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1© 2009, Cisco Systems, Inc. All rights reserved.
Module 2
Internetworking Devices
By Dr. Percy DIAS

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Physical Topology
• Physical topology is the actual layout of the
wire or media.

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Physical Topology
• Bus
–Connects all the devices using a single cable
–Main cable segment must end with a terminator that
absorbs the signal when it reaches the end (data can
bounces back)
• Star
–Most commonly used in Ethernet LAN
–Central connection point can be a hub, switch or
router, and might be desirable for security or restricted
access
–If the central device fails, the whole network becomes
disconnected

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Physical Topology
• Ring
–Frame travels around the ring, stopped at
each node
–If a node wants to transmit data, it is
permitted to add data as well as destination
address
–Each device cleans up the signal, so fewer
repeaters are needed

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Physical Topology
• Dual Ring
–If one ring fails, data can be transmitted on the
other ring
• Full-mesh
–Connect all devices to each other for
redundancy and fault tolerance
• Partial-mesh
–At least one device maintains multiple
connections to others

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Logical Topology
• Logical topology of a network refers to the
logical paths that signals travel from one
point on the network to another
• Broadcast
– Ethernet
• Token Passing
– Controls network access by passing an
electronic token sequentially to each host
– Token Ring and FDDI are examples of Token
Passing on a physical ring topology

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Collisions
• Hubs operate at Layer 1, simply repeating all
received signals out all other ports

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Networking Devices
• End user devices
– Provide services directly to the user
• Network devices
– Connect the end-user devices to allow
them to communicate
• Data leaves a source and it is transformed
into either electrical , light pulse or radio
waves (signals) that pass along the
medium

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Networking Devices

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Repeated Ethernet Signal

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Repeated Ethernet Signal
• Betty sends a clean signal
• The signal degrades by the time it reaches
the repeater
• The repeater regenerates a new, clean
signal and sends it out its other port

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Repeaters
• When a signal is sent over a wire, it degrades
• To extend the distance of LANs, repeaters were
developed
• Typically had two ports connecting two different
Ethernet segments
• Interpreted the incoming signal on one port as
1’s and 0’s
• Sent a regenerated clean signal out the other
port

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Repeaters: Layer 1

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Hubs
• Regenerate
signals
• Used as network
concentration
points
• Multiport repeater
• Layer 1 device

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Hub Operation

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Network Interface Card
• Carries a unique code called a Media
Access Control (MAC)
• MAC address controls data communication
for the host on the network
• NICs are considered layer 2 and operate at
both layer 1 and 2 (Layer 1: send and
receive signals over an attached cable)

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Bridges
Layer 2 device designed to create two or more LAN
segments, each of which is a separate collision domain
Bridges filter traffic
by looking at MAC
addresses.

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Bridges
• Filter traffic on a LAN to keep local traffic
local yet allow connectivity to other parts
• Keep track of which MAC addresses are on
each side of the bridge and makes
forwarding decisions based on MAC address
table
• When a bridge receives a frame, the
destination MAC address is looked up in the
bridge table to determine whether to filter,
flood or forward the frame onto another
segment.

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Bridges

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Flooding Unknown Unicasts

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Learning Table Entries and
Flooding Unknown Unicasts
• Switches and bridges learn entries in the
table dynamically
• They use this logic:
– Examine the source MAC address of the
frame and the interface on which it was
received
– Add that source MAC address and
corresponding interface to the table

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Learning Content Addressable Memory
(CAM) Table Entries: Two Switches
SW2

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Bridge Making Filter Decision

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Bridge Making a Forwarding Decision

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LAN Switches
• Layer 2 device
• Use a table of MAC addresses (switching table)
to determine the segment on which a frame
needs to be transmitted
• Cisco switching table sometimes referred to as
Content Addressable Memory (CAM).
• Switches operate at much higher speed than
bridges (hardware vs software)
• Each switch port acts as a separate bridge and
give each host the medium’s full bandwidth
(microsegmentation)
• Improve network performance: speed and
bandwidth

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Switch Making a Forwarding Decision

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Switch Buffering Example
Barney

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Preventing Collisions with
Switch Buffering
• Switches prevent collisions by buffering
frames
• If several PCs send frames to the same
address at the same time, the switch holds
the frames in memory - a process called
buffering
• The switch then forwards the frames one
at a time

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Routers
• Layer 3 device
• Use logical address (network layer
address)
• Can connect different layer 2 technologies
• Examine incoming packet (layer 3 data),
choose the best path, and switch them to
the proper outgoing port.

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Cisco Academy 3 Reference
• CCNA1 Online Materials
• Slide 2 - 6: 2.1.4
• Slide 8-9, 26: 2.1.3
• Slide 12-14: 5.1.6-5.1.7
• Slide 16-19, 23-24, 26: 5.1.9-5.1.11

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Cisco Academy 4 Exploration
Reference
Networking Fundamentals
• Slide 2 - 6: 9.1.1-9.1.3
• Slide 8-9,12-14, 26: 9.6.1-9.6.4
• Slide 16-19, 23-24, 26: 9.6.1-9.6.4
• Slide 14-15: 9.6.1-9.6.2