Bài giảng Network+ Certification - Chapter 19, Network Troubleshooting Scenarios

Tài liệu Bài giảng Network+ Certification - Chapter 19, Network Troubleshooting Scenarios: Chapter 19, Network Troubleshooting Scenarios |1| Chapter Overview A. Introduction to network troubleshooting B. Incident administration C. Gathering information D. Possible causes 1. Internet router problem 2. Internet communication problem 3. DNS failure 4. LAN communication problem 5. Computer configuration problem 6. User error Chapter 19, Lesson 1 “I Can’t Access a Web Site” 1. Introduction A. The process of troubleshooting network problems varies, depending on the size of the organization and the people involved. 1. Medium- to large-sized organizations usually establish a procedure that determines how technical support calls are registered, addressed, and escalated. 2. In smaller organizations, the process might be much more informal. 3. The cause of the problem might be simple, such as user error, but the procedures described in this lesson illustrate how technical staff can handle even minor problems to everyone’s sat...

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Chapter 19, Network Troubleshooting Scenarios |1| Chapter Overview A. Introduction to network troubleshooting B. Incident administration C. Gathering information D. Possible causes 1. Internet router problem 2. Internet communication problem 3. DNS failure 4. LAN communication problem 5. Computer configuration problem 6. User error Chapter 19, Lesson 1 “I Can’t Access a Web Site” 1. Introduction A. The process of troubleshooting network problems varies, depending on the size of the organization and the people involved. 1. Medium- to large-sized organizations usually establish a procedure that determines how technical support calls are registered, addressed, and escalated. 2. In smaller organizations, the process might be much more informal. 3. The cause of the problem might be simple, such as user error, but the procedures described in this lesson illustrate how technical staff can handle even minor problems to everyone’s satisfaction. 4. Sometimes the problem seems minor, but it might actually indicate a serious network problem. |2| B. Scenario: a user named Alice calls the network help desk and reports that she has been trying to access a particular Web site for several hours and keeps receiving an error message. 1. Common problem because Internet resources are prone to occasional and sometimes frequent outages 2. Can also indicate a problem with the caller’s computer or with the internal network 3. Based on the information provided, and knowing nothing about Alice’s level of expertise, the help desk technician has no way of knowing whether the problem is caused by a. User error b. A computer configuration problem c. A faulty network connection d. A malfunction of the router providing the Internet access e. A problem with the Internet or the specific Web site itself 2 Outline, Chapter 19 Network+ Certification, Second Edition |3| 2. Incident Administration A. The first step for the help desk staff is to begin to document the incident. B. Many help desks use software that lets technicians document calls and store them in a database. 1. Help desk software typically can a. Assign a priority to each call b. Escalate calls to senior technicians, if necessary c. List all of the information obtained from the caller d. Document the steps taken to solve the problem C. Prioritizing calls 1. Because the technician has little information about Alice’s problem at this point, he or she cannot accurately assign a priority to this call. 2. If the problem is the router or the network and many users are affected, it can be very serious. a. For example, if a company sells products over the Web, and the Web servers are located on site, an Internet connection failure means that the Web site is down and no orders are coming in. (1) In this case, the call might be assigned the highest priority. b. If revenue-producing work can go on without Internet access, the priority of the call might be lower. c. If the problem is Alice’s computer or her procedures, the priority of the call would be much lower, unless Alice is the company president. (1) It might seem as though political considerations should not affect the priority assigned to a technical support call, but they invariably do. D. Escalating calls 1. Technical support operations often have two or more tiers of support technicians, depending on their expertise and experience. a. First-tier technicians typically take help desk calls. b. If the problem is serious or complex, the first-tier technician escalates the call to the second tier. 2. A well-organized technical support team documents the escalated calls. a. Problems involving user error and individual workstations might remain in the first tier, whereas network outages and problems affecting multiple users might be immediately escalated. b. Calls should be escalated when a technician in the first tier makes several attempts to resolve the problem and cannot do so. c. Political concerns can determine whether a call is escalated. 3. The two-tier arrangement keeps the organization’s more experienced (and presumably more highly paid) technicians from having to field calls about elementary problems. |4| 3. Gathering Information A. The next step is for the technician to ask the user about the exact circumstances under which the problem occurred. Outline, Chapter 19 3 Network+ Certification, Second Edition 1. Until more information is available, it is impossible to assign a priority to the call or determine if it should be escalated. B. In this scenario, Alice says that she has been trying to open a Web site in Microsoft Internet Explorer, one that had always worked before, and after a few seconds she receives an error message. 1. She tried again several times over an hour, and received the same error message every time. 2. She did not write down the error message at the time, but she can re-create the error by trying to access the Web site. |5| 3. The error message is “The page cannot be displayed” and it also says “Cannot find server or DNS error.” a. This error message is a common one that can appear when (1) The Web server that the browser is trying to contact is down (2) The client computer’s Internet connection is broken (3) The client’s Domain Name System (DNS) server fails to resolve the DNS name in the requested URL C. Determining the cause of the problem is a matter of isolating the malfunctioning component or components. 1. You eliminate all of the properly functioning components until you are left with only the problematic ones. |6| 4. Possible Cause: Internet Router Problem A. Difficulty in accessing the Internet is one of the most common problems. 1. For an organization with more than a few users, setting up a router that connects to an Internet service provider (ISP) is the easiest and most economical way of providing Internet access. a. The alternative is to equip all users with their own modems, telephone lines, and Internet access accounts. (1) Expensive (2) Labor intensive: requires the network support staff to install the modem and configure the computer 2. The router could be a. A stand-alone unit connected to an ISP using a leased telephone line, such as a T-1 line b. A computer with a modem that connects to the ISP with a standard dial-up connection and is configured to share the dial-up connection with network users c. Any one of many routing solutions between these two extremes |8| B. Many things can go wrong with this type of routed Internet access solution. 1. The router’s connection to the ISP or the ISP’s connection to the Internet might malfunction. a. Outages might occur, regardless of the type of connection. b. The ISP might have network problems. 4 Outline, Chapter 19 Network+ Certification, Second Edition (1) You can do nothing about the network problems, except report them to the ISP’s technical support. 2. The router device or computer might experience a hardware or power failure. a. If the router is not functioning, the Internet access requests have nowhere to go. (1) When the Internet resource does not respond, the client application eventually times out and displays an error message. (2) This condition affects all of the users who access the Internet through that router. 3. A network problem might prevent access to the router. a. A broken network cable, a faulty cable connector, or a malfunctioning hub or other network connection device can cause problems. b. The number of users affected by this type of problem depends on the location of the fault and the type of the malfunctioning component. (1) If the cable connecting the user’s computer to the hub has been severed, only the user’s computer is affected. (2) If the hub itself is malfunctioning, all of the computers connected to it will experience the same problem. (3) If a central component is faulty, such as a backbone cable or switch, the problem could extend to many or all users. 4. The client computer might be misconfigured and unable to send Internet access requests to the router. a. The client computer might have a problem in its networking hardware, its software, or its network configuration. (1) Affects only that one computer (2) A common cause of error messages like the one Alice received |9| C. A router problem is not a likely cause of Alice’s problem. 1. If the router were malfunctioning, the help desk would probably receive calls from many users with the same problem. 2. You should eliminate the router as a possible source of the problem at the very beginning of the troubleshooting process. a. Router problems are easy to check. b. The potential seriousness of a router problem makes it a high priority for the technical support staff. D. To test the router, try to access an Internet site by using a computer that shares the same routed Internet connection. 1. In Alice’s organization, all of the users on the network share a single Internet connection. a. To determine that the connection and the router are functioning properly, the technician simply launches his own Web browser and connects to an Internet site. 2. This test narrows down the source of the problem to Alice’s procedures, her computer, or her computer’s connection to the router. |10| E. If your computer also fails to access the Internet, the problem could be Outline, Chapter 19 5 Network+ Certification, Second Edition 1. A component that both you and the user use to access the router, such as a hub, switch, local area network (LAN) router, or backbone network a. Next step: determine exactly which other users on the network have the same problem. (1) You can then isolate the problem to a particular hub, cable, or other piece of equipment, depending on how widespread the problem is. 2. The router itself a. Users cannot access Internet services if the router cannot (1) Access the Internet itself (through an ISP) (2) Forward packets between the internal network and the ISP’s Internet-connected network b. If the router is a computer, you can test the connection to the ISP by running a Web browser on that computer and trying to connect to an Internet site. (1) If you succeed in connecting to an Internet site, check the router configuration to see if it is communicating properly with both networks and is forwarding packets. (2) If the router itself cannot connect to the Internet, the problem might be the connection between the router and the ISP. 3. The connection between the router and the ISP a. All wide area network (WAN) technologies used to connect networks to ISPs require (1) Hardware at both sides of the connection (2) A service that provides the communications link between the hardware devices b. If the router uses a simple dial-up connection to the ISP, the problem could be either one of the two modems involved or the telephone line that connects them. c. You can test your line and modem by replacing them with others that you know work properly. d. Other technologies use the same testing principles, but testing is likely to be more difficult. (1) Even large organizations do not have an extra T-1 line available for testing. 4. A proxy server or other device a. Network users sometimes access Internet Web sites through a proxy server or other device that functions as a “middle man” between the client and the Web server. (1) Introduces another possible source of the problem (2) If you or other users can access the Internet through the same server, then the server, the router, and the ISP connection are functioning properly. F. If neither a component nor a connection is causing the problem, the problem is in the ISP’s network or the Internet. 6 Outline, Chapter 19 Network+ Certification, Second Edition 1. The problem might correct itself in a few minutes or hours, but if Internet access is essential to the business, the ISP should be contacted right away. 2. Dealing with the ISP might be the responsibility of a senior technical support representative, so the call probably should be escalated. G. In Alice’s case, the technician determines that the router is functioning normally because he can connect to an Internet site using his own browser. |11| 5. Possible Cause: Internet Communication Problem A. Next step: determine exactly what kinds of network communications are affected. 1. This procedure should a. Methodically test the entire data connection from Alice’s computer to the Internet b. Trace backward, component by component, until the source of the problem is detected B. Try to determine what kinds of network connections are affected while you are still on the telephone with the user. 1. First, ask the user to try connecting to a different Web site. a. Using one of the default links supplied with the browser is a good idea because these sites are nearly always in operation, and you minimize the possibility of user error. b. If you must have the user type a Web site address, dictate the exact URL to the user, and keep it simple, such as www.microsoft.com. 2. If the browser can connect to other Internet Web sites, then the network, the router, and the Internet connection are functioning properly. a. In this case, the problem can nearly always be traced to either a Web site that is down or to user error. 3. If the user’s Web browser cannot connect to any other Internet sites, you should determine if any other network communications are possible. 4. Next, ask the user to open a different client application and try to connect to the Internet. a. The application you select does not matter, as long as it connects directly to an Internet site. (1) For example, an e-mail client or a newsreader is a good choice if the user does not connect to a mail or news server on the local network. b. As a last resort, you can always have the user launch the File Transfer Protocol (FTP) client from the command line. (1) Virtually every operating system that supports Transmission Control Protocol/Internet Protocol (TCP/IP) includes an FTP client. 5. If the user cannot use a Web browser to access Internet sites but can connect to the Internet with a different client application, the problem is in the browser software running on the user’s computer. Outline, Chapter 19 7 Network+ Certification, Second Edition 6. If the user cannot connect to the Internet with any client application (and other users can), the next step is to determine which part of the computer’s Internet access architecture is failing. |12| 6. Possible Cause: DNS Failure A. A common cause of Internet access problems is the failure of the user’s computer to resolve DNS names into Internet Protocol (IP) addresses. 1. DNS servers are a vital part of any Internet communication that uses a name to refer to an Internet server. 2. The first thing that a client application does when given a name of a computer, such as www.microsoft.com, is to send the name to a DNS server for resolution. a. When you type the name of a server into a Web browser, you might experience a brief delay before the Web page loads. (1) This delay results partly from the time it takes the client application to generate a DNS Request message containing the server name, send it to a DNS server, and wait for a reply from the DNS server containing the IP addresses associated with the name. B. Checking the TCP/IP client’s DNS configuration 1. The address of the DNS server that a computer uses to resolve names is supplied as part of the system’s TCP/IP client configuration. |13| a. On a computer running Microsoft Windows 2000, the DNS server address is found in the Internet Protocol (TCP/IP) Properties dialog box. b. If the addresses in the Preferred DNS Server and Alternate DNS Server fields do not point to DNS servers that are up and running, the name resolution process will fail when the user attempts to connect to a Web server. 2. To configure the DNS server addresses on a computer running Windows 2000: a. From the Start menu’s Settings group, open the Network And Dial- Up Connections window. b. Right-click the Local Area Connection icon, and then select Properties. c. Highlight the Internet Protocol (TCP/IP) entry in the components list, and then click Properties to display the Internet Protocol (TCP/IP) Properties dialog box. (1) The other Microsoft Windows operating systems use a similar arrangement of dialog boxes, although the access procedures are slightly different. C. The easiest way to test for a DNS name resolution problem is to use an IP address instead of a server name in the URL. 1. Example: when the user’s browser fails to connect to a Web server with its name but other computers are able to access the Internet, you can use the Ping program on another computer to resolve the name of the desired server into an IP address. 8 Outline, Chapter 19 Network+ Certification, Second Edition a. Use the following command: ping servername b. The Ping command first displays the server’s name, followed by the server’s IP address, and then displays the results of the attempt to communicate with that server. c. When the attempt is successful, the program lists each reply received from the server, with information such as (1) The number of data bytes included in the message (2) The time elapsed between the transmission of the request and the receipt of the reply (3) The Time To Live (TTL) value for the transmission d. If the Ping command fails to resolve the name, you can use the Nslookup command to send a name resolution request to a DNS server that you know is operational. 2. Have the user replace the server name in the browser’s URL with the IP address you have discovered. 3. If the browser succeeds in connecting to the server with an IP address (when using a server name has failed), there is a problem with the DNS name resolution process. 4. DNS name resolution problems have two major causes: a. The computer’s TCP/IP client is configured with incorrect DNS server addresses. b. The DNS servers are not functioning properly. 5. You can check the DNS server addresses on a computer running Windows by using IPCONFIG.EXE or WINIPCFG.EXE to display the TCP/IP configuration. a. Use IPCONFIG.EXE on computers running Windows 2000 or Microsoft Windows NT. b. Use WINIPCFG.EXE on computers running Microsoft Windows 95, Microsoft Windows 98, or Microsoft Windows Me. c. If the addresses are incorrect, they must be changed in the Internet Protocol (TCP/IP) Properties dialog box. d. You should modify the computer’s TCP/IP configuration in person. 6. If the network uses Dynamic Host Configuration Protocol (DHCP) servers to configure its TCP/IP clients automatically, you should check the DHCP server configuration to see if it is supplying incorrect addresses to the network clients. a. If it is supplying incorrect addresses, do not manually change the DNS server configuration in the user’s computer; correct the DHCP server’s configuration instead. b. After you have corrected the DHCP server’s configuration, repair the user’s computer by renewing the DHCP lease with IPCONFIG.EXE or WINIPCFG.EXE. |15| D. Checking the DNS server 1. If the DNS server addresses in the user’s TCP/IP client configuration are correct, the problem might be the DNS servers or the computer’s network connection to the DNS servers. Outline, Chapter 19 9 Network+ Certification, Second Edition 2. The DNS servers might be supplied by the organization’s ISP, or they might be located on site. a. If the DNS servers belong to the ISP, all you can do is test to see if they are available. b. If you can contact the DNS servers by using the Ping command with an IP address, you know that the servers are up and running. (1) This does not necessarily mean that the servers can process DNS Request messages. c. If you can execute a Ping command by using a server name, the DNS server can resolve the server’s name into its IP address. 3. If the DNS servers belong to your organization, you can check them more thoroughly. a. In this case, the first-tier technician might escalate the call to a senior technician. b. A ping test can determine that the DNS server is functioning, but checking the status of the DNS server software itself depends on the operating system and the application software running on the computer. |16| c. Example: on a computer running Windows 2000 Server and Microsoft DNS Server, you can start by opening the Services console on the Start menu’s Administrative Tools group and checking to see that the DNS Server service is running. (1) If the service is not running, you must find out why. (2) The Startup Type field for the DNS Server service should be set to Automatic, indicating that the service loads when the computer starts. (3) If the Startup Type field is set to Manual or Disabled, this is the reason the service is not running. (4) Before you manually start the service or change the Startup Type setting to Automatic, check with your colleagues to see if someone has configured it this way for a good reason. (5) If the Startup Type is set to Automatic but the service is not running, someone manually stopped it, the service failed to start, or the service shut itself down. (6) Check the computer’s Event Viewer (also accessible from the Administrative Tools group) for log entries that might explain why the service is not running. (a) Failure of the service to start during boot time should generate a log entry indicating the reason. (7) Various types of environmental problems could cause the service to shut down, including a memory shortage or a configuration problem. (8) Troubleshooting environmental problems with the DNS Server service requires knowledge of the operating system and the DNS Server software. 10 Outline, Chapter 19 Network+ Certification, Second Edition d. If the DNS Server service is running but names are still not being resolved, look at the server software and the DNS communications process in more detail. (1) Start by examining the DNS server’s configuration files. (2) If the server’s list containing the names and addresses of the DNS root name servers has somehow been modified or erased, names cannot be resolved, even though everything else is functioning correctly. (3) The DNS server’s own network connection and Internet access are also vital to the name resolution process. (4) The server itself might be functioning properly, but the name resolution process stops if (a) Network conditions prevent the server from receiving DNS Request messages from the client (b) The server cannot access the Internet to relay the requests to other DNS servers e. If the DNS server’s configuration files show no obvious problems, you might have to use a protocol analyzer to determine if the DNS server is communicating properly with the network and the Internet. |17| (1) You should be able to see the DNS Request packets arriving at the server, and the server’s own DNS Requests being transmitted to other DNS servers on the Internet. (2) Analyzing network traffic in this way requires familiarity with what is known as a baseline. (a) You have to know what the network traffic pattern is supposed to look like (called the baseline) before you can determine what is wrong. (3) By analyzing the traffic traveling to and from the server, you might be able to determine that the problem is in the server’s communications with the local network or in its communications with the Internet. E. The procedures for diagnosing and repairing DNS name resolution problems are also useful in other scenarios. 1. Example: computers running Windows might use the Windows Internet Name Service (WINS) to resolve Network Basic Input/Output System (NetBIOS) names into IP addresses, just as they use DNS servers to resolve DNS names. 2. The same type of client and server configuration problems affecting DNS name resolution can also affect the WINS name resolution process. 3. You can check the addresses of the WINS servers in the client computer’s TCP/IP configuration and the functionality of the WINS servers in much the same way that you check the equivalent DNS resources. |18| 7. Possible Cause: LAN Communication Problem A. If the problem is not caused by an Internet communications problem or a DNS name resolution problem, examine the computer’s general network communication capabilities. Outline, Chapter 19 11 Network+ Certification, Second Edition 1. Begin by asking the user to try to access resources on the local network. a. Local network resources can include shared server drives, internal network applications (such as e-mail or database servers), and Windows Explorer (when it is used for browsing the network). |19| B. Testing the local hub 1. Have the user open My Network Places in Windows Explorer and see if computers belonging to nearby users are visible. a. If the hub is the problem, other computers nearby that are connected to the same network hub will experience the problem. 2. If there is an internal network communications difficulty, narrow down the source of the problem. a. Help desk technicians should have access to information about which computers are connected to specific hubs and LANs. (1) Preferably in the form of a map or diagram that shows the cables and connection devices that make up the network (2) This information should be developed during the initial planning stages of the network and should be maintained consistently. b. Windows Explorer displays the computers on the network in terms of domains and workgroups, which probably do not correspond to the hubs and LANs that form the network’s physical configuration. 3. If you and the user are still working together over the telephone at this point, explain carefully what must be done, without introducing unnecessary technical details. a. In this case, you might consider traveling to the user’s site. |20| C. Testing the computer connection 1. If the user cannot see the other computers connected to the same hub when he or she uses My Network Places, the problem is likely the user’s connection to the hub, the computer hardware or software, or the user’s procedures. 2. If the computer is connected to the hub with a prefabricated network cable, you can try replacing the cable with one that you know is functioning properly. 3. If the computer is connected to the hub with an internal cable run, begin by switching the network cable plugged into the user’s computer with a cable from a nearby computer that is working properly. a. If the user’s computer can now access the network, you know that the problem is in the original cable run, and you can start trying to determine exactly where the problem is. b. Begin by swapping out the patch cables at both ends of the connection with replacements that you know are working properly. (1) Swapping out equipment wherever possible is one of the most basic and most effective troubleshooting techniques. (2) If the patch cables are not causing the problem, you can test the internal cable run. c. Test the cable run with the proper cable testing equipment. 12 Outline, Chapter 19 Network+ Certification, Second Edition (1) A multifunction cable tester, a wire map tester, or even an inexpensive tone generator and locator can tell you if the cable is wired properly and signals are getting through. (2) If there is a break in the cable, use the multifunction tester to find the break in relation to the end you are testing from. (3) If you do not have cable-testing equipment, you can plug the patch cables at both ends into a different cable run that you know is working properly. 4. Problems with internal cable runs usually result when someone works in the spaces where the cables are located and accidentally damages one of the cables. a. Just moving a cable inside a drop ceiling closer to a fluorescent light fixture can be enough to induce communication problems on that connection. b. You should secure your cables well when installing them, even when they are running through relatively inaccessible areas, such as walls and ceilings. |21| D. Testing hub connections 1. If the user’s computer can see and access other computers connected to the same hub, the next step is to try to access other computers on the same LAN that are connected to different hubs. 2. If the user can access computers attached to the same hub but cannot access the other computers on the LAN connected to different hubs, the problem might be in the connection between the user’s hub and the rest of the network. 3. What to check next depends on the physical configuration of the network. a. Example: if the user’s hub is connected to another hub, the connection might not be functioning properly for several reasons. (1) The cable run connecting the two hubs might be faulty. (a) If the hubs are connected by a prefabricated cable, it might have a damaged connector or a kink that caused a break in one or more of the wires. (b) If the hubs are connected by an internally installed cable run, the cable connectors could be wired incorrectly or one of the path cables could be damaged. (c) Use the cable testing procedures described earlier to check the connection. (2) The connection between the hubs might not contain a crossover circuit. (a) The problem could be that neither end of the cable is plugged into an uplink port or that both ends are plugged into an uplink port. (b) Some hubs have a switch that you use to specify whether one of the ports functions as an uplink port. (c) If this switch is set incorrectly, the result is the same as plugging the cable into the wrong port. Outline, Chapter 19 13 Network+ Certification, Second Edition (3) One or both of the hub ports might be damaged. (a) Check the link pulse light-emitting diodes (LEDs) for the ports used to connect the two hubs together. (b) If both LEDs are not lit when the hubs are connected, the two hubs are not communicating properly. 4. All of the hub problems described in this section can also affect a switch. |22| E. Testing router connections 1. If the user can access other computers on other segments of the LAN, test the connections to other LANs. a. This assumes that the organization’s network is really an internetwork that consists of multiple LANs connected by routers. 2. You can test the computer’s connectivity by using Windows Explorer to access computers that are located on other networks. a. If the user’s computer can access resources in all of the LANs in the organization’s internetwork, the problem is not network connectivity. (1) It is time to check the computer itself. b. If the user’s computer can access resources in some LANs but not others, the problem might be in one of the routers that connect the networks. c. The difficulty of locating the malfunction depends on how complicated the internetwork configuration is. d. For example, if the network consists of 30 LANs interconnected by dozens of routers with redundant access paths, finding one malfunctioning router can be a complicated process. (1) Almost certainly has to be done by the technicians at the top of the organization’s technical support hierarchy 3. One method for isolating the router causing the user’s problem is to use the Traceroute program to see exactly where the packets generated by the computer are going. a. Traceroute is a TCP/IP command-line utility that transmits packets and displays a list of the routers that the packets pass through on the way to the destination. b. Most TCP/IP implementations include a version of Traceroute. (1) On computers running Windows, the program is called TRACERT.EXE. c. Run Traceroute with the name of the Web server the user is trying to reach. d. When the packets reach a malfunctioning router, the program should stop displaying information. (1) The last router listed in the Traceroute display should be that of the last properly functioning router in the path to the destination. e. With knowledge of your network’s configuration, you should be able to figure out which router the packets are trying to go to next. 14 Outline, Chapter 19 Network+ Certification, Second Edition (1) This is the router that either is not receiving the packets or is not forwarding them properly, causing the user’s communication failure. 4. Suppose that your network consists of a number of LANs containing user computers, all of which are connected to a single backbone LAN. a. One of the user LANs also contains the router that connects the network to the Internet. |23| b. Any of the following scenarios could cause Alice’s problem. All of these scenarios are likely to cause more than one call to the help desk. (1) If the router connecting Alice’s LAN to the backbone (Router A) fails, Alice can communicate with the computers on her own LAN, but traffic from her computer cannot reach the backbone and cannot be forwarded to any of the other LANs, including the LAN containing the router that is connected to the Internet. (a) This problem would also affect all of the other computers on Alice’s LAN. (2) If the router connecting the backbone to the LAN containing the Internet router (Router B) fails, all of the users on the LANs other than the one containing the Internet router would be able to communicate among themselves, but not with users on the Internet router LAN. (a) Also, no one could access the Internet except for the users on the LAN containing the Internet router. (3) If the problem is a hub failure on the backbone LAN, all users would experience the same result that Alice does, but the problem would also affect all of the traffic between LANs on the entire internetwork. (a) In this case, the internetwork would be reduced to a collection of unconnected LANs because the backbone is unavailable to carry traffic between them. (b) A cable break on the backbone LAN isolates the LAN served by that cable from the rest of the network. (c) This scenario would probably cause a flood of complaints. 5. Sometimes router failure is a less likely cause of communication problems because of the configuration of the internetwork. a. The internetwork in this example has only one path between each pair of LANs. b. To guard against outages caused by router failures, many internetworks have redundant routers and backbones. (1) In this case, there would have to be two major failures at the same time to cause any of the three preceding problem scenarios that were described in item 4.b above. 6. A diagnosis of router failure is comparatively rare if the help desk receives only a single user call. Outline, Chapter 19 15 Network+ Certification, Second Edition a. A problem like Alice’s is far more likely to be caused by a procedural error, a computer configuration error, or possibly a minor network problem. b. A router failure would probably result in a more general network failure that would cause a large number of simultaneous complaints, which would immediately be brought to the attention of the network’s senior support staff. (1) The help desk technicians should not try to handle a general network failure. c. When the network administrators are aware of the problem, the role of the first-tier technician is to inform users that they know of the problem and that a fix is forthcoming. (1) There is no need to troubleshoot each call when they all have the same cause. |24| 8. Possible Cause: Computer Configuration Problem A. If the user’s computer cannot access the network and neither the network nor the cable connecting the computer to the network is at fault, look at the computer itself. 1. A problem that prevents any network access would eliminate the need to troubleshoot the hub and router. 2. You might even look at computer configuration as a possible problem as soon as you determine that no network communication is possible. a. Unless the user is familiar with the configuration interface of the operating system, you should probably troubleshoot the computer in person. (1) This eliminates the difficulties than can arise from giving instructions over the telephone. 3. If the user’s problem is in the computer, the difficulty can exist at almost any level. a. It is a good idea to use the Open Systems Interconnection (OSI) reference model to list the various possible causes. |25| B. Physical layer problems 1. If the cable that connects the computer to the network is functioning properly, the problem might be in the computer’s network interface adapter. 2. One common cause of communication problems is a network interface card (NIC) that is loose in its bus slot. a. If the card is not installed firmly into the slot and secured in place with a screw or other device, a tug on the network cable can loosen the card and break the connection between the NIC and the computer. (1) If the NIC is completely disconnected, most operating systems report that the device is not functioning. (2) However, if the NIC is only slightly loosened and is not pulled completely out of the slot, the problem could be intermittent and especially difficult to detect. 16 Outline, Chapter 19 Network+ Certification, Second Edition b. The Device Manager application in most versions of Windows can report when a device is or is not functioning properly. 3. The network interface adapter might also be physically damaged by a power surge, static electricity, or a manufacturing defect. a. If the adapter’s cable connector is damaged, the contacts in the cable plug might not connect properly to the contacts in the adapter’s jack. (1) Cases like this are difficult to detect, except by ruling out all other possible causes of the problem. 4. The solution is nearly always to replace the network interface adapter, but technicians rarely do this until they have checked the configuration of the computer’s networking software. a. If the network interface adapter comes with a diagnostic program and a loopback connector is available, you can test the adapter without opening the computer. |26| C. Data-link layer problems 1. In addition to the network interface adapter, the network adapter driver implements the data-link layer protocol in the computer. 2. The driver must be configured with the same hardware settings as the network interface adapter so that the two components can communicate. a. Incorrect configuration settings are a common reason a computer cannot communicate with the network. (1) However, this generally does not occur in a computer that has been functioning properly unless someone manually changes the configuration settings or a device installation affects them. b. When something no longer works, you should ask the user what has changed on the computer. (1) Has the user installed any new hardware or software? (2) Has the user changed any configuration settings? c. In most cases, the hardware settings of both the network interface adapter and the network adapter driver are configurable, using an interface provided by the operating system. d. To manually configure the adapter, you typically have to use a special utility supplied by the manufacturer. 3. Today, most network interface adapters are installed by using Plug and Play, which automatically configures both the adapter and its driver to use the same settings. a. The settings chosen are based on an evaluation of the hardware requirements for all of the devices in the computer. (1) Installing a new piece of hardware into the computer can cause Plug and Play to alter the settings of existing devices. b. It is not common, but it is possible for Plug and Play to select hardware settings that cause either the adapter or its driver to malfunction. 4. If a new hardware device has been installed, you might have to disable or remove it to determine if it is causing the network adapter’s configuration problem. Outline, Chapter 19 17 Network+ Certification, Second Edition a. You might have to manually configure the new device to use it in the computer. b. If the adapter or driver configuration parameters have been changed manually, you should (1) Use Device Manager in Windows 2000 to delete the device from the system configuration (2) Restart the computer (3) Let Plug and Play detect the adapter and reinstall it, reconfiguring both the adapter and the driver in the process |27| D. Network and transport layer problems 1. The primary functions of the TCP/IP protocols are at the network and transport layers, and the TCP/IP client configuration is one of the chief causes of network communication problems. 2. WINS servers perform the same type of name resolution process for NetBIOS names as DNS servers do for host names, and incorrect WINS server addresses can prevent the computer from accessing some of the other computers on the network. a. A computer running Windows that is not configured with WINS server addresses can still resolve the name of other computers on its own LAN, using broadcast messages. (1) However, broadcasts cannot reach the computers on other LANs, so WINS is needed to resolve these names. b. WINS support is included in Windows 2000 only to enable the computer to communicate with other computers using NetBIOS names, such as Windows NT and Windows 98. c. Windows 2000 uses the Active Directory service, which relies on DNS servers to resolve names. 3. An incorrect IP address or subnet mask can completely prevent all network communications, and—even worse—an IP address duplicated on a second computer can prevent both computers from accessing the network. a. To test for a duplicate IP address, shut down the user’s computer and ping that computer’s IP address from another workstation. (1) If you receive a response to the Ping command, another computer is using that same IP address. 4. An incorrect or missing default gateway parameter can also cause the user’s problem. a. A computer that is not configured with a correct default gateway address can access the other computers on its own LAN but not any of the other LANs on the internetwork. b. Without a default gateway address, the computer does not know where to send packets that are destined for other networks. (1) This prevents the user’s Web browser from connecting to any sites on the Internet. 18 Outline, Chapter 19 Network+ Certification, Second Edition 5. In Windows 2000, to modify any of the TCP/IP configuration parameters listed here, use the Internet Protocol (TCP/IP) Properties dialog box. 6. If the network has DHCP servers that configure the network’s TCP/IP clients, none of the fields in the Internet Protocol (TCP/IP) Properties dialog box should have values in them. a. Manually configured TCP/IP parameters take precedence over those supplied by DHCP. (1) If someone has been “experimenting” by supplying their own TCP/IP values, remove them before reactivating the DHCP client. 7. You must know what allocation mode the DHCP servers are using. a. If they are using automatic allocation, moving the computer to a different subnet requires manually releasing the assigned IP address and renewing it so that the DHCP server can assign one from the proper subnet. (1) This is another way for the computer to have an incorrect IP address. b. If you move computers around on the network frequently, consider using dynamic allocation, which leases addresses to computers for a short period of time and renews them each time the computer starts. E. Application layer problems 1. Application layer networking protocols are generally not configurable, but problems at the application level can affect network communications. 2. Virus infections a. A virus most likely will not prevent the user from accessing a Web site, but new viruses are constantly being invented. b. If you do not already have antivirus software installed on the computer, you should install it, make sure the virus signatures are updated, and run a complete system scan, just to be safe. 3. Although it does not affect Internet access directly, having the incorrect network client installed on a computer can also cause network communication problems. a. For computers running Windows, the Client for Microsoft Networks provides the redirector that enables the computer to send resource access requests to other computers running Windows. (1) If the redirector is removed, there is a break in the protocol stack, and network communication ceases. 4. Applications that are damaged or improperly configured can interfere with network communications. a. For example, if Alice modified the configuration of her browser, causing it to access the Internet by dialing out to an ISP instead of using the LAN, she could not access any Web sites unless a modem was installed or a dial-up account was configured. Outline, Chapter 19 19 Network+ Certification, Second Edition b. This problem would be specific to the browser, however, and you would catch it when you ask Alice to try to use another application to access the Internet. |28| 9. Possible Cause: User Error A. Errors in user procedures are one of the most common causes of help desk calls. 1. Discussing user errors last does not imply that you should go through all of the testing procedures first before checking for user error. 2. In fact, you can often quickly determine that the user’s equipment and the network are functioning properly, and that the problem must be something the user did. 3. In the interest of diplomacy, be certain that user error is the problem before you broach the subject with the user. B. User error can easily be the reason for a failure to access a Web site, and it can sometimes be difficult to detect when you work with the user over the telephone. C. Incorrectly entering URLs into the browser causes many common Internet access problems. 1. When a technician has the user test the system by trying to access other sites, it is best to use existing bookmarks or favorites whenever possible. 2. The problem might be that the user is typing URLs with backslashes instead of forward slashes, or is inserting three forward slashes after the http: prefix instead of two. a. In fact, this error is causing Alice’s problem. (1) Alice thought that using three forward slashes was correct, and she was using them even when the technician was dictating the URLs of other sites over the telephone to test her Internet connectivity. (2) The technician said www, knowing that typing the http:// prefix is not necessary in most cases, but Alice added it to each URL on her own and used three forward slashes instead of two. (3) This problem could have been solved almost immediately if the technician had gone to Alice’s location and watched her type the URLs. 3. This does not mean that you should travel to the user’s location after every call to the help desk. a. In many cases, in-person help is impractical, but Alice’s case demonstrates the importance of communication between the technician and the user. D. Many other common procedural errors can interfere with a user’s network connectivity, and many errors can be very difficult to catch over the telephone. 1. Sometimes there is no substitute for watching what the user is doing. 2. User logons, for example, are a common source of difficulties. 20 Outline, Chapter 19 Network+ Certification, Second Edition a. If a user has been trying to log on repeatedly and is failing every time, you should first check to see if the user has been locked out of the account. b. Many networks are configured to disable accounts after a certain number of failed logon attempts. c. If the account is not locked, password policies might also be to blame. (1) Users might ignore a message telling them that a periodic password change is required or might attempt to reuse an expired password. d. Windows 2000 and Windows NT users sometimes try to log on to the wrong domain or log on to the local system with the wrong account. (1) The domain selector in the Logon dialog box might have been changed somehow, which is something that you are not likely to realize without actually watching the user try to log on. |29| Chapter Summary A. Administrative tasks, such as record keeping, call prioritizing, and call escalation, are essential activities in a professional technical support organization. B. The first step in troubleshooting any networking problem is to gather information from the user experiencing the problem. C. For an Internet access problem, checking the router that connects the network to the ISP is fast and easy, and is always a good idea. D. DNS name resolution problems are a common cause of Internet access failures. E. Solving a network communications problem is a matter of isolating the malfunctioning component. F. If the network is functioning properly, you should check the user’s computer for the problem. G. User error is also a common cause of Internet access difficulties, but you should approach the user diplomatically.

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