1. 15.1 Chapter 15 Connecting LANs, Backbone Networks, and Virtual LANs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 15.2 15-1 CONNECTING DEVICES In this section, we divide connecting devices into five different categories based on the layer in which they operate in a network. Passive Hubs Active Hubs Bridges Two-Layer Switches Routers Three-Layer Switches Gateways Topics discussed in this section:

3. 15.3 Figure 15.1 Five categories of connecting devices

4. 15.4 5 categories of connecting devices Defined as: Those which operate below the physical layer such as a passive hub. Those which operate at the physical layer such as the repeater or an active hub. Those which operate at the physical and data link layers such as bridge or two-layer switch. Those which operate at the physical, data link and network layers such as the router or three-layer switch. Those which can operate at all five layers such as the gateway.

5. 15.5

6. 15.6 Passive Hub Connects wires coming from different branches. Location on the Internet model – below the physical layer. Considered part of the transmission media.

7. 15.7 Repeater Repeater is a device that operates only in the physical layer. Signals can carry information for a limited distance. Weak signals are regenerated and sent out to the next hop. Increases the distance of your network.

8. 15.8 Is used to extend the physical length of a network. Regenerates the electrical signal and then send the refreshed signal. Repeaters do not have the intelligence to route traffic. Repeaters cannot connect two LANs; it connects two segments of the same LAN. Segment: creating physical network. In another words, it is a subset of larger network. Repeater

9. 15.9 Figure 15.2 A repeater connecting two segments of a LAN

10. 15.10 A repeater forwards every frame; it has no filtering capability. Note

11. 15.11 A repeater is a regenerator, not an amplifier. Note

12. Repeater Not an amplifier Amplifier does not discriminate between the intended signal and noise. Repeater – regenerates bit by bit a weakened signal. 15.12

13. 15.13 Figure 15.3 Function of a repeater

14. 15.14 Active Hubs Multiport repeater Used to create connections between stations in a physical star topology. Does not have a filtering capability; Does not have the intelligence to find from which port the frame should be sent out. It regenerates the corrupted signals and send them out from every port.

15. 15.15 Figure 15.4 A hierarchy of hubs

16. 15.16 Bridge Operates in both the Physical and Data Link Layer Allows you divide your network into smaller segments Contain logic that allows them to keep traffic for each segment separate. Check the source and destination MAC addresses in the frame. Also regenerates the signal to eliminate attenuation

17. 15.17 Bridges Bridges are used to connect 2 or more LANs. Can connect similar LANs (2 Ethernet LANs) Can also connect different LANs (Token Ring to a Ethernet). Cannot maintain the data integrity.

18. 15.18 A bridge has a table used in filtering decisions. Note

19. 15.19 Figure 15.5 A bridge connecting two LANs

20. 15.20 A bridge does not change the physical (MAC) addresses in a frame. Note

21. 15.21 Types of Bridges Simple Bridge Multiport Bridge Transparent Bridge

22. 15.22 Simple Bridge Uses a look up table that contains the physical addresses of every station connected to it. The table indicates which segment the station belongs. Addresses must be entered manually (by hand). maintenance is time consuming Least expensive hardware

23. 15.23 Multiport Bridge Used to connect more than 2 LANs together. Uses multiple tables to hold the physical addresses of each LAN.

24. 15.24 Transparent Bridge Sometimes called a Learning Bridge Does not require you to manually input the physical addresses of your stations. Automatically creates and updates the Look up table. Uses the source and destination addresses in each packet.

25. 15.25 Transparent Bridge According to IEEE 802.1d specfications, a system equipped with transparent bridges must meet 3 criteria: Frames must be forwarded from one station to another. The forwarding table is automatically made by learning frame movements in the network. Loops in the system must be prevented.

26. 15.26 Forwarding A transparent bridge must correctly forward the frames using its table.

27. 15.27 Learning Requires a table that dynamically maps addresses to ports automatically. Bridge inspects both the destination and the source address. The destination address is used for the forwarding decision (table lookup); The source address is used for adding entries to the table and for updating purposes.

28. Example of a Learning Bridge 15.28

29. 15.29 Loop Problem If redundant bridges in the system exist, the frames get forwarded twice to the receiver. Does not fill in the destination address correctly. Floods the network. TO solve the problem IEEE requires that bridges use the spanning tree algorithm

30. Loop Problem in a Learning Bridge 15.30

31. 15.31 Spanning tree Is a graph in which there is no loop. Creating a topology in which each LAN can be reached from any other LAN through one path only (no loop). To find the spanning tree: Every bridge has a built in ID ID is broadcasted to all other bridges. The smallest ID is selected as the root bridge. The algorithm finds the shortest path (lowest cost) from the root bridge to every other bridge or LAN. The ports on each bridge as designated as Forwarding ports others are blocking ports Forwarding ports: is a part of the spanning tree and a frame can be forwarded. Blocking ports: is not a part of the spanning tree and a frame cannot be forwarded.

32. 15.32 Source Routing Bridges TO prevent loop problems, source routing bridges require that the sending station defines the bridges that the frame must visit. The sending stations defines the addresses of these bridges and they are included in the frame. Source address, Destination address and all addresses of bridges

33. 15.33 Source Routing Bridges Designed by IEEE to be used with Token Ring LANs Token Ring LANs are not very common today.

34. 15.34 Layer 2 and 3 switches Layer 2 switch functions as a device that operates at the Physical and Data Link layer. Uses Mac addressing Same as a Bridge, but with many ports and better (faster) performance. Has a filtering capability based on the MAC address. Some are more sophisticated and include a buffer that to hold frames for processing. New two layer switch is called Cut-through switch forwards the frame as soon as they check the MAC addresses in the header of the frame.

35. 15.35 3 Switches/Routers Layer 3 switch is the same as a router. Routes traffic based on logical address such as IP address. Router connects LANS and WANs. Addressing tables are dynamic and are updated using routing protocols. The switching fabric in a three-layer switch allows faster table lookup and forwarding.

36. 15.36 A Router

37. 15.37 Router with LAN and WAN

38. 15.38 Internetworking devices Routers More sophisticated hardware than the Networking devices Used to connect 2 or more networks. Using addresses, it can determine several possible paths to route packets. Uses Logical Address – IP address Determines the best path.

39. 15.39 Internetworking devices Routers Whenever there are multiple paths from Source to Destination, the Router determines the best path.

40. 15.40 Internetworking devices Gateway Used as a Protocol Converter Gateways are usually routers with additional software installed. Usually a computer that operates in all OSI layers. Also able to provide some security by filtering unwanted application-layer messages.

41. 15.41 A Gateway

42. 15.42 Figure 21-13 A Gateway

43. 15.43 Internetworking devices Gateway Allows you to connect two different networks using different protocols to each other. Example : connect an AppleTalk network to a TCP/IP network – must use a Gateway.

44. 15.44 Backbone Networks A backbone network allows several LANs to be connected. In a backbone network, no station is directly connected to the backbone; the stations are part of a LAN and the backbone connects the LANs. Bus Backbone: is used as a distribution backbone to connect different building in an organization. Star Backbone: the backbone is just one switch that connect the LANs. It is used as a distribution backbone inside a building.

45. Bus Backbone 15.45

46. Star Backbone 15.46

47. 15.47 Virtual LANs - VLANs Local area Network configured by software, not by physical wiring. Divides a LAN into logical, instead of physical segments.

48. A switch using VLAN Software 15.48

49. Advantages of VLAN VLANs can reduce the relocation cost of stations going from one group to another. Physical reconfiguration takes times and costly. Can be used to create virtual work groups. Extra measure of security. People belonging to the same group can send broadcast messages with the guaranteed assurance of users in other groups will not receive these messages. 15.49