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Larger Site Networks

Larger Site Networks. Part 1. Small Site Single-hub or Single-Switch Ethernet LANs. Large Site Multi-hub Ethernet LANs Ethernet Switched Site Networks Congestion, Latency, and solutions ATM LANs Routers, Layer 3, and Layer 4 Switches. Multi-hub LANs. Multiple hubs

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Larger Site Networks

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  1. Larger Site Networks Part 1

  2. Small Site Single-hub or Single-Switch Ethernet LANs Large Site Multi-hub Ethernet LANs Ethernet Switched Site Networks Congestion, Latency, and solutions ATM LANs Routers, Layer 3, and Layer 4 Switches

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

  4. Hubs • Small LANs • Single-hub or single-switch LAN • 200 meter maximum distance span between farthest two stations with UTP Y 100 m 100 m X 200 m

  5. Hubs • Large LANs • Multiple-hub LANs • To increase maximum distance span 100 m 100 m 100 m

  6. 1. Station X transmits bit to Hub A Hub operates at the physical layer (one bit at a time) 2. Hub A broadcasts bit out all ports Two Hubs in 802.3 10Base-T B Y A X

  7. 3. Uplink Port sends bit to Hub B Uplink ports aremarked by an “X” 4. Hub B broadcasts bit to all attached stations, including Y Note that all stations on both hubs receive the bit broadcast almost simultaneously Two Hubs in 802.3 10Base-T B Y Uplink Port 3 X A

  8. Multiple Hubs in 10Base-T • Farthest stations in 10Base-T can be fivesegments (500 meters apart) • 100 meters per segment • Separated by four hubs 100m 100m 10Base-T hubs 100m 100m 500m, 4 hubs 100m

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

  10. Multiple Hubs in 10Base-T • No loops allowed • If hub or link fails, network is divided 6 3 4 No! 2 5 1 C No Loops B A

  11. Multiple Hubs in 10Base-T • Practical Limit in 10Base-T is Number of Stations • Degradation of service beyond 100 stations • Unacceptable service beyond 200 stations • Maximum possible span normally embraces more than 200 stations • In 10Base-T, the number of stations is the real limit to distance spans • Still, it is possible to have a LAN with more than a 200 meter maximum span

  12. Multiple Hubs in 100Base-TX • Limit of Two Hubs in 100Base-TX • Must be within a few meters of each other • Maximum span is 200 meters • Shorter maximum span than 10Base-T 100m 2 Collocated Hubs 100Base-TX Hubs 100m ~200 m

  13. Multiple Hubs with 1000Base-T • Limit of One Hub in Gigabit Ethernet • Maximum span is 200 meters • Same limit as 100Base-TX • Shorter maximum span than 10Base-T 100m 100m

  14. Multiple Hubs in Perspective • 10Base-T Hubs • 500 meter maximum distance span with UTP • Farther with some optical fiber links • However 10Base-T is limited by the number of stations it can support • So the maximum practical distance span is really much smaller • 100Base-TX Hubs and Gigabit Ethernet Hubs • 200 meter maximum distance span

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

  16. Ethernet Switched Networks • There are Distance Limits Between Pairs of Switches • 100 meters with UTP • Longer with optical fiber • But There is No Limit on the Number of Switches Between the Farthest Stations • So there is no maximum distance span Maximum Separation 100 m with UTP Longer with optical fiber Ethernet Switch

  17. Hierarchies • Ethernet Switches Must be Arranged in a Hierarchy • Root is the top-level • Usually, Fastest Switches are at the Top (Root) • Sizes given are only examples 100Base-X Building Switch Root Gigabit Ethernet Campus Switch 10Base-T Workgroup Switch

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

  19. Hierarchies • Vulnerable to Single Points of Failure • Switch or Link (trunk line between switches) • Divide the network into pieces X X Ethernet Switch

  20. Hierarchies • 802.1D Spanning Tree Allows Redundant Links • Automatically deactivated to prevent loops Ethernet Switch Deactivated Redundant Link

  21. Hierarchies • 802.1D Spanning Tree Allows Redundant Links • Automatically reactivated in case of failure • Slow and not completely effective X Failure Ethernet Switch Reactivated Redundant Link

  22. Hierarchies • Link Aggregation Protocol Allows Multiple Links Between Stations • If one link fails, others continue • Switch failures or cuts of all links still fatal Ethernet Switch Multiple Links

  23. Hierarchies • Single Possible Path Simplifies Switch Forwarding Decisions • When frame arrives, only one possible output port (no multiple alternative routes to select among) • Switch sends frame out that port Simple Forwarding Decision Ethernet Switch

  24. Hierarchies • Switches allow only a single path for each MAC destination address • Associated with a single port on each switch • So switch forwarding table has one and only one row for each MAC address Ethernet Switch Address A3.. B2.. Port 3 5

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

  26. Hierarchies • Overall, then, Ethernet switch forwarding is much simpler than router forwarding • So Ethernet switches are both cheaper and faster than routers Simple Forwarding Decision Ethernet Switch

  27. Hierarchies • Router networks are meshes, allowing multiple alternative routes to the destination host • Each alternative route is represented by a row in the router forwarding table • Router must evaluate each row for each packet • For each row, may have to compute match length, and metric • After looking at all rows, must choose the best alternative route

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

  29. Switch Learning • Switch Forwarding Table has Address-Port Pairs • Manual Entry is Too Time Consuming • Many addresses • Addresses change • Solution: Learn addresses automatically Address A3.. B2.. Port 3 5

  30. Switch Learning • Situation: Switch with • NIC A1-33-B6-47-DD-65 (A1) on Port 1 • NIC BF-78-C1-34-17-F4 (BF) on Port 2 • NIC C9-34-78-AB-DF-96 (C9) on Port 5 • Switch Forwarding Table is Initially Empty Ethernet Switch Address Port At Start A1 BF C9

  31. Switch Learning • A1 on Port 1 Sends to C9 on Port 5 • Switch does not know port for C9 • Broadcasts the frame, acting as a hub • Notes from source address that A1 is on Port 1 • Adds this information to switch forwarding table Ethernet Switch Address A1 Port 1 After Transmission A1 BF C9

  32. Switch Learning • C9 on Port 5 Sends to A1 on Port 1 • Table shows that A1 is on Port 1 • Switch only sends out Port 1: Acts like a switch! • Source address shows that C9 is on Port 5 • Switch adds this information to forwarding table Ethernet Switch Address A1 C9 Port 1 5 After Transmission A1 BF C9

  33. Switch Learning • Every Few Minutes, Switch Erases Switch Forwarding Table • To eliminate obsolete information • Relearning is very fast Ethernet Switch Address Port Erased A1 BF C9

  34. Switch Learning • Switches Can be in Hierarchy • Switches only learn that stations are out certain ports • Do not Learn of switch in Between Switch A Address A1 BF C9 Port 1 1 1 Port 1 Switch B A1 BF C9

  35. Switch Purchasing Decision • Hub Purchases are Simple • Number of Ports and Port Speeds • Switch Purchases are More Complex • Port speed • Number of ports • Maximum number of MAC-Port pairs in forwarding table • Queue sizes • Switching matrix aggregate throughput • Blocking or nonblocking • Reliability • Manageability

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