Switching Concepts

1
Switching Concepts

• Introduction to Ethernet/802.3 LANs
• Introduction to LAN Switching
• Switch Operation

2
Switching Concepts

• Introduction to Ethernet/802.3 LANs

3
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

4
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!

5
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

6
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

7
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

8
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

9
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

• Full duplex
• Host can transmit immediately
• 2 hosts can transmit simultaneously
• No collisions
• 100 bandwidth available
• Requires dedicated connection to a switchport

10
Switching Concepts

• Introduction to LAN Switching
• Switch Operation

11
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

12
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

13
LAN Segmentation - Switches

• Each switchport is a collision domain
  micro-segmentation
• 100 bandwidth available to each switchport
• Every switchport can send/receive simultaneously
• Host to switch/switch to switch connection
  creates full duplex link

14
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

15
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

16
Switching Methods

• 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

Increased Latency
17
Terminology

• Ignoring a frame filtering
• Copying a frame forwarding
• Microsegmentation dividing a network into
  smaller segments (using a switch)

18
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