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Layer 2 LAN Switching
153
In the figure, there are four hosts attached to a switch. The switch has
nothing in the MAC address table when it is powered on. The figure shows
the switch's MAC filter table after each device has communicated with the
switch. The following steps show how the table is propagated:
1.
Station 1 sends a frame to station 3. Station 1 has a MAC address of
0000.8c01.1111. Station 3 has a MAC address of 0000.8c01.2222.
2.
The switch receives the frame on Ethernet interface 0/0 and places the
source address in the MAC address table.
3.
Because the destination address is not in the MAC database, the frame
is forwarded out all interfaces.
4.
Station 3 receives the frame and responds to station 1. The switch
receives this frame on interface E0/2 and places the source hardware
address in the MAC database.
5.
Station 1 and station 3 can now make a point-to-point connection and
only the two devices will receive the frames. Stations 2 and 4 will not
see the frames.
If the two devices do not communicate with the switch again within a cer-
tain time limit, the switch will flush the entries from the database to keep the
database as current as possible.
Forward/Filter Decision
The layer 2 switch also uses the MAC filter table to both forward and filter
frames received on the switch. When a frame arrives at a switch interface, the
destination hardware address is compared to the forward/filter MAC data-
base. If the destination hardware address is known and listed in the database,
the frame is sent out only on the correct exit interface. The switch does not
transmit the frame out of any interface except for the destination interface.
This preserves bandwidth on the other network segments. This is called
frame filtering.
If the destination hardware address is not listed in the MAC database, the
frame is broadcasted out all active interfaces except the interface on which
the frame was received. If a device answers the broadcast, the MAC database
is updated with the device location (interface).
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