3Com OfficeConnect 3C840, User Manual

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How Bridges Learn A-3
address table, the bridge knows the destination is on the LAN and
therefore discards or filters the frame.
If the destination address is not in the address table, the bridge transmits
the data across the bridge. It does this even if the destination device is on
the local LAN because it does not recognize the destination station as
local. However, if the destination device is on the local LAN, once it replies
to the original source station, its own source address is part of the data
frame and it is learned by the bridge and added to the address table.
By operating in this way, the amount of data forwarded by the bridge is
kept to a minimum. Traffic that is for devices on the attached LAN is rarely
forwarded over the bridge.
A bridge can be configured to forget or age a station's address after a
period of inactivity, a facility that is used to ensure that stations that are
no longer attached to the LAN, do not remain in the bridge's address
table, using up space that may be required for other station's addresses.
Some bridges allow address information to be manually configured into
the bridge, provided the automatic learning facility is turned off. This will
not normally prove necessary unless specific traffic filtering is required.
You can also configure a number of other features to improve the
performance and operation of the OfficeConnect Remote 840. These
include sophisticated filtering techniques so that only certain types of
frames, or those associated with particular work groups, are passed
between specific segments.
Bridging Between
Remote Sites
The OfficeConnect Remote 840 is able to send frames between LANs that
may be separated by considerable physical distances. It achieves this by
making use WAN links. WANs can be established by using either digital
leased lines, ISDN lines, or analog (modem) lines and are usually operated
by telephone companies or other service providers.
Figure A-1 shows two LAN segments, A and B, which are connected by a
pair of OfficeConnect Remote 840 units, 1 and 2. The type of link
between the two depends on the WAN services available at each of the
remote bridge locations, and the price the network administrator is
willing to pay for those services.
840ug.book Page 3 Friday, July 7, 2000 2:23 PM