CCENT/CCNA ICND1 Official Exam Certification Guide - Chapter 7 doc

Part II: LAN Switching Chapter 7 Ethernet LAN Switching Concepts Chapter 8 Operating Cisco LAN Switches Chapter 9 Ethernet Switch Configuration Chapter 10 Ethernet Switch Troubleshooting Chapter 11 Wireless LANs 1828xbook.fm Page 165 Thursday, July 26, 2007 3:10 PM This chapter covers the following subjects: LAN Switching Concepts: Explains the basic processes used by LAN switches to forward frames. LAN Design Considerations: Describes the reasoning and terminology for how to design a switched LAN that operates well. 1828xbook.fm Page 166 Thursday, July 26, 2007 3:10 PM C H A P T E R 7 Ethernet LAN Switching Concepts Chapter 3, “Fundamentals of LANs,” covered the conceptual and physical attributes of Ethernet LANs in a fair amount of detail. That chapter explains a wide variety of Ethernet concepts, including the basics of UTP cabling, the basic operation of and concepts behind hubs and switches, comparisons of different kinds of Ethernet standards, and Ethernet data link layer concepts such as addressing and framing. The chapters in Part II, “LAN Switching,” complete this book’s coverage of Ethernet LANs, with one additional chapter (Chapter 11) on wireless LANs. This chapter explains most of the remaining Ethernet concepts that were not covered in Chapter 3. In particular, it contains a more detailed examination of how switches work, as well as the LAN design implications of using hubs, bridges, switches, and routers. Chapters 8 through 10 focus on how to access and use Cisco switches. Chapter 8, “Operating Cisco LAN Switches,” focuses on the switch user interface. Chapter 9, “Ethernet Switch Configuration,” shows you how to configure a Cisco switch. Chapter 10, “Ethernet Switch Troubleshooting,” shows you how to troubleshoot problems with Cisco switches. Chapter 11, “Wireless LANs,” concludes Part II with a look at the concepts behind wireless LANs. “Do I Know This Already?” Quiz The “Do I Know This Already?” quiz allows you to assess whether you should read the entire chapter. If you miss no more than one of these eight self-assessment questions, you might want to move ahead to the “Exam Preparation Tasks” section. Table 7-1 lists the major headings in this chapter and the “Do I Know This Already?” quiz questions covering the material in those sections. This helps you assess your knowledge of these specific areas. The answers to the “Do I Know This Already?” quiz appear in Appendix A. Table 7-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping Foundation Topics Section Questions LAN Switching Concepts 1–5 LAN Design Considerations 6–8 1828xbook.fm Page 167 Thursday, July 26, 2007 3:10 PM 168 Chapter 7: Ethernet LAN Switching Concepts 1. Which of the following statements describes part of the process of how a switch decides to forward a frame destined for a known unicast MAC address? a. It compares the unicast destination address to the bridging, or MAC address, table. b. It compares the unicast source address to the bridging, or MAC address, table. c. It forwards the frame out all interfaces in the same VLAN except for the incom- ing interface. d. It compares the destination IP address to the destination MAC address. e. It compares the frame’s incoming interface to the source MAC entry in the MAC address table. 2. Which of the following statements describes part of the process of how a LAN switch decides to forward a frame destined for a broadcast MAC address? a. It compares the unicast destination address to the bridging, or MAC address, table. b. It compares the unicast source address to the bridging, or MAC address, table. c. It forwards the frame out all interfaces in the same VLAN except for the incoming interface. d. It compares the destination IP address to the destination MAC address. e. It compares the frame’s incoming interface to the source MAC entry in the MAC address table. 3. Which of the following statements best describes what a switch does with a frame destined for an unknown unicast address? a. It forwards out all interfaces in the same VLAN except for the incoming inter- face. b. It forwards the frame out the one interface identified by the matching entry in the MAC address table. c. It compares the destination IP address to the destination MAC address. d. It compares the frame’s incoming interface to the source MAC entry in the MAC address table. 1828xbook.fm Page 168 Thursday, July 26, 2007 3:10 PM “Do I Know This Already?” Quiz 169 4. Which of the following comparisons does a switch make when deciding whether a new MAC address should be added to its bridging table? a. It compares the unicast destination address to the bridging, or MAC address, table. b. It compares the unicast source address to the bridging, or MAC address, table. c. It compares the VLAN ID to the bridging, or MAC address, table. d. It compares the destination IP address’s ARP cache entry to the bridging, or MAC address, table. 5. PC1, with MAC address 1111.1111.1111, is connected to Switch SW1’s Fa0/1 interface. PC2, with MAC address 2222.2222.2222, is connected to SW1’s Fa0/2 interface. PC3, with MAC address 3333.3333.3333, connects to SW1’s Fa0/3 interface. The switch begins with no dynamically learned MAC addresses, followed by PC1 sending a frame with a destination address of 2222.2222.2222. If the next frame to reach the switch is a frame sent by PC3, destined for PC2’s MAC address of 2222.2222.2222, which of the following are true? a. The switch forwards the frame out interface Fa0/1. b. The switch forwards the frame out interface Fa0/2. c. The switch forwards the frame out interface Fa0/3. d. The switch discards (filters) the frame. 6. Which of the following devices would be in the same collision domain as PC1? a. PC2, which is separated from PC1 by an Ethernet hub b. PC3, which is separated from PC1 by a transparent bridge c. PC4, which is separated from PC1 by an Ethernet switch d. PC5, which is separated from PC1 by a router 7. Which of the following devices would be in the same broadcast domain as PC1? a. PC2, which is separated from PC1 by an Ethernet hub b. PC3, which is separated from PC1 by a transparent bridge c. PC4, which is separated from PC1 by an Ethernet switch d. PC5, which is separated from PC1 by a router 1828xbook.fm Page 169 Thursday, July 26, 2007 3:10 PM 170 Chapter 7: Ethernet LAN Switching Concepts 8. Which of the following Ethernet standards support a maximum cable length of longer than 100 meters? a. 100BASE-TX b. 1000BASE-LX c. 1000BASE-T d. 100BASE-FX 1828xbook.fm Page 170 Thursday, July 26, 2007 3:10 PM LAN Switching Concepts 171 Foundation Topics This chapter begins by covering LAN concepts—in particular, the mechanics of how LAN switches forward Ethernet frames. Following that, the next major section focuses on campus LAN design concepts and terminology. It includes a review of some of the Ethernet types that use optical cabling and therefore support longer cabling distances than do the UTP-based Ethernet standards. LAN Switching Concepts Chapter 3 introduced Ethernet, including the concept of LAN hubs and switches. When thinking about how LAN switches work, it can be helpful to think about how earlier products (hubs and bridges) work. The first part of this section briefly looks at why switches were created. Following that, this section explains the three main functions of a switch, plus a few other details. Historical Progression: Hubs, Bridges, and Switches As mentioned in Chapter 3, Ethernet started out with standards that used a physical electrical bus created with coaxial cabling. 10BASE-T Ethernet came next. It offered improved LAN availability, because a problem on a single cable did not affect the rest of the LAN—a common problem with 10BASE2 and 10BASE5 networks. 10BASE-T allowed the use of unshielded twisted-pair (UTP) cabling, which is much cheaper than coaxial cable. Also, many buildings already had UTP cabling installed for phone service, so 10BASE-T quickly became a popular alternative to 10BASE2 and 10BASE5 Ethernet networks. For perspective and review, Figure 7-1 depicts the typical topology for 10BASE2 and for 10BASE-T with a hub. Figure 7-1 10BASE2 and 10BASE-T (with a Hub) Physical Topologies Larry Archie Bob Solid Lines Represent Co-ax Cable 10BASE2, Single Bus Larry Archie Bob Solid Lines Represent Twisted Pair Cabling 10BASE-T, Using Shared Hub - Acts like Single Bus Hub 1 1828xbook.fm Page 171 Thursday, July 26, 2007 3:10 PM 172 Chapter 7: Ethernet LAN Switching Concepts Although using 10BASE-T with a hub improved Ethernet as compared to the older standards, several drawbacks continued to exist, even with 10BASE-T using hubs: ■ Any device sending a frame could have the frame collide with a frame sent by any other device attached to that LAN segment. ■ Only one device could send a frame at a time, so the devices shared the (10-Mbps) bandwidth. ■ Broadcasts sent by one device were heard by, and processed by, all other devices on the LAN. When these three types of Ethernet were introduced, a shared 10 Mbps of bandwidth was a huge amount! Before the introduction of LANs, people often used dumb terminals, with a 56-kbps WAN link being a really fast connection to the rest of the network—and that 56 kbps was shared among everyone in a remote building. So, in the days when 10BASE-T was first used, getting a connection to a 10BASE-T Ethernet LAN was like getting a Gigabit Ethernet connection for your work PC today. It was more bandwidth than you thought you would ever need. Over time, the performance of many Ethernet networks started to degrade. People developed applications to take advantage of the LAN bandwidth. More devices were added to each Ethernet. Eventually, an entire network became congested. The devices on the same Ethernet could not send (collectively) more than 10 Mbps of traffic because they all shared the 10 Mbps of bandwidth. In addition, the increase in traffic volumes increased the number of collisions. Long before the overall utilization of an Ethernet approached 10 Mbps, Ethernet began to suffer because of increasing collisions. Ethernet bridges were created to solve some of the performance issues. Bridges solved the growing Ethernet congestion problem in two ways: ■ They reduced the number of collisions that occurred in the network. ■ They added bandwidth to the network. Figure 7-2 shows the basic premise behind an Ethernet transparent bridge. The top part of the figure shows a 10BASE-T network before adding a bridge, and the lower part shows the network after it has been segmented using a bridge. The bridge creates two separate collision domains. Fred’s frames can collide with Barney’s, but they cannot collide with Wilma’s or Betty’s. If one LAN segment is busy, and the bridge needs to forward a frame onto the busy segment, the bridge simply buffers the frame (holds the frame in memory) until the segment is no longer busy. Reducing collisions, and assuming no significant change in the number of devices or the load on the network, greatly improves network performance. 1828xbook.fm Page 172 Thursday, July 26, 2007 3:10 PM LAN Switching Concepts 173 Figure 7-2 Bridge Creates Two Collision Domains and Two Shared Ethernets Adding a bridge between two hubs really creates two separate 10BASE-T networks—one on the left and one on the right. The 10BASE-T network on the left has its own 10 Mbps to share, as does the network on the right. So, in this example, the total network bandwidth is doubled to 20 Mbps, as compared with the 10BASE-T network at the top of the figure. LAN switches perform the same basic core functions as bridges, but with many enhanced features. Like bridges, switches segment a LAN into separate parts, each part being a separate collision domain. Switches have potentially large numbers of interfaces, with highly optimized hardware, allowing even small Enterprise switches to forward millions of Ethernet frames per second. By creating a separate collision domain for each interface, switches multiply the amount of available bandwidth in the network. And, as mentioned in Chapter 3, if a switch port connects to a single device, that Ethernet segment can use full- duplex logic, essentially doubling the speed on that segment. Figure 7-3 summarizes some of these key concepts, showing the same hosts as in Figure 7-2, but now connected to a switch. In this case, all switch interfaces are running at 100 Mbps, with four collision domains. Note that each interface also uses full duplex. This is possible NOTE A switch’s effect of segmenting an Ethernet LAN into one collision domain per interface is sometimes called microsegmentation. 1 Collision Domain Sharing 10 Mbps 1 Collision Domain Sharing 10 Mbps 1 Collision Domain Sharing 10 Mbps Bridge Fred Wilma Barney Fred Barney Wilma Betty Betty 1828xbook.fm Page 173 Thursday, July 26, 2007 3:10 PM 174 Chapter 7: Ethernet LAN Switching Concepts because only one device is connected to each port, essentially eliminating collisions for the network shown. Figure 7-3 Switch Creates Four Collision Domains and Four Ethernet Segments The next section examines how switches forward Ethernet frames. Switching Logic Ultimately, the role of a LAN switch is to forward Ethernet frames. To achieve that goal, switches use logic—logic based on the source and destination MAC address in each frame’s Ethernet header. To help you appreciate how switches work, first a review of Ethernet addresses is in order. The IEEE defines three general categories of Ethernet MAC addresses: ■ Unicast addresses: MAC addresses that identify a single LAN interface card. ■ Broadcast addresses: A frame sent with a destination address of the broadcast address (FFFF.FFFF.FFFF) implies that all devices on the LAN should receive and process the frame. ■ Multicast addresses: Multicast MAC addresses are used to allow a dynamic subset of devices on a LAN to communicate. NOTE The IP protocol supports the multicasting of IP packets. When IP multicast packets are sent over an Ethernet, the multicast MAC addresses used in the Ethernet frame follow this format: 0100.5exx.xxxx, where a value between 00.0000 and 7f.ffff can be used in the last half of the address. Ethernet multicast MAC addresses are not covered in this book. Fa0/4 Fa0/3 Fred 0200.1111.1111 Barney 0200.2222.2222 Fa0/2 Fa0/1 Wilma 0200.3333.3333 Betty 0200.4444.4444 Each Circle Is 1 Collision Domain, 100 Mbps Each 1828xbook.fm Page 174 Thursday, July 26, 2007 3:10 PM [...]... July 26, 20 07 3:10 PM 194 Chapter 7: Ethernet LAN Switching Concepts Exam Preparation Tasks Review All the Key Topics Review the most important topics from this chapter, noted with the key topics icon Table 7- 5 lists these key topics and where each is discussed Table 7- 5 Key Topics for Chapter 7 Key Topic Element Description Page Number List Some of the benefits of switching 175 Figure 7- 4 Example of... forwarding logic 176 Figure 7- 5 Example of switch filtering logic 177 Figure 7- 6 Example of how a switch learns MAC addresses 178 Table 7- 2 Summary of three switch internal forwarding options 181 List Some of the benefits of switching 182 List Summary of logic used to forward and filter frames and to learn MAC addresses 182 List Definitions of collision domain and broadcast domain 185 Table 7- 3 Four LAN design... broadcast domain, as shown in Figure 7- 1 0 Figure 7- 1 0 Sample Network with Two Broadcast Domains and No VLANs Dino Fred Wilma Betty 1 87 1828xbook.fm Page 188 Thursday, July 26, 20 07 3:10 PM 188 Chapter 7: Ethernet LAN Switching Concepts Alternately, you can create multiple broadcast domains using a single switch Figure 7- 1 1 shows the same two broadcast domains as in Figure 7- 1 0, now implemented as two different... STP A simple example makes the need for STP more obvious Remember, switches flood frames sent to both unknown unicast MAC addresses and broadcast addresses 179 1828xbook.fm Page 180 Thursday, July 26, 20 07 3:10 PM 180 Chapter 7: Ethernet LAN Switching Concepts Figure 7- 7 shows that a single frame, sent by Larry to Bob, loops forever because the network has redundancy but no STP Figure 7- 7 Network with... 10BASE-T TIA/EIA CAT3 or better, two pair 100 m (328 feet) 100BASE-TX TIA/EIA CAT5 UTP or better, two pair 100 m (328 feet) 100BASE-FX 62.5/125-micron multimode fiber 400 m (1312.3 feet) 1000BASE-CX STP 25 m (82 feet) 1000BASE-T TIA/EIA CAT5e UTP or better, four pair 100 m (328 feet) 1000BASE-SX Multimode fiber 275 m (853 feet) for 62.5-micron fiber 550 m (1804.5 feet) for 50-micron fiber 1000BASE-LX Multimode... Figure 7- 8 illustrates collision domains 183 1828xbook.fm Page 184 Thursday, July 26, 20 07 3:10 PM 184 Chapter 7: Ethernet LAN Switching Concepts Figure 7- 8 Collision Domains NOTE The LAN design in Figure 7- 8 is not a typical design today Instead, it simply provides enough information to help you compare hubs, switches, and routers Each separate segment, or collision domain, is shown with a dashed-line... 50-micron fiber 1000BASE-LX Multimode fiber 550 m (1804.5 feet) for 5 0- and 62.5-micron fiber 1000BASE-LX 9-micron single-mode fiber 10 km (6.2 miles) 1828xbook.fm Page 193 Thursday, July 26, 20 07 3:10 PM LAN Design Considerations Most engineers simply remember the general distance limitations and then use a reference chart (such as Table 7- 4 ) to remember each specific detail An engineer must also consider... to examine and understand the address table The table lists MAC addresses and the interface the switch should use when forwarding packets sent to that MAC address For example, the table lists 0200.3333.3333 off Fa0/3, which is the interface out which the switch should forward frames sent to Wilma’s MAC address (0200.3333.3333) 175 1828xbook.fm Page 176 Thursday, July 26, 20 07 3:10 PM 176 Chapter 7: ... incoming frames and examining the source MAC address in the frame If a frame enters the switch and the source MAC address is not in the MAC address table, the switch creates an entry in the table The MAC address is placed in the table, along with the interface from which the frame arrived Switch learning logic is that simple 177 1828xbook.fm Page 178 Thursday, July 26, 20 07 3:10 PM 178 Chapter 7: Ethernet... table For example, in Figure 7- 6 , the switch forwards the first frame out Fa0/2, Fa0/3, and Fa0/4, even though 0200.2222.2222 (Barney) is only off Fa0/2 The switch does not forward the frame back out Fa0/1, because a switch never forwards a frame out the same 1828xbook.fm Page 179 Thursday, July 26, 20 07 3:10 PM LAN Switching Concepts interface on which it arrived (As a side note, Figure 7- 6 does not . Transmission 1828xbook.fm Page 177 Thursday, July 26, 20 07 3:10 PM 178 Chapter 7: Ethernet LAN Switching Concepts Figure 7- 6 depicts the same network as Figure 7- 4 , but before the switch has built. table. 1828xbook.fm Page 175 Thursday, July 26, 20 07 3:10 PM 176 Chapter 7: Ethernet LAN Switching Concepts Figure 7- 4 Sample Switch Forwarding and Filtering Decision Figure 7- 5 shows a different. Cabling 10BASE-T, Using Shared Hub - Acts like Single Bus Hub 1 1828xbook.fm Page 171 Thursday, July 26, 20 07 3:10 PM 172 Chapter 7: Ethernet LAN Switching Concepts Although using 10BASE-T with a