3Com LANPLEX 2500, Руководство по эксплуатации

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Address Learning 6-3
Address Learning
In Express Switching mode, the LANplex 2500 system learns the source
addresses only of stations not connected to the bridge’s backbone port. The
bridge listens to the frames transmitted by stations and stores their
addresses in a table. The address table is referenced by the filtering/
forwarding function of the bridge.
Packet Filtering
and Forwarding
As with traditional bridging, described in Chapter 5, the destination system
looks the address of each packet arriving at a bridge port (Ethernet or FDDI)
in the address table.
■
If an address match is found, then the system forwards packet directly to
the destination port.
■
If no match is found, the bridge behaves differently from a traditional
bridge. For example:
■
A traditional bridge would flood the packet to all ports except the one
on which it was received.
■
In Express Switching mode, the bridge never floods unicast or directed
packets. A packet received on the backbone port is discarded. A packet
received on a nonbackbone port is forwarded
only to the backbone
port.
Using Express
Switching
Using Express Switching mode in your network has these advantages:
■
Packets with unknown destination addresses are never flooded. Therefore,
each nonbackbone segment can be assured of being
private to the stations
that are directly connected to it and that its bandwidth cannot be intruded
upon by extraneous packets.
■
There is no limitation on the number of addresses that can be seen on the
backbone network. Therefore, the system can support very large backbone
networks.