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Introduction to IT and
Communications Technology
Ethernet Switching
CE00378-1
Justin Champion
C208 – 3292
Content

Looking at
 Switching
 Bridges
 Collision
Domains
Switch operation

A switch works with the inspection of the MAC
address
 If
the switch has a particular device attached the data
will be forwarded to that machine
Bridge operation
Bridges
Full duplex
Transmit and receive concurrently due to separate cables for each
Network latency

This is the measure of the transfer time of a
Ethernet frame
 The

latency of the network is an important issue
Particularly with Multimedia low Latency is needed
Cut-through

These switches
 Will
start to forward the frame as soon as the
destination MAC address has been read
Reduces latency in the network
 Reduces reliability of the switch

Store-and-forward

The frames will be stored in the switch
 They
are then forwarded to the destination
When stored the frames can be checked for errors
 This increases latency on the network
 Does increase reliability


Prevents corrupted packets being forwarded
Spanning-tree-operation

This is a protocol used between routers
 It
is responsible for generating routes between points
 Ensuring that looping does not take place
 Storing alternative routes, in the event of failure
STP states

Each port which uses Spanning Tree Protocol (STP)

Will always be in one of the following 5 states

Blocking




Listening



The port is preparing to transfer Ethernet frames
Building a bridging table
Forwarding


Building an active knowledge of the topology connected
Learning


The port will only receive bridge protocol data unit (BDPU) frames
These are used to exchange information with already operating ports on the
network
When a switch is first turned on it will be in this state
Sending or receiving Ethernet frames
Disabled

This port will not be involved with the use of STP
Collision domains

A collision domain
 Is
the part of the cable where multiple devices may
potentially communicate at once

A good network will have small collision domains and therefore
less likely to have collisions corrupting data
Collision domain segmentation

Network devices which operate at different layers effect the
collision domain

Layer 1 devices – repeaters, Hubs


Layer 2 devices – Switches, Bridges


Will extend the domain
Will break up the domain
Layer 3 devices – routers

Will break up the domain
Increasing a collision domain
If a computer anywhere in this network
communicates every device will receive
the frame!
Repeaters extend
the size of the
collision domain
Four repeaters (part of 5-4-3-2-1 rule)





5 segments of
network media
4 repeaters or hubs
3 host segments of
the network
2 link sections with no
hosts
1 large collision
domain
Round-trip delay calculation
Extension of a collision domain

Layer 1 devices just repeat the signal with NO
interpretation
 This
extends the collision domain, by forwarding all
traffic
Limiting the collision domain

The following items will limit the collision
domain
 These
devices processes the data before
forwarding
Bridge
 Router
 Switch

A bridge can segment a collision
domain
A bridge can segment a collision
domain
Bridge
A bridge can segment a collision
domain
Broadcasts in a bridged environment

Level 2 devices
 These
will segment the collision domain
However they will have no effect on the broadcast domain
 A broadcast packet will be sent on all ports to all devices

Effect of broadcast radiation on hosts in an IP network
Average number of broadcasts and multicasts for IP
Broadcast domain segmentation
Data flow through a network
Segments
Summary

Looked at Ethernet devices
 Operations
of switches
 Types of switches
Store and forward
 Cut through

 Network
latency
 Collision domains
 Broadcast domains