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 AddressPort A1BFC9 Ethernet Switch At Start

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 Address A1 Port 1 A1BFC9 Ethernet Switch After Transmission

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 Address A1 C9 Port 1 5 A1BFC9 Ethernet Switch After Transmission

6. Switch Learning Every Few Minutes, Switch Erases Switch Forwarding Table –To eliminate obsolete information –Relearning is very fast AddressPort A1BFC9 Ethernet Switch Erased

7. Switch Learning Switches Can be in Hierarchy –Switches only learn that stations are out certain ports –Do not Learn of switch in Between A1BFC9 Address A1 BF C9 Port 1 Port 1 Switch A Switch B

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 Switch Matrix Queues Output Ports Input Ports Frames

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

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 Switch Matrix Queues Output Ports Input Ports Frames

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 Switch Matrix Queues Output Ports Input Ports Frames

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) Switch Matrix Queues Output Ports Input Ports Frames

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

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 Switch Matrix Queues Output Ports Input Ports Frames

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