3Com 520 - Manual (Page 56)

A-2 B R I D G I N G A N D R O U T I N G
Guidelines For Choosing Bridging or Routing
The list below outlines some of the reasons why you
might choose to configure the OfficeConnect Remote
as a bridge or a router. Read through the rest of this
appendix for more explanation and to help decide
which of the above conditions apply to your network.
■ A bridge is simpler to configure but a router can
provide more security on a busy network and filter
unwanted data transmissions more effectively.
■ If your network consists of only one or two links
between different sites and your network is not
heavily loaded, in most circumstances you can
configure your OfficeConnect Remote units as
bridges.
■ If your network structure is complicated and consists
of a mixture of leased line and ISDN links, or if it uses
several different protocols, you may obtain better
performance from the OfficeConnect Remote units if
you configure them as routers.
■ If you are connecting to a routed corporate network
that is already running IP and/or IPX protocols or if you
are using the OfficeConnect Remote to connect to the
Internet you must configure the unit as a router.
How Bridges Learn
When a bridge is first powered on, it does not know
the number or the locations of stations that are
connected to the LAN. To minimize the amount of
data passed over the bridge it must learn the
whereabouts (address) of stations to ensure that it
passes only the data that is intended to be passed
over the bridge.
Like the envelope of a letter, the header of each
frame of data transmitted on the network has a From
(source) address and To (destination) address. This
ensures that data reaches its destination on the LAN
and that the receiving station can reply. The bridge
reads every frame of data received at the LAN port
and extracts the source address of the frame. From
this information it builds an address table of stations
it knows to be on the LAN.
To decide if data should be passed over the bridge,
the bridge examines the destination address of the
frame. If the address is already in its address table,
the bridge knows the destination is on the LAN and
therefore rejects 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
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