CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Switching Concepts
Introduction to Ethernet/802.3 LANs
Introduction to LAN Switching
Switch Operation
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Switching Concepts
Introduction to Ethernet/802.3 LANs
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Ethernet Technology Overview
• Ethernet multi-access
broadcast technology
• Uses CSMA/CD
• Collisions impact on
network performance
• Layer 2 devices can
improve performance
• Media includes CAT5(e),
fibre, wireless
• Speeds from 10Mbps to
10,000 Mbps
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Network Growth
• Bandwidth needs have increased
– Internet/intranet/email
– Multimedia
– Increasing use of enterprise servers
• Ethernet has developed to meet challenge
– 10Mbps, 100Mbps, 1000Mbps, 10Gbit
– Coaxial, Twisted Pair, Fibre Optic, Wireless
– Repeaters, hubs, bridges, switches, routers
• BUT you must understand the features of all this
technology to gain best performance in your network
design!
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Hubs
• Layer 1 devices
• Regenerate, retime,
amplify signals
• 1 collision/bandwidth
domain
• Broadcasts propagated
out of every port
• Only 1 device can
transmit at a time
• Only 50-60% bandwidth
available
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Bridges
• Layer 2 device
• Splits network into 2
collision/bandwidth
domains
• Broadcasts are forwarded
• Local traffic stays local
• Checks Layer 2 MAC
addresses in 802.3 frame
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Switches
• Layer 2 device
• Learns MAC addresses of
devices attached to each port
• Each switchport is a collision
domain
• More collision domains BUT
smaller collision domains
• Broadcasts still sent out of
every port
• Each switchport has dedicated
bandwidth
• 100% bandwidth available
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Transmission Time & Latency
•
•
•
•
•
•
•
Bit time – time taken to recognise 1 bit
Minimum frame size - 64 bytes – 512 bits
Maximum frame size – 1518 bytes – 12,144 bits
Transmission time is always 512 bit times
10Mbps – 64 byte frame - 51,200 ns (100ns bit time)
100Mbps – 64 byte frame 5,120 ns (10 ns bit time)
1000Mbps – 64 byte frame – 512 ns (1ns bit time)
Times above do not include
• Time taken to propagate signal along medium
• Delays introduced by hubs/switches/routers/NICs etc
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Types of Transmission
Half-duplex
• Host checks medium
for signal – if clear
host transmits
• Only 1 host can
transmit at a time
• Collisions – jam
signal generated,
back-off algorithm
before retransmission
• 50-60% bandwidth
available
Chapter 4 – Switching Concepts
Full duplex
• Host can transmit
immediately
• 2 hosts can transmit
simultaneously
• No collisions
• 100% bandwidth
available
• Requires dedicated
connection to a
switchport
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Switching Concepts
Introduction to LAN Switching
Switch Operation
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Overview
• Maximum availability for the least cost
– Reduce the effects of collisions on available
bandwidth
– Reduce the effect of broadcasts on available
bandwidth
– Deploy network hardware (media/switches/routers) to
overcome bottlenecks & meet bandwidth
requirements
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
LAN Segmentation - bridges
• A bridge splits a LAN into
2 segments
• It creates 2 collision
domains
• Adds 10-30% latency
• Learns MAC addresses
• Keeps local traffic local
• Forwards broadcasts
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
LAN Segmentation - Switches
• Each switchport is a
collision domain – “microsegmentation”
• 100% bandwidth
available to each
switchport
• Every switchport can
send/receive
simultaneously
• Host to switch/switch to
switch connection creates
full duplex link
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
LAN Switch Operation - 1
• When a switch starts up it sends a broadcast out
of all ports to learn host MAC addresses
• When a frame is received for an unknown
destination a broadcast is sent to discover
• Addresses are added to a switching table
mapping them to the port on which they were
learned
• When a frame is received for a known
destination it is switched to the appropriate port
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
LAN Switch Operation - 2
• Switches contain RAM – known as CAM
“Content Addressable Memory”
• Stores MAC address table
• Used as frame buffer
• Used to queue frames in asymmetric switching –
switchports operating at different speeds e.g. 10
and 100 Mbps
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Cut-through Switching
• Fast-forward – as soon as destination address is read
switching starts
• Fragment-free – after 64 bytes have been received
(minimum valid frame size) frame is switched
Store & Forward Switching
• Entire frame is received before switching
Chapter 4 – Switching Concepts
Increased Latency
Switching Methods
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Terminology
• Ignoring a frame – filtering
• Copying a frame – forwarding
• Microsegmentation – dividing a network into
smaller segments (using a switch)
Chapter 4 – Switching Concepts
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Broadcasts
• Bridges & switches cannot block layer 2 or layer
3 broadcasts
• Adding bridges or switches to a network extends
the broadcast domain but creates additional
collision domains – a 24 port switch creates 24
collision domains
• Routers can inspect layer 3 packets and create
broadcast domains – a router with 3 ports
creates 3 broadcast domains
Chapter 4 – Switching Concepts