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Introduction to Filtering
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Introduction
to Filtering
This router provides programmable filtering which gives you the ability to control under
what conditions Ethernet frames are forwarded from one network to another. There are
many reasons why this might need to be accomplished, some of which are security, protocol
discrimination, bandwidth conservation, and general restrictions.
Filtering may be accomplished by using two different methods. The first method is to filter
or forward frames based solely on their source or destination MAC address. This method
of filtering is useful when bridging between LANs and for providing remote access security
in any type of network. The Ethernet MAC (Media Access Control) address is checked
against the addresses in the filtering list and the frame is filtered or forwarded accordingly.
The second method of filtering is pattern filtering where each frame is checked against a
filter pattern. The filter pattern may be defined to perform a check of any portion of the
Ethernet frame. Separate filter patterns may be defined for bridged frames, IP routed
frames, and IPX routed frames.
For more information on filtering, please refer to the Programmable Filtering section of the
router Reference Manual located on the accompanying CD-ROM.
MAC Address Filtering
MAC address filtering is provided by three built-in functions.
The first function is “Filter if Source”; the second is “Filter if Destination.” The third
function allows you to change the filter operation from “positive” to “negative.” The
positive filter operation causes frames with the specified MAC addresses to be filtered. The
negative filter operation causes frames with the specified MAC addresses to be forwarded.
You may easily prevent any station on one segment from accessing a specific resource on
the other segment; for this, “positive” filtering and the use of “Filter if Destination” would
be appropriate. If you want to disallow a specific station from accessing any service, “Filter
if Source” could be used.
You may easily prevent stations on one segment from accessing all but a specific resource
on the other segment; for this, “negative” filtering and the use of “Forward if Destination”
would be appropriate. If you want to disallow all but one specific station from accessing any
service on the other segment, the use of “Forward if Source” could be used.