109 Ethernet CCNA Exploration Chapter 9 31/03/2017EthernetCCNA ExplorationChapter 9S Ward Abingdon and Witney College
2Jane’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:
3Ethernet Ethernet OSI Model Layers 1 (physical) and 2 (data link) TCP/IP Model Network Access layerApplicationPresentationSessionTransportNetworkData linkPhysicalApplicationTransportInternetNetwork AccessEthernet
4Ethernet Most common LAN technology today Star Topology (Physical)Point-to-Point Topology (Logical) see p. 323Different media (copper cable, optical fibre)Different bandwidths100Mbps - Fast Ethernet1000Mbps - Gigabit EthernetSame addressing scheme – mac/physicalSame basic frame format
5Ethernet History 802.2 Ethernet 802.3 First LAN was Ethernet, designed at Xerox1980 Ethernet standard published by DIX (Digital, Intel, Xerox)1985 IEEE modified Ethernet standard and published as 802.3Ethernet802.3802.2MACLLC
6SublayersLogical Link Control sublayer links to upper layers; is independent of equipmentMedia Access Control sublayer provides addressing; frame format, error detection, CSMA/CDPhysical Layer handles bits; puts signals on the medium, detects signalsMACLLC
7Advantages of Ethernet Simplicity and ease of maintenanceAbility to incorporate new technologies(e.g. fiber optic, higher bandwidths)ReliabilityLow cost of installation and upgrade100BaseT (Fast Ethernet, UTP)1000BaseT(Gigabit Ethernet, UTP)1000BaseX (Gigabit Ethernet, Fiber)
8Shared MediumPhysical 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
9Hubs 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 timeCollisions are managed by CSMA/CDPoor performance if a lot of traffic and therefore a lot of collisionsCollisions avoided by using switches and full duplex operation
10Hubs and SwitchesSwitch forwards frames only to the intended destination (known address)- Dedicated portsHub forwards frames through all ports (except incoming port)- Floods the network
11Half Duplex Transmission Hubs (dumb hub)One-way traffic, i.e. walkie talkieNecessary on a shared mediumIf PC1 is transmitting, but also detects incoming signals, then there is a collision
12Full Duplex Transmission Switches (smart/intelligent ‘hub’)Two way traffic, i.e. telephonePC can transmit and receive at same timeNot on shared mediums – full bandwidth usedSwitches minimize possibility of collisionsNo collisions – 99.9% free
13Review of Hubs and Switches Shared mediumShared bandwidthCollisionsPoint to point linksDedicated bandwidthUse full duplex – no collisionsHubSwitch
14Fast 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 5edistance is still 100 metresLater came 1000 Mbps - Gigabit Ethernet (1999)all 4 pairs of wires needed to operate, Cat5e, 100mi.e. Voice over IP (VoIP) and multimedia servicesGigabit Ethernet requires fully switched (no hubs) and full duplex operation (send and receive)
15LAN, MAN, and WANEthernet was developed for local area networks (LANs) confined to a single building or group of buildings on one siteUsing fiber optics and gigabit speeds, Ethernet can be used for Metropolitan Area Networks (MANs) throughout a town or cityEthernet can even be used over larger areas so distinction between LAN and WAN is no longer clear
16An Ethernet Frame – 7 fields Packet from Network layer is encapsulatedPacketFrame headerPacketTrailerPreambleDestination addressStart of frame delimiterSource addressLength /type7162Packet DataFrame Check Seq.4Field sizes in bytes. Preamble and StartFD are not counted in frame size. Frame size is 64 to 1518 bytes (VLAN’s 1522b).
17Frame Fields – see pgs.325-326 Preamble and start of frame delimiter: acts as a wake-up call, helps synchronization, shows where frame startsDestination Address:MAC address of destination, 6 bytes hold 12 hexadecimal digits; switches use this address to forward framesSource Address:MAC address of sender, 6 bytes hold 12 hexadecimal digits; switches use this address to add entries in their lookup tables
18Frame Fields (continued) Length or type field:2 bytes define exact length of data fieldlength or type values usedused later in CRC processupper-layer protocol type is addedEthernet II is frame format used in TCP/IP networks – 802.3
19Frame Fields (continued) Data and Pad fieldscontains Layer 3 PDU = an IP packetif packet is less than 64 bytes, then field length is made up to 64 bytes with a “pad” of zerosFrame Check Sequence fieldused for CRC (cyclic redundancy check) to detect corrupt framesSender=results of CRCReceiver=generates a CRCIf calculations match – no errorsIf calculations do not match – frame is dropped
20Ethernet MAC Address Unique identification for a device (or NIC) Burned into ROM -- copied to RAMFirst 3 bytes identify manufacturer (Organizationally Unique Identifier-OUI)Nic (device) reads destination MAC address to see if it should process frameSwitch reads destination MAC address to see where it should forward frame
21Writing/Reading a MAC Address Hex digits are written in different ways:A-3C-78-0000:05:9A:3C:78:000005.9A3C.7800All of these are the same mac addressA = manufacturer’s ID, assigned by IEEE and 3C = assigned by manufacturer(1st - ipconfig/all to get mac address)
22Different AddressesMAC addresses are used to identify devices within a network (switches)MAC addresses are Layer 2 addresses in frame headerIP addresses are used to pass data between networks (routers)IP addresses are Layer 3 addresses in packet headerThe addresses identify the network and device
23Packets on a long journey… Packet header with IP addresses is created by source host and stays the same throughout the journeyFrame 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)
24Unicast, Multicast, Broadcast Unicast: message sent to one particular hostit must contain the destination host’s IP address and MAC addressBroadcast: 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 hexMulticast: message for a group of devices using IP address range to
25More on Collisions Ethernet originally used shared coaxial cable If hosts transmitted at the same time, there was a collisionLater networks used hubs and UTP cable, but medium is still shared and collisions occurred
26Hubs and Collision Domains Collision domain – area where collisions occurAdd more hubs and PCs – collision domain gets bigger = more traffic, more collisionsHosts connected by hubs share bandwidthOnly one PC can send
27CSMA/CDCarrier Sense: ‘Listen’ to see if there are signals on the cableMultiple Access: Hosts share the same cable and all have access to itCollision Detection: Detect and manage any collisions of signals when they occurThis is the ‘first come, first served’ method of letting hosts put signals on the medium
28Listen for signalsAre there signals on the cable?Yes.
29Wait if there are signals Wait until there are no more signals
30Listen for signalsAre there signals on the cable now?No.
31Put signals on cablePut my signals on the cable.
32Listen for collisions: no No collision.All is well.My message was sent.
33Listen for collisions: yes There is a collision.Stop sending signals.Send jamming signal.My message is lost.
34Listen again No signals now. Wait for a random length of time. Send message again.
35CSMA/CDCollisions happen if a host transmits when there is a signal on the cable but the host does not yet know about itLatency is the time a signal takes to travel to the far end of a cableThe longer the cable and the more intermediate devices, then more latencyAll clear
36CSMA/CDIf 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 laterIf the host has sent 64 bits and then detects a collision, it is too late; it will not resendLatency must be small enough so that all collisions are detected in timeThis limits cable length and the number of intermediate devices
37Some DefinitionsLatency or propagation delay: the time it takes for a signal to pass from source to destinationBit 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
38Interframe SpacingThe time between the end of one frame and the start of the next frameGives the medium a chance to stabilizeGives devices time to process the frameDevices wait a minimum of 96 bit times after a frame has arrived before they can send9.6 microseconds for 10 Mbps Ethernet0.96 microseconds for 100 Mbps Ethernet
39How Switch Tables WorkSwitch builds a switching (lookup) table matching its port numbers to the MAC addresses of devices connected to itWhen a frame arrives, it reads the destination MAC address, looks it up in the table, finds the right port and forwards the frame
40Switch Does FloodingIf 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 protocolIP to MAC address mapping, arp requests andarp replies
41Switch Learns Addresses switch learns addresses by looking at the source MAC address of an incoming framethen 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 hostCheck out
42Address Resolution Protocol (ARP) Table – Layer 2 protocol A host PC wants to send a messageIt knows the destination IP address and puts it in the packet headerIt looks in its own ARP table and finds the corresponding MAC addressIt puts the MAC address in the frame header
43Address Resolution Protocol A host wants to send a messageIt knows the destination IP addressThe destination MAC address is not in its ARP tableHost broadcasts “Calling , what is your MAC address?”replies “My MAC address is…”Host sends message and updates ARP table
44Remote AddressesHost can see that destination IP address is on another networkIt finds the IP address of the default gatewayIt sends an ARP request for the matching MAC address of the default gatewayDefault gateway router replies and gives its own MAC addressHost sends message via router and updates its ARP table
45Proxy 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 addressThe router will reply, giving its own MAC addressRouter: "send it to me, and I'll get it to where it needs to go"The host will send the message via the router
46ARP 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) addressesOpen command prompt windowU:\>arp/?U:\>arp –a [look at your command output]Interface = ??Internet Addresses = ??
47The End Complete Packet Tracer Labs in Chapter 9 Open cisco netacad; launch chapter 9; type in lab #’s