Tài liệu WAN Introduction pdf

15 WAN Introduction CERTIFICATION OBJECTIVES 15.01 Wide Area Networking Overview 15.02 HDLCp 15.03 PPP ✓ Two-Minute Drill Q&A Self Test CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 Blind Folio 15:1 D:\omh\CertPrs8\934-9\ch15.vp Monday, August 04, 2003 12:15:12 PM Color profile: Generic CMYK printer profile Composite Default screen T he last few chapters introduced you to configuring IP features on your Cisco router. This chapter introduces you to wide area networking (WAN) concepts and some basic point-to-point configurations, including HDLC and PPP. The two chapters following this, Frame Relay and ISDN, focus on packet-switched and dialup connections, respectively. CERTIFICATION OBJECTIVE 15.01 Wide Area Networking Overview Typically, LAN connections are within a company and WAN connections allow you to connect to remote sites. Typically, you don’t own the infrastructure for WAN connections—another company, such as a telephone company, provides the infrastructure. WAN connections are usually slower than LAN connections. A derivative of WAN solutions is the metropolitan area network (MAN). MANs sometimes use high-speed LAN connections in a small geographic area between different companies, or divisions within a company. MANs are becoming more and more popular in large cities and even provide connections over a LAN medium, such as Ethernet. One of the major factors when choosing a WAN or MAN provider is cost. These connections are billed in multiple ways: flat monthly lease cost, per-packet cost, per-minute cost, and many other methods. On top of this, you have many solutions to choose from to solve your WAN connection problems. In order to choose the right solution, you’ll need to weigh your connection requirements, your traffic patterns, and the cost of the solution. Equipment and Components WAN connections are made up of many types of equipment and components. Figure 15-1 shows some of these WAN terms. Table 15-1 has a list of the terms and definitions. As you may recall from Chapter 2, a DCE terminates a connection between two sites and provides clocking and synchronization for that connection; it connects to a DTE. The DCE category includes equipment such as CSU/DSUs, NT1s, and modems. A DTE is an end-user device, 2 Chapter 15: WAN Introduction CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 The most important factor in choosing a WAN service is cost. It is important to remember the WAN terms in Table 15-1. D:\omh\CertPrs8\934-9\ch15.vp Monday, August 04, 2003 12:15:12 PM Color profile: Generic CMYK printer profile Composite Default screen such as a router or PC, that connects to the WAN via the DCE equipment. In some circumstances, the function of the DCE might be built into the DTE’s physical interface. For instance, certain Cisco routers can be purchased with built-in NT1s or CSU/DSUs in their WAN interfaces. Wide Area Networking Overview 3 CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 FIGURE 15-1 WAN terms Term Definition CPE (customer premises equipment) This is your network’s equipment, which includes the DCE (modem, NT1, CSU/DSU) and your DTE (router, access server). Demarcation point This is where the responsibility of the carrier is passed on to you; this could be inside or outside your local facility. Please note that this is a logical boundary, not necessarily a physical boundary. Local loop This is the connection from the carrier's switching equipment to the demarcation point. CO (central office) switch This is the carrier's switch within the toll network. Toll network This is the carrier's internal infrastructure for transporting your data. TABLE 15-1 WAN Terms and Definitions D:\omh\CertPrs8\934-9\ch15.vp Monday, August 04, 2003 12:15:14 PM Color profile: Generic CMYK printer profile Composite Default screen Connection Types As mentioned at the beginning of this section, you have two major concerns when choosing a WAN solution: cost and the type of solution. There are many WAN solutions to choose from, including the following: analog modems and ISDN for dialup connections, ATM, dedicated point-to-point leased lines (dedicated circuits), DSL, Frame Relay, SMDS, wireless (including cellular, laser microwave, radio, and satellite), and X.25. As you can see from this list, you have a lot of choices. Not all of these solutions will be available in every area, and not every solution is ideal for your needs. Therefore, one of your first tasks is to have a basic understanding of some of these services. Chapter 1 provided a brief overview of some of these services. This chapter covers some of these services briefly, and Chapters 16 and 17 expand on some of the others. Typically, WAN connections fall under one of four categories: ■ Leased lines, such as dedicated circuits or connections ■ Circuit-switched connections, such as analog modem and digital ISDN dialup connections ■ Packet-switched connections, such as Frame Relay and X.25 ■ Cell-switched connections, such as ATM and SMDS The following three sections will introduce you to these three connection types. Leased-Line Connections A leased-line connection is basically a dedicated circuit connection between two sites. It simulates a single cable connection between the local and remote sites. Leased lines are best suited when both of these conditions hold: ■ The distance between the two sites is small, making them cost-effective. ■ You have a constant amount of traffic between two sites and need to guarantee bandwidth for certain applications. Even though leased lines can provide guaranteed bandwidth and minimal delay for connections, other available solutions, such as ATM, can provide the same features. The main disadvantage of leased lines is their cost—they are the most expensive WAN solution. Leased lines use synchronous serial connections, with their data rates ranging from 2,400 bps all the way up to 45 Mbps, in what is referred to as a DS3 connection. A 4 Chapter 15: WAN Introduction CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 Know about the four types of WAN connections: leased lines, circuit- switched connections, packet-switched connections, and cell-switched connections. D:\omh\CertPrs8\934-9\ch15.vp Monday, August 04, 2003 12:15:14 PM Color profile: Generic CMYK printer profile Composite Default screen synchronous serial connection allows you to simultaneously send and receive information without having to wait for any signal from the remote side. Nor does a synchronous connection need to indicate when it is beginning to send something or the end of a transmission. These two things, plus how clocking is done, are the three major differences between synchronous and asynchronous connections—asynchronous connections are typically used for dialup connections, such as modems. If you purchase a leased line, you will need the following equipment: ■ DTE A router with a synchronous serial interface: this provides the data link framing and terminates the WAN connection. ■ DCE A CSU/DSU to terminate the carrier’s leased-line connection: this provides the clocking and synchronization for the connection. Figure 15-2 shows an example of the equipment required for a leased-line connection. The CSU/DSU is responsible for handling the physical layer framing, clocking, and synchronization of the connection. Data link layer protocols that you can use for Wide Area Networking Overview 5 CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 FIGURE 15-2 Leased line example Remember that leased lines are used for short-distance connections and when you have a constant amount of traffic between sites with a need of guaranteed bandwidth. D:\omh\CertPrs8\934-9\ch15.vp Monday, August 04, 2003 12:15:17 PM Color profile: Generic CMYK printer profile Composite Default screen dedicated connections include PPP, SLIP, and HDLC. SLIP is rarely used and is restricted to IP traffic. SLIP has been replaced by PPP. Circuit-Switched Connections Circuit-switched connections are dialup connections, as are used by a PC with a modem when dialing up an ISP. Circuit-switched connections include the following types: ■ Asynchronous serial connections These include analog modem dialup connections and the standard telephone system, which is commonly referred to as Plain Old Telephone Service (POTS) by the telephone carriers. ■ Synchronous serial connections These include digital ISDN BRI and PRI dialup connections; they provide guaranteed bandwidth. Asynchronous serial connections are the cheapest form of WAN services but are also the most unreliable of the services. For instance, every time you make a connection using an analog modem, there is no guarantee of the connection rate you’ll get. With these connections, the top connection rate in the U.S. is 53 Kbps, but depending on the quality of the connection, you might get something as low as 300 bps. The Federal Communications Commission (FCC) restricts analog data rates to 53 Kbps or less. Other countries might support higher data rates. The main problem with circuit-switched connections is that they are expensive if you need to make connections over long distances, with a per-minute charge that varies, depending on the destination. Therefore, the more data you have to send, the more time it will take, and the more money it will cost. Asynchronous circuit-switched connections are typically used for home office and low-speed backup connections, as well as temporary low-speed connections for additional boosts in bandwidth when your primary link becomes congested or when it fails. ISDN (discussed in Chapter 17) provides a digital circuit-switched connection with guaranteed data rates. With leased lines, as soon as the circuit is installed and you have configured your DTE, the line remains up unless there is a problem with the carrier’s network or the DCE equipment. This is different from circuit-switched connections. These connections are temporary—you make a phone call to the remote DTE and when the line comes up, you transmit your data. Once you are done transmitting your data, the phone connection is terminated. 6 Chapter 15: WAN Introduction CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 Analog connections are restricted by the FCC to 53 Kbps. D:\omh\CertPrs8\934-9\ch15.vp Monday, August 04, 2003 12:15:17 PM Color profile: Generic CMYK printer profile Composite Default screen If you will be using a circuit-switched analog connection, you’ll need this equipment: ■ DTE A router with an asynchronous serial interface ■ DCE A modem If you will be using a circuit-switched digital connection, you’ll need this equipment: ■ DTE A router with an ISDN interface ■ DCE An NT1 for a BRI or a CSU/DSU for a PRI Figure 15-3 shows an example of an analog circuit-switched connection. With this connection, you’ll typically use PPP or HDLC for the encapsulation: SLIP is rarely used. Packet-Switched Connections With leased lines and circuit-switched connections, a physical circuit is used to make the connection between the two sites. With a leased line, the same circuit path is always used. With circuit-switched connections, the circuit path is built every time a phone call is made, making it highly probable that the same circuit path will not be used for every phone call. Packet-switched connections use logical circuits to make connections between two sites. These logical circuits are referred to as virtual circuits (VCs). One advantage that Wide Area Networking Overview 7 CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 FIGURE 15-3 Analog circuit-switched connection Remember that circuit- switched connections are typically used to back up primary connections, provide additional bandwidth boosts, and afford remote access to dialup users. D:\omh\CertPrs8\934-9\ch15.vp Monday, August 04, 2003 12:15:18 PM Color profile: Generic CMYK printer profile Composite Default screen a logical circuit has over a physical one is that a logical circuit is not tied to any particular physical circuit. Instead, a logical circuit is built across any available physical connection. Another advantage of logical circuits is that you can build multiple logical circuits over the same physical circuit. Therefore, with a single physical connection to a carrier, you can connect to multiple sites. This is not possible with leased lines: for each location you want to connect to, you need a separate physical circuit, making the cost of the solution much higher that one that uses logical circuits. Technologies that use packet switching and logical circuits include ATM, Frame Relay, SMDS, and X.25. From a cost perspective, packet-switched solutions fall somewhere between circuit-switched solutions and leased lines. The oldest of these four technologies is X.25, which is an ITU-T standard. X.25 is a network layer protocol that runs across both synchronous and asynchronous physical circuits, providing a lot of flexibility for your connection options. X.25 was actually developed to run across unreliable connections. It provides both error detection and correction, as well as flow control, at both the data link layer (by LAPB) and the network layer (by X.25). In this sense, it performs a function similar to what TCP, at the transport layer, provides for IP. Because of its overhead, X.25 is best delegated to asynchronous, unreliable connections. If you have a synchronous digital connection, another protocol, such as ATM or Frame Relay, is much more efficient. Frame Relay is a digital packet-switched service that can run only across synchronous digital connections at the data link layer. Because it uses digital connections (which have very few errors), it does not perform any error correction or flow control as X.25 does. Frame Relay will, however, detect errors and drop bad frames. It is up to a higher- layer protocol, such as IP’s TCP, to resend the dropped information. If you are setting up a Frame Relay connection, you’ll need the following equipment. ■ DTE A router with a synchronous serial interface ■ DCE A CSU/DSU to connect to the carrier Figure 15-4 shows an example of a Frame Relay connection. In this example, the router needs only a single physical connection to the carrier to connect to multiple sites: this is accomplished via virtual circuits. Frame Relay supports speeds from fractional T1 or E1 connections (56–64 Kbps) up to a DS3 (45 Mbps). Frame Relay is discussed in Chapter 16. ATM and SMDS are also packet-switched technologies that use digital circuits. Unlike Frame Relay and X.25, however, these services use fixed-length (53 byte) packets, called cells, to transmit information. Therefore, these services are commonly called cell-switched services. They have an advantage over Frame Relay in that they 8 Chapter 15: WAN Introduction CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 D:\omh\CertPrs8\934-9\ch15.vp Monday, August 04, 2003 12:15:18 PM Color profile: Generic CMYK printer profile Composite Default screen can provide guaranteed throughput and minimal delay for a multitude of services, including voice, video, and data. However, they do cost more than Frame Relay services. SMDS, which was developed by BellCore, is precursor to ATM and has been replaced by the latter service. ATM (sort of an enhanced Frame Relay) can offer a connection guaranteed bandwidth, limited delay, limited number of errors, Quality of Service (QOS), and more. Frame Relay can provide some minimal guarantees to connections, but not the degree of precision that ATM can. Whereas Frame Relay is limited to 45 Mbps connections, ATM can scale to very high speeds; OC-192 (SONET), for instance, affords about 10 Gbps of bandwidth. WAN Interfaces on Cisco Routers Cisco supports a wide variety of serial cables for their serial router interfaces. Here are some of the cable types supported for synchronous serial interfaces: EIA/TIA-232, Wide Area Networking Overview 9 CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 FIGURE 15-4 Frame Relay packet-switched connection Remember that packet- switched and cell-switched services are typically used when a router has only a single WAN interface but needs to connect to multiple remote sites. D:\omh\CertPrs8\934-9\ch15.vp Monday, August 04, 2003 12:15:19 PM Color profile: Generic CMYK printer profile Composite Default screen EIA/TIA-449, EIA/TIA-530, V.35, and X.21. The end that connects to the DCE device is defined by these standards. However, the end that connects to the Cisco router is proprietary in nature. Cisco’s cables have two different end connectors that connect to the serial interfaces of their routers: ■ DB-60 Has 60 pins ■ DB-26 Has 26 pins and is flat, like a USB cable Note that these connectors are for synchronous serial connections. Cisco has other cable types, typically RJ-45, for asynchronous connections. Encapsulation Methods There are many different methods for encapsulating data for serial connections. Table 15-2 shows the most common ones. The following sections cover HDLC and PPP in more depth. 10 Chapter 15: WAN Introduction CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 Synchronous serial interfaces have either a DB-60 or DB-26 connector for connecting to Cisco routers. Protocol Explanation High-Level Data Link Control (HDLC) Based on ISO standards, it is used with synchronous and asynchronous connections. Synchronous Data Link Control Protocol (SDLC) Used in IBM SNA environments, it has been replaced by HDLC. Link Access Procedure Balanced (LAPB) Used in X.25, it has extensive error detection and correction. Link Access Procedure D Channel (LAPD) It is used by ISDN to signal call setup and teardown of phone connections. Link Access Procedure Frame mode bearer services (LAPF) It is used in Frame Relay between a DTE and a DCE and is similar to LAPD. Point-to-Point Protocol (PPP) Based on RFC standards, PPP is the most common encapsulation used for dialup. It provides for authentication, handling multiple protocols, compression, and error detection. TABLE 15-2 Common Encapsulation Methods Know the data link encapsulation types listed in Table 15-2. D:\omh\CertPrs8\934-9\ch15.vp Monday, August 04, 2003 12:15:19 PM Color profile: Generic CMYK printer profile Composite Default screen [...]... Actually, Cisco supports only its own implementation of HDLC FIGURE 15-5 WAN frame types D:\omh\CertPrs8\934-9\ch15.vp Monday, August 04, 2003 12:15:19 PM Color profile: Generic CMYK printer profile CertPrs8 / Composite Default screen 12 Chapter 15: CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 WAN Introduction Configuring HDLC As mentioned in the preceding section,... CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 WAN Introduction Frame Type The first component of PPP is the frame type that it uses The frame type defines how network layer packets are encapsulated in a PPP frame as well as the format of the PPP frame PPP is typically used for serial WAN connections because of its open-standard character It works on both asynchronous... 192.168.2.2 The ping should be successful If you want to allow connectivity for all devices, you’ll need to add a static route on both the 2500 (to reach 192.168.1.0/24) and the 2600 (to reach 192.168.3.0/24) Now you should be more comfortable with configuring PPP on a router CERTIFICATION SUMMARY One of the major factors in choosing a WAN service is cost The CPE is your WAN equipment The demarcation point is... 12:15:20 PM Color profile: Generic CMYK printer profile CertPrs8 / Composite Default screen 18 Chapter 15: FIGURE 15-6 CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 WAN Introduction PAP and CHAP authentication The configuration of PAP is straightforward First, you need to determine which side will be the client side (sends the username and password) and which will be... match the password on the remote side On your router’s WAN interface, you need to enable PPP with the encapsulation ppp command Then, you can specify PAP authentication with the ppp authentication pap command The previous client and server code listings performs a one-way authentication— the client authenticates to the server and not vice versa If you want to perform two-way authentication, where each... contains a multimedia demonstration of configuring PPP authentication using PAP on a router CHAP One big problem with PAP is that it sends the username and password across the WAN connection in clear text If someone is tapping into the WAN connection and eavesdropping on the PPP communication, they’ll see the actual password that is being used In other words, PAP is not a secure method of authentication... 04, 2003 12:15:21 PM Color profile: Generic CMYK printer profile CertPrs8 / Composite Default screen 20 Chapter 15: CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 WAN Introduction Both sides then take the source’s username, the matching password, and the challenge and run them through the MD5 hashing function The source then takes the result of this function and sends... 04, 2003 12:15:21 PM Color profile: Generic CMYK printer profile CertPrs8 / Composite Default screen 22 Chapter 15: CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 WAN Introduction Se0 PAP: Authenticating peer RouterB Se0 PAP: O AUTH-ACK id 2 len 5 Se0 PAP: I AUTH-ACK id 3 len 5 %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to up In this example,... 04, 2003 12:15:21 PM Color profile: Generic CMYK printer profile CertPrs8 / Composite Default screen 24 Chapter 15: CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 WAN Introduction authentication to CHAP: ppp authentication chap Shut down the interface: shutdown Bring the interface back up: no shutdown Exit Configuration mode: end Examine the status of the interface:... 04, 2003 12:15:22 PM Color profile: Generic CMYK printer profile CertPrs8 / Composite Default screen 26 Chapter 15: CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 WAN Introduction interface serial0, ppp authentication chap Bounce the interface: shutdown, no shutdown, and end Re-examine the router’s configuration: show running-config Examine the status of the interface: . WAN Introduction CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 The most important factor in choosing a WAN. A 4 Chapter 15: WAN Introduction CertPrs8 / CCNA Cisco Certified Network Associate Study Guide / Deal / 222934-9 / Chapter 15 Know about the four types of WAN connections: