In Figure , four hosts are all
connected to a switch. Switches forward frames based on Layer 2
addresses (i.e. Ethernet MAC addresses). The switch learns these
addresses by examining the Layer 2 Source Address of the Ethernet
frame, as the switch receives it.
Note: For more information on how switches learn addresses,
review Semester 1 information.
Assume that the switch has just been powered on, which means that
the switch’s Source Address Table (SAT), also known as the switch
table, is empty, as shown in Figure .
The switch table is how the switch associates what devices are
connected to which switch port. A switch associates which Hosts are on
which port by “learning,” which is why they are sometimes called
“learning switches.”
When a switch is first turned on, it will not have any entries in
its switching table. Each time it sees a Frame it will examine the MAC
Source Address. If the MAC Source Address is not in its table , the
switch will enter the address into its switch table associated with
the port that it was heard on.
In example in Figure , Host
A sends a Ethernet frame to Host D. When the switch receives the
Ethernet frame from Host A destined for Host D, it stores the frame in
its memory buffer. The switch examines the MAC Source Address to see
if that particular MAC address is in its SAT table. In this case, it
is not, so it adds it to the table, for switch port 1. It has now “learned”
that Host A, (MAC address 0000.0c11.1111) can be reached via switch
port 1.
Next, the switch examines the Ethernet frame Destination MAC
address and searches for this address in its SAT table. Because, this
address does not exist in the SAT table, the switch floods it
out all ports. A switch will either filter or flood
frames based on its SAT table. As you just learned, a frame gets flooded
(sent out all ports except for the incoming port) when the switch does
not have an entry for the destination MAC address in its SAT table.
A switch filters a frame, when it does have the Ethernet
frame’s destination MAC address in its SAT table. This means that
the switch knows which port the destination MAC address can be
reached, and only forwards the frame out that port.
Note: If the SAT table shows that the destination MAC address
is on the same port as the incoming frame, then the switch does not
send the frame out any port, including the incoming port. If a switch
were to ever send an Ethernet frame out an incoming port, this would
cause duplicate frames on the network.
Now, you will see what happens when Host D sends information back
to Host A. Host D sends a frame with the Destination MAC address of
Host A, 0000.0c11.1111. 
The switch examines the MAC Source Address to see if the MAC
address of Host D is in its SAT table. In this case, it is not, so it
adds it to the table, for switch port 4. It has now “learned” that
Host D, (MAC address 0000.0c44.444) can be reached via switch port 4.
Next, the switch examines the Destination MAC address of the Ethernet
frame, which is 0000.0c11.1111. The switch looks up this address in
its SAT table and acknowledges that it is in its table. The switch
forwards this frame out switch port 1 to reach Host A.
This is an example of a switch filtering a frame.
Question: What would happen if Host A needed to send an
Ethernet frame to Host D?
Answer: Because Host D’s MAC address (0000.0c44.4444) is
in the switch’s SAT table, the switch would know to forward this
frame only out switch port 4. As shown in Figure ,
this is another example of the switch filtering the frame.
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