To prevent broadcast information from being p assed across the link, use th e following filter mask:
0-FFFFFFFFFFFF
This prevents any frame with a destination address field set to the broad cast 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 s pecific 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 fou r positions constitute 4 bits, or a nibble, of the entire expres sion; 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 s ingle quotes represent a single don’t care for those bit positions in the first
nibble. The X that is located outside of the singl e quotes represents a don’t care condition for the later nibble.
NOTE: With this mask, both broadcast fram es and multicast frames will be filtered.