1. Larger Site Networks Chapter 8 Copyright 2001 Prentice Hall Revision 2: July 2001

2. Multi-hub LANs Multiple hubs Multiple hubs in 10Base-T Multiple hubs in 100Base-TX Multiple hubs in Gigabit Ethernet

3. 3 Hubs zChapter 7 ySingle-hub or single-switch LAN y200 meter maximum distance span between farthest two stations with UTP 100 m X Y 200 m

4. 4 Hubs zChapter 8 yMultiple-hub LANs yTo increase maximum distance span 100 m

5. 5 Multiple Hubs in 10Base-T zFarthest stations in 10Base-T can be five segments (500 meters apart) y100 meters per segment ySeparated by four hubs 100m 500m, 4 hubs 10Base-T hubs

6. 6 Multiple Hubs in 10Base-T zNo loops allowed yOnly one possible path between any two stations No Loops A B C 1 2 3 4 5 6 AB=1,2,3,4,5 AC=1,2,3,4,6 BC=5,4,6 First two have too many hubs No!

7. 7 Multiple Hubs in 10Base-T zPractical Limit in 10Base-T is Number of Stations yDegradation of service beyond 100 stations yUnacceptable service beyond 200 stations yMaximum possible span normally embraces more than 200 stations yIn 10Base-T, the number of stations is the real limit to distance spans yStill, it is possible to have a LAN with more than a 200 meter maximum span

8. 8 Multiple Hubs in 100Base-TX zLimit of Two Adjacent Hubs in 100Base-TX yMust be within a few meters of each other yMaximum span is 200 meters yShorter maximum span than 10Base-T 100m 2 Collocated Hubs 100Base-TX Hubs ~200 m

9. 9 Multiple Hubs with 1000Base-T zLimit of One Hub in Gigabit Ethernet yMaximum span is 200 meters ySame limit as 100Base-TX yShorter maximum span than 10Base-T 100m

10. Switched Ethernet Site Networks No Maximum Distance Spans Hierarchies and Single Possible Paths High Speeds and Low Prices

11. 11 Ethernet Switched Networks zThere is No Limit on the Number of Switches Between the Farthest Stations ySo there is no maximum distance span No Limit On Number of Switches or Span Ethernet Switch

12. 12 Hierarchies zEthernet Switches Must be in a Hierarchy yUsually, Fastest Switches are at the Top (Root) Gigabit Ethernet Campus Switch 100Base-X Building Switch 10Base-T Workgroup Switch Root New

13. 13 Hierarchies zOnly a Single Possible Path (2,1,3,4) Between Any Two Stations Single Possible Path Ethernet Switch A 1 3 4 5 B 2

14. 14 Hierarchies zVulnerable to Single Points of Failure ySwitch or Link (trunk line between switches) yDivide the network into pieces X X Ethernet Switch

15. 15 Hierarchies z802.1D Spanning Tree Allows Redundant Links yAutomatically deactivated to prevent loops yReactivated if there is a failure Deactivated Redundant Link Ethernet Switch

16. 16 Hierarchies zLink Aggregation Protocol Allows Multiple Links Between Stations yIf one link fails, others continue ySwitch failures or cuts of all links still fatal Multiple Links Ethernet Switch

17. 17 Hierarchies zSingle Possible Path Simplifies Switch Forwarding Decisions yWhen frame arrives, only one possible output port (no multiple alternative routes to select among) ySwitch sends frame out that port Simple Forwarding Decision Ethernet Switch

18. 18 Hierarchies zSwitches allow only a single path for each MAC destination address yAssociated with a single port on each switch ySo switch forwarding table has one and only one row for each MAC address Ethernet Switch Address A3.. B2.. Port 3 5

19. 19 Hierarchies zEthernet switch only has to find the single row that matches the destination MAC address yOnly has to examine half the rows on average; less if the table is alphabetized yComparison at each row is a simple match of the frame and row MAC addresses; much less work that row comparison in routers yOverall, this is much less work than routers must do Address A3.. B2.. Port 3 5

20. More on Switched Ethernet Switch Learning Purchase Considerations VLANs Intelligent Switched Network Design Box

21. 21 Switch Learning zSituation: Switch with yNIC A1-33-B6-47-DD-65 (A1) on Port 1 yNIC BF-78-C1-34-17-F4 (BF) on Port 2 yNIC C9-34-78-AB-DF-96 (C9) on Port 5 zSwitch Forwarding Table is Initially Empty AddressPort A1BFC9 Box Ethernet Switch At Start

22. 22 Switch Learning zA1 on Port 1 Sends to C9 on Port 5 ySwitch does not know port for C9 yBroadcasts the frame, acting as a hub yNotes from source address that A1 is on Port 1 yAdds this information to switch forwarding table Address A1 Port 1 A1BFC9 Box Ethernet Switch After Transmission

23. 23 Switch Learning zC9 on Port 5 Sends to A1 on Port 1 yTable shows that A1 is on Port 1 ySwitch only sends out Port 1: Acts like a switch! ySource address shows that C9 is on Port 5 ySwitch adds this information to forwarding table Address A1 C9 Port 1 5 A1BFC9 Box Ethernet Switch After Transmission

24. 24 Switch Learning zEvery Few Minutes, Switch Erases Switch Forwarding Table yTo eliminate obsolete information yRelearning is very fast AddressPort A1BFC9 Box Ethernet Switch Erased

25. 25 Switch Learning zSwitches Can be in Hierarchy ySwitches only learn that stations are out certain ports yDo not Learn of switch in Between A1BFC9 Address A1 BF C9 Port 1 Port 1 Switch A Switch B Box

26. 26 Switch Purchasing Decisions zMaximum Number of MAC address-port entries ySmall switches may not be able to store many MAC addresses yFor addresses that cannot be stored, switch must act like a hub, broadcasting and so creating latency Box Address A1 C9 Port 1 5

27. 27 Switch Purchasing Decisions zQueue Size yIncoming frames are placed in queues if they cannot be processed immediately xMay have several queues yIf queues are too small, frames will be lost during brief peak loads Switch Matrix Queues Output Ports Input Ports Frames Box

28. 28 Switch Purchasing Decisions zSwitching Matrix yReceives input from multiple input ports, via queues ySwitches each frame to the correct output port Queues Output Ports Input Ports Frames Box Switch Matrix

29. 29 Switch Purchasing Decisions zSwitching Matrix Aggregate Throughput yThe number of bits it can switch per second yNonblocking if aggregate throughput equals the number of ports times the speed of the ports. yCan handle the load even if all ports are receiving input simultaneously Switch Matrix Queues Output Ports Input Ports Frames Box

30. 30 Switch Purchasing Decisions zNonblocking Calculation y12 input ports y100 Mbps each yMaximum possible input: 1,200 Mbps (1.2 Gbps) yNonblocking switch needs 1.2 Gbps of aggregate switching capacity Switch Matrix Queues Output Ports Input Ports Frames Box

31. 31 Switch Purchasing Decisions zReliability through Redundancy yRedundant power supplies and cooling fans yMay even have redundant switch matrix for backup Switch Matrix Queues Output Ports Input Ports Frames Box

32. 32 Switch Purchasing Decisions zManageability yCan be managed remotely from the network administrator’s desk xNetwork administrator can check on status of switch xNetwork administrator can modify how the switch functions yWe will see remote management in Chapter 12 yRemote management greatly reduces labor Switch Matrix Queues Output Ports Input Ports Frames Box

33. 33 Ethernet Virtual LANs zHubs versus Switches yHubs broadcast bits out all ports ySwitches usually send a frame out a one port zMore fundamentally yIn unicasting, a message is only intended to go to one machine, as when a client sends a message to a server ySwitches assume unicasting; it is the basis for sending a frame out a single port Box

34. 34 Ethernet Virtual LANs zBroadcasting ySometimes, station needs to send a frame to all other stations; this is broadcasting yFor example, servers send a frame to advertise their presence with a broadcast message every minute or so Box

35. 35 Ethernet Virtual LANs zBroadcasting with Ethernet Switches yBroadcaster sets the destination MAC address to all ones (48 ones) yWhen switch broadcast such frames yCan create congestion Broadcast Frame Ethernet Switch Box

36. 36 Ethernet Virtual LANs zIn multicasting, messages are only intended to go to some stations yFor instance, from a server only to the client PCs it serves yIf Ethernet switches can implement multicasting, traffic overload would be avoided Multicast Frame Box

37. 37 Ethernet Virtual LANs zEthernet switches do implement multicasting yA server and the clients it serves are treated as a single virtual LAN (VLAN) yCan only communicate among themselves, as if they were on their own LAN Frame Marketing VLAN Server Marketing VLAN Client Box

38. 38 Ethernet Virtual LANs zVLAN Benefits yVLANs reduce traffic on the switched network yOther benefits xThey provide weak security because clients cannot reach all servers (easily defeated but good first line of defense) xVLANs give ease of management because if a user changes organizational membership, VLAN membership is easily changed centrally Box New

39. 39 Ethernet Virtual LANs zVLAN Problems yVLANs have not been standardized xA network of switches from different vendors cannot implement VLANs yStandardization is beginning xUsing tagging (Chapter 7) xTag Control Information field has a 12-bit VLAN ID (VID) number, allowing 2 12 VLANs to be identified Box New

40. 40 Ethernet Virtual LANs zVLAN Interconnection yFor cross-VLAN communication, routers actually connect multiple switches Box New Ethernet Switch

41. 41 When are Frames Forwarded? zCut-Through Ethernet Switches yForward after seeing only part of a frame xMinimum is destination address to determine output port xMay need to see tag fields for priority, VLAN xMay wait until 46 octets of data plus PAD yFast operation Box PreSFDDASALenDataPADFCS Forward the Frame

42. 42 When are Frames Forwarded? zStore-and-Forward Ethernet Switches yForwarded only after receiving full frame yAllows error checking (CRC field) zHybrid Ethernet Switches yStart in cut-through mode but check errors yIf many errors, go to store-and-forward mode Box PreSFDDASALenDataPADFCS Forward the Frame

43. 43 Bad Switch Organization zOne Server for All Clients yAll traffic goes to and from server yBottlenecks: no simultaneous conversations yNo major benefits compared to hub Bottleneck Box Ethernet Switch

44. 44 Bad Switch Organization zMultiple Servers for Clients yAllows simultaneous conversations yBrings switching’s main benefit Box Ethernet Switch

45. Congestion, Latency, and Remedies Peak Loads Congestion and Latency Overprovisioning Capacity Priority Quality of Service Traffic Shaping

46. 46 The Peak Load Problem zCapacity Sufficient Most of the Time yOtherwise, get bigger switches and trunk lines! zBrief Traffic Peaks can Exceed Capacity yFrames will be delayed in queues or even lost if queue gets full Capacity Traffic Peak

47. 47 Overprovisioning zOverprovisioning: Install More Capacity than Will be Needed Nearly All of the Time yWasteful of capacity yStill, usually the cheapest solution today because of its simplicity Overprovisioned Capacity Traffic Peak

48. 48 Priority zAssign Priorities to Frames yHigh priority for time-sensitive applications (voice) yLow priority for time-insensitive applications (e-mail) yIn traffic peaks, high-priority frames still get through yLow-priority applications do not care about a brief delay for their frames High-Priority Frame Goes Low-Priority Frame Waits Briefly

49. 49 Priority zStandardizing Priority y802 Tag Fields are standardizing priority for Ethernet and other 802 LAN technologies yPriority is also being standardized by the IETF for IPv4 and IPv6 (Diffserv for differentiated services) y802 and IETF are harmonizing efforts for end-to-end priority High-Priority Frame Goes Low-Priority Frame Waits Briefly

50. 50 Full Quality of Service (QoS) zPriority Makes no Quantitative Promises of Maximum Latency, etc. zQuality of Service (QoS) Makes Quantitative Promises for such things zReserves capacity; if not used, this capacity is wasted High Guarantee Low or No Guarantee

51. 51 Full QoS is Not a Cure-All zTraffic with no guarantees will not benefit zIt may not get through at all zOften, voice traffic is given strong guarantees while data traffic is given low or no guarantees High Guarantee Reserved Capacity Low or No Guarantee

52. 52 Traffic Shaping zOverprovisioning, Priority, and QoS are Ways to Cope with Brief Congestion zTraffic Shaping Prevents recognizes that congestion is beginning, acts to stop it zSwitch Tells Some Sources to Slow or Stop if Congestion is Beginning, based on Policies Source A Source B Network Slow or Stop Continue

53. ATM Switches Cells Scalable QoS Perspective Virtual Circuits

54. 54 ATM Switches zAsynchronous Transfer Mode zBasic Standards Set by ITU-T yPartner with ISO in OSI standards yATM standards developed within OSI architecture zATM Forum Sets Detailed Standards yGroup of mostly ATM vendors yMoves quickly yAlso tests for interoperability

55. 55 ATM Switches zHas fixed-length frames are called cells yAlways 5 octet header, 48 octet payload, ySo always 53 octets total zSmall cell reduces latency (delay) at each switch ySwitch may only be able to send frame out after whole frame is read yWith short frames, this is not a problem Payload (48 octets) Header (5 octets) ATM Cell

56. 56 ATM Switches zHighly Scalable yComparable to Ethernet zVery sophisticated yOffers quality of service guarantees yVery expensive to purchase and manage zATM has high overhead (extra characters) y5 overhead octets for 48 data octets (10% overhead) yActually even worse (see Module E)

57. 57 ATM Switches zUnfortunately, very expensive yHas lost the desktop yIt is usually cheaper to use high-capacity Ethernet switches with overprovisioning, so that latency does not grow to the point where QoS is critical yIn LANs, usually used only where service quality is critical, typically when voice is being carried. Even losing there.

58. 58 ATM QoS Categories zATM Offers Varying Levels of QoS zParameters yPeak cell rate (maximum burst speed) yMaximum burst size (bits per burst) ySustainable cell rate (always allowed) yCell Delay Variation Tolerance (CDVT): how exact cell-to-cell timing is; Critical for voice and video yCell Loss Ratio: Losses during transmission

59. 59 ATM QoS Categories zATM Offers Varying Levels of QoS zFor Voice and Video yITU-T Class A yATM Forum Service Category: Constant Bit Rate (CBR) yLow latency yLow Cell Delay Variation Tolerance yStrong guarantees for voice and video!

60. 60 ATM QoS Categories zFor IP and LAN Data yITU-T Class D ySeveral ATM Forum Service Categories xDeveloped several categories over Time xAvailable bit rate (ABR) weak: send if capacity is available xUnspecified bit rate (UBR) weak: simpler than ABR, but can get almost no share of capacity xGuaranteed frame rate (GFR) gets roughly fair share of capacity during congestion

61. 61 ATM QoS Categories zFor IP and LAN Data ySeveral ATM Forum Service Categories xABR, UBR, and even GFR give very low status to data transmission xNot even as good as Ethernet priority of service xYet costs far more xSo ATM QoS makes little sense if used entirely for data Has other data transmission benefits, however

62. 62 ATM QoS Categories zOther Categories zFor Videoconferencing yMay need momentary bandwidth increase if there is a burst of motion on the screen yNeeds Low Cell Delay Variation Tolerance yATM: Class B yATM Forum Service Category: Variable Bit Rate-Real Time (VBR-RT) yNot widely used or implemented

63. 63 ATM QoS Categories zOther Categories zFor Connection-Oriented Data yATM: Class C yATM Forum Service Category: Variable Bit Rate-Not Real Time (VBR-NRT) yMost data not connection-oriented yNot widely used, implemented

64. 64 ATM Switches: Virtual Circuits zOften Arranged in a Mesh yBut all traffic between two stations still is consigned to a path called a virtual circuit that is set up before the first frame transmission Virtual Circuit ATM Cell

65. 65 ATM Switches zVirtual Circuits Mean that there is Only a Single Possible Path between Any Two Stations yVirtual circuits simplify switch operation and so lower switch cost Virtual Circuit ATM Cell

66. 66 ATM Switches zPermanent Virtual Circuits (PVCs) ySet up once, for each pair of sites ySimplest and least expensive administratively because rarely changed yMost widely used form of virtual circuit zSwitched Virtual Circuit (SVC) ySet up at time of use yFlexible but expensive

67. 67 ATM Switches zATM Frame Header yDoes NOT have a destination address field yInstead, has two fields that together contain a hierarchical virtual circuit number yLike a route number on a bus--names the route, not the destination Virtual Circuit Number ATM Header

68. 68 ATM Switches zHierarchical Virtual Circuit Number yVirtual Path Identifier xHigher-level number; Often specifies a site yVirtual Channel Identifier xLower-level number; Often specifies a computer at a site Virtual Circuit Number ATM Header

69. 69 ATM Switches zVirtual Circuit yAll traffic between two sites can be given the same VPI number xBut difference VCI values ySwitch needs only one VPI table entry for all this traffic yDramatically reduces number of table entries in switches between sites and therefore makes lookups very fast

70. 70 ATM Switches zATM Reliability yVirtual circuit reduces communication to a single path yIf a switch or trunk line along the path fails, communication stops yBut ATM switches also have addresses, which are used to set up a new virtual circuit fairly rapidly Not in Book

71. 71 Switches Versus Routers zSwitches zFast zInexpensive zNo benefits of alternative routing zRouters zSlow zExpensive zbenefits of alternative routing “Switch where you can; route where you must”

72. 72 Early Site Networks zOrganization yLANs (subnets) based on hubs yRouters link hubs yHierarchy of Routers Router Hub

73. 73 The Switching Revolution zSwitches Push Routers to the Edge ySwitches replace most routers in site networks yBecause switches are cheaper than routers yRouting’s sophistication is still needed at the edge External Switch Router

74. 74 The Switching Revolution zLayer 3 Switches yTraditional switches operate at Layer 2; Switch based on MAC addresses yLayer 3 switches switch based on internet layer IP addresses External Layer 3 Switch

75. 75 The Switching Revolution zLayer 3 Switches yLayer 3 switches are replacing many Layer 2 switches in site networks because of their ability to switch based on IP addresses External Layer 3 Switch

76. 76 The Switching Revolution zLayer 3 Switches versus Routers yLayer 3 switches are much faster than routers yLayer 3 switches cost less than routers External Layer 3 Switch

77. 77 The Switching Revolution zLayer 3 Switches versus Routers yAt the internet layer, Layer 3 switches normally only support IP and sometimes IPX; Routers route many more internet layer protocols, including those of AppleTalk, SNA, and others yAt the data link layer, Layer 3 switches normally support only Ethernet on LANs. Routers support many Layer 2 LAN protocols. Router Layer 3 Switch

78. 78 The Switching Revolution zLayer 3 Switches versus Routers yLayer 3 switches rarely support Layer 2 WAN protocols yRouters usually are still needed at the edge of the site network, to communicate with external links External Layer 3 Switch

79. 79 The Switching Revolution zRouters yForward based on IP addresses and other internet layer addresses yExpensive and slow yHandle multiple internet layer protocols yHandle multiple LAN and WAN subnet protocols zLayer 3 Switches yForward based on IP addresses, sometimes IPX addresses yInexpensive and Fast yDo not handle multiple internet layer protocols yDo not handle multiple LAN and WAN subnet protocols

80. 80 The Switching Revolution zLayer 4 Switches yExamine port fields in TCP and UDP yThese fields describe the application yTherefore, can switch based on application (to give priority by application, etc.) Layer 4 Switch