1. CMPE 150- Introduction to Computer Networks 1 CMPE 150 Fall 2005 Lecture 19 Introduction to Computer Networks

2. CMPE 150- Introduction to Computer Networks 2 Announcements Homework 3 due today. Lab this week: –Layer 2. –Ethernet. Holiday this Friday (11.11). Monday, 11.14 class meets in E2 room 506.

3. CMPE 150- Introduction to Computer Networks 3 Today Data layer switching.

4. CMPE 150- Introduction to Computer Networks 4 LAN Interconnection Extend range of LAN. Support more users. Security and robustness. Heterogeneity. Hub Switch

5. CMPE 150- Introduction to Computer Networks 5 LAN Interconnection: Example Multiple LANs connected by a backbone.

6. CMPE 150- Introduction to Computer Networks 6 Bridges Connect LANs. Operate at the DLL. –Route based on DLL addresses. Routers route based on layer 3 addresses. Distinction between bridges, switches, and routers gets fuzzier as technology advances.

7. CMPE 150- Introduction to Computer Networks 7 Repeaters and Bridges Repeaters: –Extend scope of LANs. –Serve as amplifiers. –No storage/routing capabilities. Bridges: –Also extend scope of LANs. –Routing/storage capabilities.

8. CMPE 150- Introduction to Computer Networks 8 More on Bridges Have multiple interfaces, 1 per LAN. Use destination address to forward unicast frames; if destination is on the same LAN, drops frame; otherwise forwards it. Forward all broadcast frames. Have storage and routing capability.

9. CMPE 150- Introduction to Computer Networks 9 More on Bridges No additional encapsulation. But they may have to do header conversion if interconnecting different LANs (e.g., 802.3 to 802.4 frame). May interconnect more than 2 LANs. LANs may be interconnected by more than 1 bridge.

10. CMPE 150- Introduction to Computer Networks 10 Interconnecting Different LANs

11. CMPE 150- Introduction to Computer Networks 11 Bridges from 802.x to 802.y Operation of a LAN bridge from 802.11 to 802.3.

12. CMPE 150- Introduction to Computer Networks 12 Interconnecting Different LANs Conversion between different frame formats. –E.g., different frame lengths. Speed mismatch. –Buffering. Security. –Different security services provided by different DLLs/MACs. QoS?

13. CMPE 150- Introduction to Computer Networks 13 Routing Determining where to send frame so that it reaches the destination. Routing by learning: adaptive or backward learning.

14. CMPE 150- Introduction to Computer Networks 14 Routing with Bridges Bridge decides to relay frame based on destination MAC address. If only 2 LANs, decision is simple. If more complex topologies, routing is needed, i.e., frame may traverse more than 1 bridge.

15. CMPE 150- Introduction to Computer Networks 15 Routing with Bridges B 1 4 5 8 Frames for 5->8. Frames for 1->4 LAN A LAN B

16. CMPE 150- Introduction to Computer Networks 16 Routing with Bridges (Cont’d) Listens to all frames on LAN A and accepts those addressed to stations on LAN B. Retransmits frames onto B. Does the same for B-to-A traffic.

17. CMPE 150- Introduction to Computer Networks 17 Transparent Bridges Plug them in and they work! How do they work? –Promiscuous mode operation. –Upon receiving frame, decide whether to forward it or not. –Routing table mapping destination addresses to outgoing interface.

18. CMPE 150- Introduction to Computer Networks 18 Local Internetworking Example: 4 LANs and 2 bridges.. In promiscuous mode, bridge accepts every incoming frame.. Decides to forward or discard frame.. A LAN1 B LAN1 C LAN2 …

19. CMPE 150- Introduction to Computer Networks 19 Backward Learning Routing algorithm used by transparent bridges. When a bridge starts, its routing table is empty. –So, it floods: every incoming frame is forwarded in all outgoing interfaces, except the one the frame was received. –In promiscuous mode, bridge sees all frames. –They look at the frame’s source and “remember” which LAN it came from. –Entries are timestamped and expire after a certain interval. Allows for topology changes.

20. CMPE 150- Introduction to Computer Networks 20 Backward Learning (Cont’d) If source LAN = destination LAN, discard frame. If source LAN <>destination LAN, forward frame. –If destination LAN unknown, flood frame.

21. CMPE 150- Introduction to Computer Networks 21 Loops Sometimes, more than 1 bridge interconnect 2 LANs. Topology loops are introduced.

22. CMPE 150- Introduction to Computer Networks 22 Loops: Example Alternate routes: loops. Example: –LAN A, bridge 101, –LAN B, bridge 104, –LAN E, bridge 107, –LAN A. LAN A LAN B E 2 45 101 103 104 1 107

23. CMPE 150- Introduction to Computer Networks 23 Loop: Problems A B LAN 1 LAN 2 B1 B2 1. Station A sends frame to B; bridges B1 and B2 don’t know B. 2. B1 copies frame onto LAN1; B2 does the same. 3. B2 sees B1’s frame to unknown destination and copies it onto LAN 2. 4. B1 sees B2’s frame and does the same. 5. This can go on forever.

24. CMPE 150- Introduction to Computer Networks 24 Loop Resolution Goal: remove “extra” paths by removing “extra” bridges. Spanning tree: –Given graph G(V,E), there exists a tree that spans all nodes where there is only one path between any pair of nodes, i.e., NO loops. –LANs are represented by nodes and bridges by edges.

25. CMPE 150- Introduction to Computer Networks 25 Definitions (1) Bridge ID: unique number (e.g., MAC address + integer) assigned to each bridge. Root: bridge with smallest ID. Cost: associated with each interface; specifies cost of transmitting frame through that interface. Root port: interface to minimum-cost path to root.

26. CMPE 150- Introduction to Computer Networks 26 Definitions (2) Root path cost: cost of path to root bridge. Designated bridge: on any LAN, bridge closest to root, i.e., the one with minimum root path cost.

27. CMPE 150- Introduction to Computer Networks 27 Spanning Tree Algorithm (1) 1. Determine root bridge. 2. Determine root port on all bridges. 3. Determine designated bridges.

28. CMPE 150- Introduction to Computer Networks 28 Spanning Tree Algorithm (2) Initially all bridges assume they are the root and broadcast message with its ID, root path cost. Eventually, lowest-ID bridge will be known to everyone and will become root. Root bridge periodically broadcasts it’s the root.

29. CMPE 150- Introduction to Computer Networks 29 Spanning Tree Algorithm (3) Directly connected bridges update their cost to root and broadcast message on other LANs they are attached. This is propagated throughout network. On any (non-directly connected) LAN, bridge closest to root becomes designated bridge.

30. CMPE 150- Introduction to Computer Networks 30 Spanning Tree: Example B3 LAN 2 LAN 1 LAN 3LAN 4 LAN 5 B5 B4 B1 B2 10 5 5 5 5 5 5 B3 LAN 2 LAN 1 LAN 3LAN 4 LAN 5 B5 B4 B1 B2 10 5 5 5 5 5 5

31. CMPE 150- Introduction to Computer Networks 31 Spanning Tree: Example B1 LAN 1 LAN 2 B2 LAN 3LAN 4 LAN 5 B4 B5 B3. Only designated bridges on each LAN allowed to forward frames.. Bridges continue exchanging info to react to topology changes.

32. CMPE 150- Introduction to Computer Networks 32 Remote Bridges Remote bridges can be used to interconnect distant LANs.