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More on Switched Ethernet

More on Switched Ethernet. Switch Learning Purchase Considerations VLANs Intelligent Switched Network Design. Switch Learning. Switch Forwarding Table has Address-Port Pairs Manual Entry is Too Time Consuming Many addresses Addresses change Solution: Learn addresses automatically.

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More on Switched Ethernet

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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. Switch Purchasing Decisions • Hub Purchases are Simple • Number of Ports and Port Speeds • Switch Purchases are More Complex

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

  10. Switch Purchasing Decisions • Queue Size • Incoming frames are placed in queues if they cannot be processed immediately • May have several queues • If queues are too small, frames will be lost during brief peak loads Frames Input Ports Queues Switch Matrix Output Ports

  11. Switch Purchasing Decisions • Switching Matrix • Receives input from multiple input ports, via queues • Switches each frame to the correct output port Switch Matrix Frames Input Ports Queues Output Ports

  12. Switch Purchasing Decisions • Switching Matrix Aggregate Throughput • The number of bits it can switch per second • Nonblocking if aggregate throughput equals the number of ports times the speed of the ports. • Can handle the load even if all ports are receiving input simultaneously Frames Input Ports Queues Switch Matrix Output Ports

  13. Switch Purchasing Decisions • Nonblocking Calculation • 12 input ports • 100 Mbps each • Maximum possible input: 1,200 Mbps (1.2 Gbps) • Nonblocking switch needs 1.2 Gbps of aggregate switching capacity Frames Input Ports Queues Switch Matrix Output Ports

  14. Switch Purchasing Decisions • Does Not Have to Be Completely Nonblocking • Because not all input ports will send at once • However, for switches high in the hierarchy, must be close to nonblocking (Tolly: at least 75%) • Even for workgroup switches, should never be less than 25% nonblocking (Tolly) Frames Input Ports Queues Switch Matrix Output Ports

  15. Switch Purchasing Decisions • Reliability through Redundancy • Redundant power supplies and cooling fans • May even have redundant switch matrix for backup Frames Input Ports Queues Switch Matrix Output Ports

  16. Switch Purchasing Decisions • Manageability • Can be managed remotely from the network administrator’s desk • Network administrator can check on status of switch • Network administrator can modify how the switch functions • We will see remote management in Chapter 12 • Remote management greatly reduces labor Frames Input Ports Queues Switch Matrix Output Ports

  17. Switch Purchasing Decision • Recap • 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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