1. L2 Switching Overview 2009 年 12 月 主讲：孟宁 电话： 0512-68839302 E-mail ： mengning@ustc.edu.cn 主页： http://staff.ustc.edu.cn/~mengning 地址：苏州工业园区独墅湖高等教育区仁爱路 166 号明德楼 A302 室

2. Content ♦ OSI Protocol Stack ♦ Network Device ♦ Packet Flow ♦ Physical Layer ♦ Auto Negotiation ♦ MII Registers ♦ Frame Format ♦ Bridging ♦ Question ♦ Reference

3. OSI Protocol Stack Application (Message/Stream) Providing user-application services and procedures Presentation Arranging data in a mutually agreed format such as data encryption and data compression Session Controlling process communication responsible for buffering, and synchronization Network (Packet) Supporting routing services for transferring user packet data and network management data Transport (Segment/Datagram) Proposing end to end control about partitioning and reassembling messages Data Link (Frame) Organizing frame format; provides flow initialization, flow control, flow termination and error control Physical (Bits) Transferring data bits across link; defines physical characteristics of media OSI7 OSI6 OSI5 OSI4 OSI3 OSI2 OSI1

4. Network Device(1) OSI7 OSI6 OSI5 OSI4 OSI3 OSI2 OSI1 OSI7 OSI6 OSI5 OSI4 OSI3 OSI2 OSI1 Hub/Repeater OSI1

5. Network Device(2) OSI7 OSI6 OSI5 OSI4 OSI3 OSI2 OSI1 OSI7 OSI6 OSI5 OSI4 OSI3 OSI2 OSI1 Bridge/Switch OSI1 OSI2

6. Network Device(3) OSI7 OSI6 OSI5 OSI4 OSI3 OSI2 OSI1 OSI7 OSI6 OSI5 OSI4 OSI3 OSI2 OSI1 Router/Router Switch OSI3 OSI2 OSI4

7. Packet Flow Coaxial Cable Twisted Pair Optical Fiber Ethernet FDDI Token Ring Token Bus IP ARP RARP ICMP Telnet FTP POP I MAP SMTP HTTP SNMP TCP UDP ( Session Control ) ( Presentation Format ) Email: POP, IMAPSMTP HTTP: WWW FTP: File Transfer Protocol Telnet: Remote Login SNMP: Network Management ARP: Address Resolution for MAC Address based on IP Address RARP: Address Resolution for IP Address based on MAC Address

8. Physical Layer -Twisted Pair(1) ♦ Crossover –Data Circuit Equipment (DCE) –Data Terminating Equipment (DTE) –MDI-II/MDI-X PC Hub Switch Router PCHubSwitchRouter      

9. Physical Layer -Twisted Pair(2) ♦ Cable type Category Description Transfer Rate CAT 1 CAT 2 CAT 3 CAT 4 CAT 5 Voice and Data Voice 4 Mbps 10 Mbps 16 Mbps 100 Mbps Ethernet Token Ring Fast Ethernet CAT 5eGigabit Ethernet 1 Gbps < 4 Mbps CAT 6 Gigabit Ethernet 1 Gbps

10. Physical Layer - Layout LLC MAC Signal/Pin PHY ( Physical Layer Interface ) IEEE802.3ab TX_CLK RX_CLK TX_EN TXD TX_ER RX_DV RXD RX_ER CRS COL MDC MDIO MII/GMII 即媒体独立接口

11. Auto Negotiation ♦ AN is an optional standard function defined in the 1998 IEEE 802.3 specification. ♦ Its purpose is to enable device interface to achieve their highest possible mode of operation over a line segment. ♦ It does this by providing a means for the interfaces to exchange information about their capability (such as speed and duplex) over a link. ♦ PHY transceiver uses MII to store information about its capability.

12. MII Registers ♦ 32 PHY registers ♦ 16-bit value of each register ♦ Register table

13. Frame Format(1) PreambleDelimiterDASAType Upper Layer Protocol ( ULP ) CRC Maximum Frame size 1518 bytes Minimum Frame size 64 bytes Ethernet Header Ethernet Payload Ethernet Trailer DA: Destination Address SA: Source Address Type: EtherType CRC: Cyclic Redundancy Check PreambleDelimiterDASAType Upper Layer Protocol ( ULP ) 7 bytes 1 byte 6 bytes 6bytes 2bytes < 1500 bytes 4 bytes CRC Maximum Frame size 1518 bytes Minimum Frame size 64 bytes Ethernet Header Ethernet Payload Ethernet Trailer DA: Destination Address SA: Source Address Type: EtherType CRC: Cyclic Redundancy Check

14. Frame Format(2) Preamble: 1010 1010... 1010 1010 7 bytes Purpose: achieve receiver synchronization with sender Delimiter: 1010 1011 1 byte Purpose: denote frame starting point

15. Frame Format(3) DA: MAC Address SA: MAC Address ( MAC: Media Access Control ) A B C mac_a mac_b mac_c mac_c mac_a DA SA mac_a mac_c

16. Frame Format(4) 6 bytes Vendor ID ( e.g. Cisco 00:00:0c ) Serial No Note: 1. MAC address saved in NIC for PC should not be modified by user. It is created by NIC vendor. 2. For network device such as bridge or router, it is saved in NVRAM. There exist a distinct MAC address for each bridge ( or router ) port.

17. Frame Format(5) Ethernet ICMP IPARPRARP EtherType = 0800 for IP EtherType = 0806 for ARP EtherType = 8035 for RARP OSI 3 OSI 2

18. Bridging ♦ Filtering Database ♦ Forwarding Algorithm ♦ Learning Algorithm ♦ Operation Process

19. Filtering Database MAC AddressCorresponding Port E Age (DA or SA ) 2 1 B MAC AddressCorresponding Port Age (DA or SA ) 2 1 3 DC A 3 100 B 1 65 C 2 44 D 3 96 E 1 29 A 2 53 B 2 40 C 2 9 D 2 100 E 1 65 A

20. Forwarding Algorithm End Start Receive frame from incoming port X DA in Filtering Database Broadcast incoming frame to all ports except for X Filtering incoming frame Forward incoming frame to Corresponding Port X equal to Corresponding Port yes no yes no

21. Learning Algorithm SA in Filtering Database Reset Age to Default Age Write SA 、 X and Default Age into Filtering Database End Start yes no

22. Operation Process(1) MAC port MAC port MAC port DASA Ethernet Header Ethernet Frame EC IIIIII A B D 1 2 1 3 2 1 2

23. Operation Process(2) EC IIIIII A BD 1 2 1 3 2 1 2 MAC port MAC port MAC port B -> D DBDBDBDBDB B 2 B 1 B 2 C -> A ACACACACAC C 2 C 1 E -> A AEAE E 3 E 2 AEAEAE

24. 常见二层协议 ♦VLAN 虚拟局域网 ♦STP 生成树协议 - RSTP MSTP ♦LA 链路聚合，也称为 trunking ♦IGMP Snooping 多播组的二层支持 ♦PNAC 基于端口的认证 802.1X ♦......

25. 谢谢大家！