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Chapter 2
Layer-2 Switching
Let me give you an example of how a forward/filter table is populated:
1.
Host A sends a frame to Host B. Host A's MAC address is
0000.8c01.000A; Host B's MAC address is 0000.8c01.000B.
2.
The switch receives the frame on the E0/0 interface (switch interface
addressing is covered in Appendix B) and places the source address in
the MAC address table.
3.
Since the destination address is not in the MAC database, the frame is
forwarded out all interfaces.
4.
Host B receives the frame and responds to Host A. The switch receives
this frame on interface E0/1 and places the source hardware address in
the MAC database.
5.
Host A and Host B can now make a point-to-point connection and
only the two devices will receive the frames. Hosts C and D will not see
the frames, nor are their MAC addresses found in the database
because they haven't yet sent a frame to the switch.
If Host A and Host B don't communicate to the switch again within a cer-
tain amount of time, the switch will flush their entries from the database to
keep it as current as possible.
Forward/Filter Decisions
When a frame arrives at a switch interface, the destination hardware address
is compared to the forward/filter MAC database. If the destination hardware
address is known and listed in the database, the frame is only sent out the
correct exit interface. The switch doesn't transmit the frame out any inter-
face except for the destination interface. This preserves bandwidth on the
other network segments and is called
frame filtering
.
If the destination hardware address is not listed in the MAC database,
then the frame is broadcast out all active interfaces except the interface the
frame was received on. If a device answers the broadcast, the MAC database
is updated with the device's location (interface).
If a host or server sends a broadcast on the LAN, the switch will broad-
cast the frame out all active ports by default. Remember, the switch only
creates smaller collision domains, but it's still one large broadcast domain
by default.
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