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Ethernet

CCNA Exploration

Chapter 9

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Jane’s educated guess as to why the word ‘Ethernet’?

“Aristotle was a Greek philosopher born in 384 BC. He was one of the greatest thinkers of the world and his written works encompassed all major areas of thought. Aristotle mistakenly believed that the Earth was at the center of the universe and made up of only four elements: earth, water, air, and fire. He also thought that celestial bodies such as the sun, moon, and stars, were perfect and divine, and made of a fifth element called ETHER.” Source: http://www.windows2universe.org/people/ancient_epoch/aristotle.html

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Ethernet OSI Model Layers 1 (physical) and 2 (data link) TCP/IP Model Network Access layer

Application

Presentation

Session

Transport

Network

Data link

Physical

Application

Transport

Internet

Network AccessEthernet

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Ethernet Most common LAN technology today

Star Topology (Physical) Point-to-Point Topology (Logical) see p. 323

Different media (copper cable, optical fibre) Different bandwidths

100Mbps - Fast Ethernet 1000Mbps - Gigabit Ethernet

Same addressing scheme – mac/physical Same basic frame format

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Ethernet History

First LAN was Ethernet, designed at Xerox 1980 Ethernet standard published by DIX (Digital,

Intel, Xerox) 1985 IEEE modified Ethernet standard and

published as 802.3

Ethernet802.3802.2

MACLLC

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Sublayers

Logical Link Control sublayer links to upper layers; is independent of equipment

Media Access Control sublayer provides addressing; frame format, error detection, CSMA/CD

Physical Layer handles bits; puts signals on the medium, detects signals

MACLLC

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Advantages of Ethernet

Simplicity and ease of maintenance Ability to incorporate new technologies

(e.g. fiber optic, higher bandwidths) Reliability Low cost of installation and upgrade 100BaseT (Fast Ethernet, UTP) 1000BaseT(Gigabit Ethernet, UTP) 1000BaseX (Gigabit Ethernet, Fiber)

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Shared Medium

Physical bus topology10Base5 (thick coaxial cable, distance 500m) and10Base2 (thin coaxial cable, distance 185m)

Physical star topology10BaseT (UTP cable, distance 100m, hubs)

Collisions happen – but managed with CSMA/CD

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Hubs and Switches

“Legacy Ethernet” 10Base5, 10Base2 or 10BaseT (1990) with hubs;

designed to work with collisions; devices transmit at the same time

Collisions are managed by CSMA/CD Poor performance if a lot of traffic and

therefore a lot of collisions Collisions avoided by using switches and full

duplex operation

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Hubs and Switches

Switch forwards frames only to the intended destination (known address)

- Dedicated ports

Hub forwards frames through all ports (except incoming port)

- Floods the network

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Half Duplex Transmission

Hubs (dumb hub) One-way traffic, i.e.

walkie talkie Necessary on a shared

medium If PC1 is transmitting,

but also detects incoming signals, then there is a collision

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Full Duplex Transmission

Switches (smart/intelligent ‘hub’)

Two way traffic, i.e. telephone PC can transmit and receive

at same time Not on shared mediums – full

bandwidth used Switches minimize possibility

of collisions No collisions – 99.9% free

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Review of Hubs and Switches

Shared medium Shared bandwidth Collisions

Point to point links Dedicated bandwidth Use full duplex – no collisions

Hub

Switch

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Fast and Gigabit Ethernet

Moving from hubs to switches came higher bandwidth: 100 Mbps - Fast Ethernet (1995) only 2 pairs of wires needed to operate, Cat5 or 5e distance is still 100 metres

Later came 1000 Mbps - Gigabit Ethernet (1999) all 4 pairs of wires needed to operate, Cat5e, 100m i.e. Voice over IP (VoIP) and multimedia services

Gigabit Ethernet requires fully switched (no hubs) and full duplex operation (send and receive)

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LAN, MAN, and WAN

Ethernet was developed for local area networks (LANs) confined to a single building or group of buildings on one site

Using fiber optics and gigabit speeds, Ethernet can be used for Metropolitan Area Networks (MANs) throughout a town or city

Ethernet can even be used over larger areas so distinction between LAN and WAN is no longer clear

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An Ethernet Frame – 7 fields

Packet

Packet TrailerFrame header

Packet from Network layer is encapsulated

Preamble Destination address

Start of frame

delimiter

Sourceaddress

Length/type

7 1 6 6 2

PacketData

FrameCheckSeq.

46-1500 4

Field sizes in bytes. Preamble and StartFD are not counted in frame size. Frame size is 64 to 1518 bytes (VLAN’s 1522b).

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Frame Fields – see pgs.325-326 Preamble and start of frame delimiter:

acts as a wake-up call, helps synchronization, shows where frame starts

Destination Address: MAC address of destination, 6 bytes hold 12

hexadecimal digits; switches use this address to forward frames

Source Address: MAC address of sender, 6 bytes hold 12

hexadecimal digits; switches use this address to add entries in their lookup tables

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Frame Fields (continued)

Length or type field: 2 bytes define exact length of data field length or type values used used later in CRC process upper-layer protocol type is added

Ethernet II is frame format used in TCP/IP networks – 802.3

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Frame Fields (continued) Data and Pad fields

contains Layer 3 PDU = an IP packet if packet is less than 64 bytes, then field length is

made up to 64 bytes with a “pad” of zeros Frame Check Sequence field

used for CRC (cyclic redundancy check) to detect corrupt frames

Sender=results of CRC Receiver=generates a CRC

If calculations match – no errors If calculations do not match – frame is dropped

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Ethernet MAC Address

Unique identification for a device (or NIC) Burned into ROM -- copied to RAM First 3 bytes identify manufacturer

(Organizationally Unique Identifier-OUI) Nic (device) reads destination MAC address

to see if it should process frame Switch reads destination MAC address to see

where it should forward frame

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Writing/Reading a MAC Address Hex digits are written in different ways:

00-05-9A-3C-78-00 00:05:9A:3C:78:00 0005.9A3C.7800

All of these are the same mac address 00-05-9A = manufacturer’s ID, assigned by

IEEE and 3C-78-00 = assigned by manufacturer

http://standards.ieee.org/develop/regauth/oui/oui.txt (1st - ipconfig/all to get mac address)

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Different Addresses MAC addresses are used to identify devices

within a network (switches) MAC addresses are Layer 2 addresses in frame

header IP addresses are used to pass data between

networks (routers) IP addresses are Layer 3 addresses in packet

header The addresses identify the network and device

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Packets on a long journey…

Packet header with IP addresses is created by source host and stays the same throughout the journey

Frame header is stripped off and replaced by each router, so MAC addresses are different for every hop of the journey (routers’ macs)

If parts of the journey are not over Ethernet, then there will be a different addressing system used (i.e. LocalTalk or IPX/SPX protocols)

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Unicast, Multicast, Broadcast

Unicast: message sent to one particular host it must contain the destination host’s IP address and

MAC address Broadcast: message for all hosts on a network

“Host” part of IP address is all binary 1s. i.e. 192.168.1.255 MAC address is all binary 1s,

FF:FF:FF:FF:FF:FF in hex Multicast: message for a group of devices

using IP address range 224.0.0.0 to 239.255.255.255

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More on Collisions

Ethernet originally used shared coaxial cable If hosts transmitted at the same time, there

was a collision Later networks used hubs and UTP cable, but

medium is still shared and collisions occurred

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Hubs and Collision Domains

Collision domain – area where collisions occur Add more hubs and PCs – collision domain

gets bigger = more traffic, more collisions Hosts connected by hubs share bandwidth

Only one PCcan send

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CSMA/CD

Carrier Sense: ‘Listen’ to see if there are signals on the cable

Multiple Access: Hosts share the same cable and all have access to it

Collision Detection: Detect and manage any collisions of signals when they occur

This is the ‘first come, first served’ method of letting hosts put signals on the medium

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Listen for signals

Are there signals on the cable?Yes.

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Wait if there are signals

Wait until there are no more signals

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Listen for signals

Are there signals on the cable now?No.

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Put signals on cable

Put my signals on the cable.

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Listen for collisions: no

No collision.All is well.My message was sent.

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Listen for collisions: yes

There is a collision.Stop sending signals.Send jamming signal.My message is lost.

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Listen again

No signals now.Wait for a random length of time.Send message again.

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CSMA/CD Collisions happen if a host transmits when

there is a signal on the cable but the host does not yet know about it

Latency is the time a signal takes to travel to the far end of a cable

The longer the cable and the more intermediate devices, then more latency

All clear

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CSMA/CD If a host detects a collision while it is sending

the first 64 bits of a frame, then CSMA/CD works and the frame will get resent later

If the host has sent 64 bits and then detects a collision, it is too late; it will not resend

Latency must be small enough so that all collisions are detected in time

This limits cable length and the number of intermediate devices

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Some Definitions

Latency or propagation delay: the time it takes for a signal to pass from source to destination

Bit time: the time it takes for a device to put one bit on the cable (Or for the receiving device to read it)

Slot time: the time for a signal to travel to the far end of the largest allowed network; maximum time required to detect a collision

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Interframe Spacing

The time between the end of one frame and the start of the next frame

Gives the medium a chance to stabilize Gives devices time to process the frame Devices wait a minimum of 96 bit times after

a frame has arrived before they can send 9.6 microseconds for 10 Mbps Ethernet 0.96 microseconds for 100 Mbps Ethernet

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How Switch Tables Work

Switch builds a switching (lookup) table matching its port numbers to the MAC addresses of devices connected to it

When a frame arrives, it reads the destination MAC address, looks it up in the table, finds the right port and forwards the frame

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Switch Does Flooding

If the switch does not find the destination address in its table, then it floods the frame through all ports except the incoming port to find the destination address (floods the network)

Broadcast messages also get flooded in networks, i.e. address resolution protocol IP to MAC address mapping, arp requests and

arp replies

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Switch Learns Addresses switch learns addresses by looking at the

source MAC address of an incoming frame then matches the address to the port where

the frame came in and puts the information in its table (RAM table)

entries are time stamped and removed from the table when time runs out (“aging”)

entries can be refreshed when another frame comes in from the same host

Check out http://computer.howstuffworks.com/lan-switch11.htm

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Address Resolution Protocol (ARP) Table – Layer 2 protocol

A host PC wants to send a message It knows the destination IP address and puts

it in the packet header It looks in its own ARP table and finds the

corresponding MAC address It puts the MAC address in the frame header

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Address Resolution Protocol

A host wants to send a message It knows the destination IP address The destination MAC address is not in its ARP

table Host broadcasts “Calling 192.168.1.7, what is

your MAC address?” 192.168.1.7 replies “My MAC address is…” Host sends message and updates ARP table

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Remote Addresses

Host can see that destination IP address is on another network

It finds the IP address of the default gateway It sends an ARP request for the matching

MAC address of the default gateway Default gateway router replies and gives its

own MAC address Host sends message via router and updates

its ARP table

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Proxy ARP See http://www.visualland.net/view.php?cid=862

If a host cannot tell that the destination IP address is on another network, it will send an ARP request asking for the matching MAC address

The router will reply, giving its own MAC address

Router: "send it to me, and I'll get it to where it needs to go"

The host will send the message via the router

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ARP Broadcasts arp is a protocol of IPv4 protocol suite IPv6 LANs use NDP (neighbor discovery

protocol) to translate 128-bit IPv6 (logical) addresses into 48-bit hardware (physical) addresses

Open command prompt window U:\>arp/? U:\>arp –a [look at your command output]

Interface = ??

Internet Addresses = ??

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The End

Complete Packet Tracer Labs

in Chapter 9-Open cisco netacad; launch

chapter 9; type in lab #’s