复旦大学：《计算机网络 Computer Networking》课程电子教案（PPT课件讲稿）08 Software School

Computer Networking Lecture8- Switching and Bridging Dejian Ye Software School Fudan University

1 Dejian Ye Software School Fudan University Computer Networking Lecture 8 – Switching and Bridging

Scale yak yak What breaks when we keep adding people to the same wire? 2

2 Scale What breaks when we keep adding people to the same wire? 2 yak yak…

Scale yak yak What breaks when we keep adding people to the same wire? Only solution: split up the people onto multiple wires >But how can they talk to each other? 3

3 Scale What breaks when we keep adding people to the same wire? Only solution: split up the people onto multiple wires »But how can they talk to each other? 3 yak yak…

Problem: how to Connect lans? yak yak When should these boxes forward packets between wires? How do you specify a destination? How does your packet find its way?

4 Problem: How to Connect LANs? When should these boxes forward packets between wires? How do you specify a destination? How does your packet find its way? 4 yak yak…

Outline Bridging Internetworks >)Methods for packet forwarding 5

5 Outline Bridging Internetworks » Methods for packet forwarding 5

Building Larger LANs: Bridges Extend reach of a single shared medium Connect two or more"segments"by copying data frames between them >)Only copy data when needed key difference from repeaters/hubs >)Reduce collision domain compared with single LAN >)Separate segments can send at once> much greater bandwidth Challenge: learning which packets to copy across links LAN 1 LAN 2 6

6 Building Larger LANs: Bridges Extend reach of a single shared medium Connect two or more “segments” by copying data frames between them » Only copy data when needed → key difference from repeaters/hubs » Reduce collision domain compared with single LAN » Separate segments can send at once → much greater bandwidth Challenge: learning which packets to copy across links 6 LAN 1 LAN 2

Transparent Bridges Design goals >)Self-configuring without hardware or software changes ))Bridge do not impact the operation of the individual lans Three parts to making bridges transparent: 1)Forwarding frames 2)Learning addresses/host locations 3)Spanning tree algorithm

7 Transparent Bridges Design goals: » Self-configuring without hardware or software changes » Bridge do not impact the operation of the individual LANs Three parts to making bridges transparent: 1) Forwarding frames 2) Learning addresses/host locations 3) Spanning tree algorithm 7

Frame Forwarding A machine with MAC Address lies in the direction of num ber port of the bridge A2103209A591 36 For every packet, the bridge"looks up 992390842201 the entry for the packets destination MAC address and forwards the packet 8711098900AA 2 on that port 2 >Other packets are broadcast -why 301B2369011C 16 695909311 Timer is used to flush old entries

8 Frame Forwarding A machine with MAC Address lies in the direction of number port of the bridge For every packet, the bridge “looks up” the entry for the packets destination MAC address and forwards the packet on that port. » Other packets are broadcast – why? Timer is used to flush old entries 8 8711C98900AA 2 MAC Address Port A21032C9A591 1 99A323C90842 2 301B2369011C 2 695519001190 3 15 Age 36 01 16 11 Bridge 1 3 2

Learning bridges Manually filling in bridge tables? >) Time consuming, error-prone Keep track of source address of packets arriving on every link, showing what segment hosts are on ))Fill in the forwarding table based on this information ost host hos hosthost host host bridge hosthost host host host host

9 Learning Bridges Manually filling in bridge tables? »Time consuming, error-prone Keep track of source address of packets arriving on every link, showing what segment hosts are on »Fill in the forwarding table based on this information 9 host host host host host host host host host host host host Bridge

Spanning tree bridges More complex topologies can provide redundancy. >) But can also create loops What is the problem with loops? Solution: spanning tree hosthost hosthost host host Bridge Bridge hosthost host host hosthost 10

10 Spanning Tree Bridges More complex topologies can provide redundancy. »But can also create loops. What is the problem with loops? Solution: spanning tree 10 host host host host host host host host host host host host Bridge Bridge