Chương 8:
ETHERNET SWITCHING
Overview
• Shared Ethernet works extremely well
under ideal conditions. When the number
of devices trying to access the network is
low, the number of collisions stays well
within acceptable limits.

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• when the number of users on the network
increases, the increased number of
collisions can cause intolerably bad
performance. Bridging was developed to
help ease performance problems that
arose from increased collisions. Switching
evolved from bridging to become the key
technology in modern Ethernet LANs.

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• The concept of collision domains and
broadcast domains is concerned with the
ways that networks can be designed to limit

the negative effects of collisions and
broadcasts. This module explores the effects
of collisions and broadcasts on network traffic
and then describes how bridges and routers
are used to segment networks for improved
performance.

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• Students completing this module should be
able to:
– Define bridging and switching.
– Define and describe the content-addressable
memory (CAM) table.
– Define latency.
– Describe store-and forward and cut-through
switching modes.
– Explain Spanning-Tree Protocol (STP).
– Define collisions, broadcasts, collision domains,
and broadcast domains.
– Identify the Layer 1, 2, and 3 devices used to
create collision domains and broadcast domains.
– Discuss data flow and problems with broadcasts.
– Explain network segmentation and list the
devices used to create segments.
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8.1. Ethernet Switching
8.1.1 Layer 2 bridging
• As more nodes are added to an Ethernet
physical segment, contention for the media
increases. Ethernet is a shared media,
which means only one node can transmit
data at a time. The addition of more nodes
increases the demands on the available
bandwidth and places additional loads on
the media.
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• By increasing the number of nodes on a
single segment, the probability of collisions
increases, resulting in more retransmissions.
A solution to the problem is to break the large
segment into parts and separate it into
isolated collision domains.
• To accomplish this a bridge keeps a table of
MAC addresses and the associated ports.
The bridge then forwards or discards frames
based on the table entries. The following
steps illustrate the operation of a bridge:
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– The bridge has just been started so the
bridge table is empty. The bridge just waits
for traffic on the segment. When traffic is
detected, it is processed by the bridge.
– Host A is pinging Host B. Since the data is
transmitted on the entire collision domain
segment, both the bridge and Host B
process the packet.
– The bridge adds the source address of the
frame to its bridge table. Since the address
was in the source address field and the
frame was received on port 1, the frame
must be associated with port 1 in the table.
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– The destination address of the frame is
checked against the bridge table. Since the
address is not in the table, even though it is
on the same collision domain, the frame is
forwarded to the other segment. The
address of Host B has not been recorded yet
as only the source address of a frame is
recorded.
– Host B processes the ping request and
transmits a ping reply back to Host A. The
data is transmitted over the whole collision
domain. Both Host A and the bridge receive
the frame and process it.
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– The bridge adds the source address of
the frame to its bridge table. Since the
source address was not in the bridge
table and was received on port 1, the
source address of the frame must be
associated with port 1 in the table.

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– Host A is now going to ping Host C. Since
the data is transmitted on the entire
collision domain segment, both the bridge
and Host B process the frame. Host B
discards the frame as it was not the
intended destination.
– The bridge adds the source address of the
frame to its bridge table. Since the address
is already entered into the bridge table the
entry is just renewed.

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– The destination address of the frame is
checked against the bridge table to see if its
entry is there. Since the address is not in the
table, the frame is forwarded to the other
segment. The address of Host C has not
been recorded yet as only the source
address of a frame is recorded.

– Host C processes the ping request and
transmits a ping reply back to Host A. The
data is transmitted over the whole collision
domain. Both Host D and the bridge receive
the frame and process it. Host D discards the
frame, as it was not the intended destination.
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– The destination address of the frame is
checked against the bridge table to see
if its entry is present. The address is in
the table but it is associated with port 1,
so the frame is forwarded to the other
segment.
– The bridge adds the source address of
the frame to its bridge table. Since the
address was in the source address field
and the frame was received on port 2,
the frame must be associated with port 2
in the table.
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– When Host D transmits data, its MAC
address will also be recorded in the
bridge table
• These are the steps that a bridge uses to
forward and discard frames that are
received on any of its ports.

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8.1.2. Layer 2 switching
• Generally, a bridge has only two ports and
divides a collision domain into two parts.
All decisions made by a bridge are based
on MAC or Layer 2 addressing and do not
affect the logical or Layer 3 addressing.
Thus, a bridge will divide a collision
domain but has no effect on a logical or
broadcast domain.
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• A switch is essentially a fast, multi-port bridge,
which can contain dozens of ports.  Rather
than creating two collision domains, each port
creates its own collision domain. In a network
of twenty nodes, twenty collision domains
exist if each node is plugged into its own
switch port. If an uplink port is included, one
switch creates twenty-one single-node
collision domains. A switch dynamically builds
and
maintains
a
Content-Addressable
Memory (CAM) table, holding all of the
necessary MAC information for each port.
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8.1.3. Switch operation

• A switch is simply a bridge with many
ports. When only one node is connected to
a switch port, the collision domain on the
shared media contains only two nodes.
The two nodes in this small segment, or
collision domain, consist of the switch port
and the host connected to it. These small
physical
segments
are
called
microsegments.
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• In a network that uses twisted-pair cabling,
one pair is used to carry the transmitted
signal from one node to the other node. A
separate pair is used for the return or
received signal. It is possible for signals to
pass through both pairs simultaneously.
The capability of communication in both
directions at once is known as full duplex.


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