1. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.1 Computer Networks and Internets, 5e By Douglas E. Comer Lecture PowerPoints By Lami Kaya, LKaya@ieee.org

2. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.2 Chapter 17 LAN Extensions: Fiber Modems, Repeaters, Bridges, and Switches

3. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.3 Topics Covered 17.1 Introduction 17.2 Distance Limitation and LAN Design 17.3 Fiber Modem Extensions 17.4 Repeaters 17.5 Bridges and Bridging 17.6 Learning Bridges and Frame Filtering 17.7 Why Bridging Works Well 17.8 Distributed Spanning Tree 17.9 Switching and Layer 2 Switches 17.10 VLAN Switches 17.11 Bridging Used with Other Devices

4. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.4 17.1 Introduction This chapter –discusses two important concepts: mechanisms that can extend a LAN across a longer distance and LAN switching –introduces repeaters, bridges, and the spanning tree algorithm used to prevent forwarding loops

5. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.5 17.2 Distance Limitation and LAN Design Distance limitation is a fundamental part of LAN designs When designing a network technology, engineers choose a combination of factors that can be achieved at a given cost –Capacity –Maximum delay –Distance Hardware is designed to emit a fixed amount of energy if wiring is extended beyond the design limits stations will not receive a sufficiently strong signal, and errors will occur

6. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.6 17.3 Fiber Modem Extensions Extension mechanisms do not increase the signal strength –nor do they merely extend cables Most extension mechanisms use standard interface –and insert additional hardware components that can relay signals across longer distances The simplest LAN extension mechanism consists of an optical fiber and a pair of fiber modems –used to connect a computer to a remote Ethernet Figure 17.1 illustrates the interconnection –Each of the fiber modems contains hardware to perform two chores: accept packets over the Ethernet interface and send them over the optical fiber and accept packets that arrive over the optical fiber and send them over the Ethernet interface

7. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.7 17.3 Fiber Modem Extensions

8. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.8 17.4 Repeaters A repeater is an analog device used to propagate LAN signals over long distances –A repeater does not understand packets or signal coding –Instead, it merely amplifies the signal received and transmits the amplified version as output Repeaters were used extensively with the original Ethernet, and have been used with other LAN technologies –Recently, repeaters have been introduced with infrared receivers to permit a receiver to be located at a longer distance from a computer –Consider a situation in which the infrared receiver for a cable television controller must be in a different room than the controller A repeater can extend the connection, as Figure 17.2 illustrates

9. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.9 17.4 Repeaters

10. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.10 17.5 Bridges and Bridging A bridge is a mechanism that connects two LAN segments The bridge listens in promiscuous mode on each segment –i.e., receives all packets sent on the segment When it receives an intact frame from one segment –the bridge forwards a copy of the frame to the other segment Two LAN segments connected by a bridge appear to behave like a single LAN –a computer connected to either segment can send a frame to any computer on the both segments A broadcast frame is delivered to all computers –Thus, computers do not know whether they are connected to a single LAN segment or a bridged LAN Figure 17.3 illustrates the conceptual architecture

11. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.11 17.5 Bridges and Bridging

12. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.12 17.6 Learning Bridges and Frame Filtering Bridges do not blindly forward a copy of each frame from one LAN to another –Instead, a bridge uses MAC addresses to perform filtering A bridge examines the destination address in a frame –and does not forward the frame onto the other LAN segment unless necessary If the LAN supports broadcast or multicast –the bridge must forward a copy of each broadcast or multicast frame to make the bridged LAN operate like a single LAN How can a bridge know which computers are attached to which segments? –Most bridges are called adaptive or learning bridges because they learn the locations of computers automatically –To do so, a bridge uses source addresses

13. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.13 17.6 Learning Bridges and Frame Filtering When a frame arrives from a given segment –the bridge extracts the source address from the header –and adds the address to a list of computers attached to the segment Bridge must then extract the MAC address from the frame –and use the address to determine whether to forward the frame A bridge learns that a computer is present on a segment as soon as the computer transmits a frame Consider the bridge in Figure 17.3 Consider also Figure 17.4, which –lists a sequence of packet transmissions –the location information that the bridge has accumulated at each step –and the disposition of the packet (i.e., the segments over which the packet is sent)

14. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.14 17.6 Learning Bridges and Frame Filtering Fig 17.3 Fig 17.4

15. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.15 17.7 Why Bridging Works Well A bridged network can exhibit higher overall performance than a single LAN A bridge permits simultaneous transmission on each segment In Figure 17.3, for example –computer A can send a packet to computer B –at the same time computer X sends a packet to computer Y –Although it receives a copy of each packet the bridge will not forward either of them because each packet has been sent to a destination on the same segment as the source –the bridge merely discards the two frames without forwarding them A bridge permits simultaneous activity on attached segments –a pair of computers on one segment can communicate at the same time as a pair of computers on another segment

16. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.16 17.8 Distributed Spanning Tree Consider Figure 17.5 (below) –It shows four LAN segments currently connected by three bridges and a fourth bridge about to be inserted –We assume that computers (not shown in the diagram) are also plugged into each of the hubs

17. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.17 17.8 Distributed Spanning Tree Before the fourth bridge is inserted –the network operates as expected any computer can send a unicast frame to another computer or send a broadcast or multicast frame to all computers If a fourth bridge is inserted –a problem arises because a loop will exist Unless at least one bridge is prevented from forwarding broadcasts –copies of a broadcast frame will continue to flow around the cycle forever –And computers attached to hubs will receive many copies To prevent cycles, a Distributed Spanning Tree (DST) is used –the algorithm views bridges as nodes in a graph –and imposes a tree on the graph (a tree is a graph that does not contain cycles)

18. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.18 17.8 Distributed Spanning Tree The original (DEC in 1985) was designed for Ethernet networks –known as Spanning Tree Protocol (STP) STP consists of three steps: – Root election bridges multicast a packet that contains their bridge ID, and the bridge with the smallest ID is chosen To permit a manager to control the election, a bridge ID consists of two parts: a 16-bit configurable priority number and a 48-bit MAC address – Shortest path computation Each bridge computes a shortest path to the root bridge. Links included in the shortest paths of all bridges form the spanning tree –Forwarding An interface that connects to the shortest path is enabled for forwarding packets; an interface that does not lie on the shortest path is blocked, In STP, Ethernet bridges communicate amongst themselves using a multicast address that is reserved for STP

19. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.19 17.8 Distributed Spanning Tree Variations of STP have been designed and standardized –IEEE created a standard named 802.1d (in 1990) –the standard was updated in 1998 IEEE standard 802.1q provides a way to run STP on a set of logically independent networks –that share a physical medium without any confusion or interference Cisco created a proprietary version of STP, Per-VLAN Spanning Tree (PVST) for use on a VLAN switch IEEE standard 802.1w introduced the Rapid STP (RSTP) has been incorporated in 801.1d-2004 (in 1998), and now replaces STP, some versions are –Multiple Instance STP (MISTP) –Multiple STP (MSTP)

20. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.20 17.9 Switching and Layer 2 Switches An Ethernet switch, sometimes called a Layer 2 switch is an electronic device that resembles a hub –a switch provides multiple ports that each attach to a single computer –and a switch allows computers to send frames to one another The difference between a hub and a switch arises from the way the devices operate: –a hub operates as an analog device that forwards signals among computers –while a switch is a digital device that forwards packets –We can think of a hub as simulating a shared transmission medium –We think of a switch as simulating a bridged network that has one computer per LAN segment Figure 17.6 illustrates the conceptual use of bridges in a switch

21. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.21 17.9 Switching and Layer 2 Switches

22. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.22 17.9 Switching and Layer 2 Switches A switch does not contain separate bridges –a switch consists of an intelligent interface attached to each port –and a central fabric that provides simultaneous transfers An interface contains –a processor, memory, and other hardware needed to accept a packet –consult a forwarding table –and send the packet across the fabric to the correct output port An interface can buffer arriving packets when an output port is busy Figure 17.7 illustrates the architecture –Physically, switches are available in many sizes (ports) Advantage of using a switched LAN instead of a hub is parallelism –Although a hub can only support one transmission at a time a switch permits multiple transfers to occur at the same time, provided the transfers are independent

23. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.23 17.9 Switching and Layer 2 Switches

24. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.24 17.10 VLAN Switches Virtual Local Area Network (VLAN) switches The concept is straightforward: –allow a manager to configure a single switch to emulate multiple, independent switches A manager can specify a set of ports on the switch and designates them to be on virtual LAN 1 –designates another set of ports to be on virtual LAN 2, and so on When a computer on virtual LAN 2 broadcasts a packet –only those computers on the same virtual LAN receive a copy –(i.e., once configured, a VLAN switch makes it appear that there are multiple switches)

25. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.25 17.10 VLAN Switches Dividing computers into separate broadcast domains does not appear important –until one considers a large company or a service provider In each case, it may be important to guarantee that a set of computers can communicate –without others receiving the packets and without receiving packets from outsiders For example, a company may choose to provide a firewall between computers in the CEO's office and other computers in the company

26. © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.26 17.11 Bridging Used with Other Devices Bridging is a fundamental concept that has been incorporated into many devices For example, a DSL or cable modem provides a form of bridging: the modem –provides an Ethernet connection at a subscriber's residence –and transfers Ethernet packets between the subscriber's location and the provider's network Some wireless technologies also use a form of bridging to transfer frames from a mobile device to a provider's network