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09 Ethernet CCNA Exploration Chapter 9

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1 09 Ethernet CCNA Exploration Chapter 9
31/03/2017 Ethernet CCNA Exploration Chapter 9 S Ward Abingdon and Witney College

2 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:

3 Ethernet 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 Access Ethernet

4 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

5 Ethernet History 802.2 Ethernet 802.3
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 Ethernet 802.3 802.2 MAC LLC

6 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 MAC LLC

7 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)

8 Shared Medium Physical bus topology 10Base5 (thick coaxial cable, distance 500m) and 10Base2 (thin coaxial cable, distance 185m) Physical star topology 10BaseT (UTP cable, distance 100m, hubs) Collisions happen – but managed with CSMA/CD

9 Hubs and Switches “Legacy Ethernet” Collisions are managed by CSMA/CD
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

10 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

11 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

12 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

13 Review of Hubs and Switches
Shared medium Shared bandwidth Collisions Point to point links Dedicated bandwidth Use full duplex – no collisions Hub Switch

14 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)

15 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

16 An Ethernet Frame – 7 fields
Packet from Network layer is encapsulated Packet Frame header Packet Trailer Preamble Destination address Start of frame delimiter Source address Length /type 7 1 6 2 Packet Data Frame Check Seq. 4 Field sizes in bytes. Preamble and StartFD are not counted in frame size. Frame size is 64 to 1518 bytes (VLAN’s 1522b).

17 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

18 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

19 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

20 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

21 Writing/Reading a MAC Address
Hex digits are written in different ways: A-3C-78-00 00:05:9A:3C:78:00 0005.9A3C.7800 All of these are the same mac address A = manufacturer’s ID, assigned by IEEE and 3C = assigned by manufacturer (1st - ipconfig/all to get mac address)

22 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

23 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)

24 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 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 to

25 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

26 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 PC can send

27 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

28 Listen for signals Are there signals on the cable? Yes.

29 Wait if there are signals
Wait until there are no more signals

30 Listen for signals Are there signals on the cable now? No.

31 Put signals on cable Put my signals on the cable.

32 Listen for collisions: no
No collision. All is well. My message was sent.

33 Listen for collisions: yes
There is a collision. Stop sending signals. Send jamming signal. My message is lost.

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

35 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

36 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

37 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

38 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

39 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

40 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

41 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

42 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

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

44 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

45 Proxy ARP See
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

46 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 = ??

47 The End Complete Packet Tracer Labs in Chapter 9
Open cisco netacad; launch chapter 9; type in lab #’s

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