Most Ethernet LAN switches use a very cool system called transparent
bridging to create their address lookup tables. Transparent bridging
is a technology that allows a switch to learn everything it needs to know
about the location of nodes on the network without the network
administrator having to do anything. Transparent bridging has five parts:
Here's how it works:
Click on the menu terms to learn more about how transparent
The switch is added to the network,
and the various segments are plugged into the switch's ports.
A computer (Node A) on the first
segment (Segment A) sends data to a computer (Node B) on another
segment (Segment C).
The switch gets the first packet of
data from Node A. It reads the MAC address and saves it to the lookup
table for Segment A. The switch now knows where to find Node A anytime
a packet is addressed to it. This process is called learning.
Since the switch does not know where
Node B is, it sends the packet to all of the segments except the one
that it arrived on (Segment A). When a switch sends a packet out to
all segments to find a specific node, it is called flooding.
Node B gets the packet and sends a
packet back to Node A in acknowledgement.
The packet from Node B arrives at the
switch. Now the switch can add the MAC address of Node B to the lookup
table for Segment C. Since the switch already knows the address of
Node A, it sends the packet directly to it. Because Node A is on a
different segment than Node B, the switch must connect the two
segments to send the packet. This is known as forwarding.
The next packet from Node A to Node B
arrives at the switch. The switch now has the address of Node B, too,
so it forwards the packet directly to Node B.
Node C sends information to the switch
for Node A. The switch looks at the MAC address for Node C and adds it
to the lookup table for Segment A. The switch already has the address
for Node A and determines that both nodes are on the same segment, so
it does not need to connect Segment A to another segment for the data
to travel from Node C to Node A. Therefore, the switch will ignore
packets traveling between nodes on the same segment. This is filtering.
Learning and flooding continue as the
switch adds nodes to the lookup tables. Most switches have plenty of memory
in a switch for maintaining the lookup tables; but to optimize the use
of this memory, they still remove older information so that the switch
doesn't waste time searching through stale addresses. To do this,
switches use a technique called aging. Basically, when an entry
is added to the lookup table for a node, it is given a timestamp. Each
time a packet is received from a node, the timestamp is updated. The
switch has a user-configurable timer that erases the entry after a
certain amount of time with no activity from that node. This frees up
valuable memory resources for other entries. As you can see,
transparent bridging is a great and essentially maintenance-free way
to add and manage all the information a switch needs to do its job!
In our example, two nodes share
segment A, while the switch creates independent segments for Node B and
Node D. In an ideal LAN-switched network, every node would have its own
segment. This would eliminate the possibility of collisions and also the
need for filtering.