Автор работы: Пользователь скрыл имя, 26 Февраля 2013 в 09:53, статья
The Internet is a worldwide, publicly accessible network of networks. It enables individuals and businesses alike, through interconnected computer networks, to share information, resources, and services.
In the beginning, the Internet was used strictly for scientific, educational, and military research. In 1991, regulations changed to allow businesses and consumers to connect as well. The Internet has grown rapidly, and is now global. New technologies are continuously being developed that make the Internet easier and more attractive to use. Online applications are available to the Internet user, including email, web browsing, streaming music and video, gaming, and instant messaging.
To troubleshoot at Layer 1, first check that all devices have the proper electrical supply, and that the devices are turned on. This may seem to be an obvious solution, but many times the person reporting the problem may overlook a device that is within the network path from source to destination. If there are any LEDs that display the status of the connectivity, verify with the customer that they are indicating correctly. If on-site, visually inspect all network cabling and reconnect cables to ensure a proper connection. If the problem is with wireless, verify that the wireless access point is operational and that wireless settings are configured correctly.
When remotely troubleshooting a problem, the technician should advise the caller through each step, what to look for, and what to do if an error is found. If it is determined that all Layer 1 issues have been addressed, it is time to travel up the OSI model to Layer 2.
Layer 2 Troubleshooting
Network switches and host NICs perform Layer 2 functions. Layer 2 problems can be caused by faulty equipment, incorrect device drivers, or an improperly configured switch. When remotely troubleshooting a problem, it may be difficult to isolate a Layer 2 problem.
An on-site technician can check whether the NIC is installed and working properly. Reseating the NIC, or replacing a suspected faulty NIC with a known good NIC, helps to isolate the problem. The same process can be done with any network switch.
Layer 3 Troubleshooting
At Layer 3, the technician needs to investigate the logical addressing used in the network, such as the IP address scheme. If the network is using IP addressing, the technician verifies that the device has the proper settings, such as:
At Layer 3, several utilities can assist with the troubleshooting process. Three of the most common command line tools are:
ipconfig - Shows IP settings on the computer
ping - Tests basic network connectivity
traceroute - Determines if the routing path between the source and destination is available
Most network problems can usually be resolved using these Layer 1, 2, and 3 troubleshooting techniques.
Layer 4 Troubleshooting
If Layers 1 through 3 all appear to be operating normally and the technician can successfully ping the IP address of the remote server, it is time to check the higher layers. For example, if a network firewall is used along the path, it is important to check that the application TCP or UDP port is open and no filter lists are blocking traffic to that port.
Layers 5 through 7 Troubleshooting
The technician should also check the application configuration. For example, if troubleshooting an email issue, ensure that the application is configured with the correct sending and receiving email server information. It is also necessary to ensure that domain name resolution is functioning as expected.
For remote technicians, higher layer issues can be checked by using other network utility tools, such as a packet sniffer, to view traffic as it crosses the network. A network application, such as Telnet, can also be used to view configurations.
Help Desk Troubleshooting Scenarios
The number and types of calls received by the help desk can vary extensively. Some of the most common calls include problems with email, host configuration, and connectivity.
Email Issues
A common cause of many email problems is using the wrong POP, IMAP, or SMTP server names. It is best to check with the email administrator to confirm the proper name of the POP or IMAP server and SMTP server. In some cases, the same server name for both POP/IMAP and SMTP are used. Also, confirm that the username and password are correct. Since the password is not usually displayed, it is a good idea to carefully re-enter it.
When troubleshooting these issues over the phone, it is important to step the customer through the configuration parameters carefully. Many customers are unfamiliar with the terminology and the settings of the various configuration parameters. If possible, connect to the customer device via remote management software. This allows the technician to perform the necessary steps for the customer.
Host Configuration Issues
A common issue that can prevent connectivity to the Internet or other network resources is improperly configured host addressing information. This can include an incorrect IP address, subnet mask, or default gateway.
In environments where the IP addressing information is manually configured, it is possible that the IP configuration was simply entered incorrectly. In environments where hosts are configured to dynamically receive an IP address from an assignment server, such as a DHCP server, the server may fail or become unreachable due to network issues.
If a host is configured to receive an address dynamically, and an assignment server is unavailable or unreachable, a link-local address will be automatically assigned to the local host by the operating system. IPv4 addresses in the address block 169.254.0.1 to 169.254.255.254 (169.254.0.0 /16) are designated as link-local addresses. A link-local process will randomly select an IP address within the 169.254.0.0/16 range. But what prevents two hosts from randomly selecting the same IP address?
Once the link-local process selects an IP address, it sends an ARP query with that IP onto the network to see if any other devices are using that address. If there is no response, the IP address is assigned to the device, otherwise another IP address is selected, and the ARP query is repeated. Microsoft refers to link-local addresses as Automatic Private IP Addressing (APIPA).
If multiple hosts on the same network obtain a link-local address, client/server and peer-to-peer applications between those hosts will work properly. However, because link-local addresses are in the private Class B address space, communication outside of the local network is not possible.
When troubleshooting both manually and dynamically configured hosts, use the host command ipconfig /all to verify that the host is using the appropriate IP configuration.
Customer Connectivity Issues
Connectivity problems are more common with new customers trying to connect for the first time. However, sometimes existing customers encounter connectivity issues. First-time customers may have problems with installing the hardware as well as software configuration settings. Existing customers notice connectivity problems when they cannot open a web page or connect to instant messaging or email.
There are many reasons why a customer has no connectivity, including the following:
In many cases, the problem is simply a faulty cable, or a cable plugged into an incorrect port. These types of issues can be resolved by checking the cable connection or replacing the cable.
Other problems, such as software issues, may be more difficult to detect. One example is an incorrectly loaded TCP/IP stack, preventing IP from operating correctly. The TCP/IP stack can be tested and verified using a loopback address. The loopback is a special address, the reserved IPv4 address 127.0.0.1, which hosts use to direct traffic to themselves. The loopback address creates a shortcut method for TCP/IP applications and services that run on the same device to communicate.
You can ping the loopback address to test the configuration of TCP/IP on the local host. If you are unable to get a response when pinging the loopback address, suspect an improperly configured or installed TCP/IP stack.
Addresses 127.0.0.1 through 127.255.255.254 are reserved for testing purposes. Any address within this block will loop back within the local host. No address within this block should ever appear on any network. Despite the fact that the entire 127.0.0.0/8 network range is reserved, the only address typically used for loopback testing is the 127.0.0.1 address.
Creating and Using Help Desk Records
When a Level 1 help desk technician receives a call, there is a process followed to gather information. There are also specific systems for storing and retrieving relevant information. It is extremely important to gather the information correctly in the event that a call has to be escalated to Layer 2 or require an on-site visit.
The information gathering and recording process starts as soon as the technician answers the phone. When the customer identifies who they are, the technician accesses the relevant customer information. Typically, a database application is used to manage the customer information.
The information is transferred to a trouble ticket, or incident report. This document can be a piece of paper in a paper filing system or an electronic tracking system designed to follow the troubleshooting process from beginning to end. Each person who works on the problem is expected to record what was done on the trouble ticket. When an on-site call is required, the trouble ticket information can be converted to a work order that the on-site technician can take to the customer site.
When a problem is resolved, the solution is documented in the customer work order or trouble ticket, and in a knowledge-base document for future reference.
Occasionally, the Level 1help desk technician may receive a call that cannot be resolved quickly. In this instance, the technician is responsible for passing the call to a Level 2 technician who is more qualified to resolve the issue. Passing the call to a higher level technician is known as the call escalation process.
Both Level 1 and Level 2 help desk technicians attempt to solve customer problems using the telephone, web tools, and possibly remote desktop sharing applications.
If the help desk technicians are not able to fix the problem remotely, it is often necessary to send a Level 3 on-site technician to the customer premise location. It is the job of the on-site technician to visit the customer premise to physically work on the problem equipment. The help desk technician can make an appointment with the customer for the on-site technician to perform the repairs, or it may be the responsibility of the on-site technician to arrange the appointment.
To properly troubleshoot the problem, the on-site technician reviews the trouble ticket to see what was previously done. This review gives the technician some background information and a logical starting point. It also helps the technician decide which tools and supplies to bring, rather than having to leave the customer site to obtain supplies.
On-site technicians typically work on the network at the customer location, although there are instances where the technician is unable to make the needed repairs and must bring the damaged equipment back to the ISP site for additional troubleshooting.
Customers Site Procedures
There are four steps an on-site technician performs before beginning any troubleshooting or repair at the customer site:
Step 1. Provide proper identification to the customer.
Step 2. Review the trouble ticket or work order with the customer to verify that the information is correct.
Step 3. Communicate the current status of any identified problems and the actions the technician expects to take at the customer site that day.
Step 4. Obtain permission from the customer to begin the work.
The technician must verify all items on the trouble ticket. Once the technician is familiar with all issues, the work can begin. The technician is responsible for checking all device and network settings, and running any necessary utilities. The technician may also have to swap out suspected faulty hardware with known good hardware to determine if a hardware problem exists.
When performing any troubleshooting tasks the customer site, especially when installing new or replacing existing equipment, it is important to minimize the risk of injury by following good safety practices. Many employers offer safety training as part of their employee services.
Ladders
Use ladders to reach high locations to install networking cable and to install or troubleshoot wireless access points in places that are difficult to reach. To reduce the risk of falling off the ladder or dropping equipment while climbing on the ladder, work with a partner whenever possible.
High or Dangerous Locations
Sometimes network equipment and cables are located in high and dangerous places, such as on the side of a building, on roof tops, or in an internal structure such as an elevator shaft, that is not accessible by a ladder. Work performed at this type of location must be done very carefully. Using a safety harness reduces the risk of falling.
Electrical Equipment
If there is a risk of damaging or coming in contact with any electrical lines when handling hardware, consult with the electrician of the customer about measures that can be taken to reduce the risk of electrical shock. Coming in contact with electrical equipment may result in serious personal injury.
Awkward Spaces
Network equipment is often located in narrow and awkward spaces. Ensure that the work area is properly lighted and ventilated. Determine the best way to lift, install, and remove equipment to minimize the risks.
Heavy Equipment
Networking devices can be large and heavy. Plan to have the correct equipment and trained personnel when heavy equipment needs to be installed or moved at a customer site.
After the technician makes any configuration changes or installs new equipment, the technician must observe the results to ensure proper operation. When finished, the technician communicates the nature of the identified problem to the customer, what solution was applied, and any follow-up procedures. Before the problem can be considered fully resolved, the technician must obtain the acceptance of the customer. The technician can then close the trouble ticket and document the solution.
A copy of the documentation is left with the customer. The document includes the original help desk call problem and the actions taken to solve the problem. The technician records the solution, and the customer acceptance is indicated on the trouble ticket. For future reference, the technician also records the problem and the solution in the help desk documentation and FAQs.
In some cases, an on-site technician can uncover network problems that require upgrades or reconfiguration of the network devices. When this occurs, it may be outside of the scope of the original trouble ticket. These issues are usually communicated to both the customer and the ISP network personnel for further action.
Troubleshooting Tools
It is very difficult to troubleshoot any type of network connectivity issue without a network diagram that depicts the IP addresses, IP routes, and devices, such as firewalls and switches. Logical and physical topologies are extremely useful in troubleshooting.
Physical Network Topologies
A physical network topology shows the physical layout of the devices connected to the network. Knowing how devices are physically connected is necessary for troubleshooting problems at the Physical Layer, such as cabling or hardware problems. Physical network topologies typically include:
Logical Network Topologies
A logical network topology shows how data is transferred on the network. Symbols are used to represent network elements such as routers, servers, hubs, hosts, and security devices. Logical network topologies typically include:
In addition to network diagrams, other tools may be needed to effectively troubleshoot network performance issues and failures.
Network Documentation and Baseline Tools
Network documentation and baseline tools are available for Windows, Linux, and UNIX operating systems. CiscoWorks can be used to draw network diagrams, keep network software and hardware documentation up to date, and help to cost-effectively measure baseline network bandwidth use. These software tools often provide monitoring and reporting functions for establishing the network baseline.
Network Management System Tools
Network Management System (NMS) tools monitor network performance. They graphically display a physical view of the network devices. If a failure occurs, the tool can locate the source of the failure and determine whether it was caused by malware, malicious activity, or a failed device. Examples of commonly used network management tools are CiscoView, HP Openview, SolarWinds, and WhatsUp Gold.
Knowledge Bases
Network device vendor knowledge bases have become indispensable sources of information. When online knowledge bases are combined with Internet search engines, a network administrator has access to a vast pool of experience-based information.
Protocol Analyzers
A protocol analyzer decodes the various protocol layers in a recorded frame and presents this information in a relatively easy-to-use format. Protocol analyzers can capture network traffic for analysis. The captured output can be filtered to view specific traffic or types of traffic based on certain criteria; for example, all traffic to and from a particular device. Protocol analyzers, such as Wireshark, provide detailed troubleshooting information about the data being communicated on the network. An example of the types of information that can be viewed using a protocol analyzer is the setup and termination of a TCP session between two hosts.
Sometimes failures in the lower layers of the OSI model cannot be easily identified with software tools. In these instances, it may be necessary to use hardware troubleshooting tools, such as cable testers, multimeters, and network analyzers.
Cable Testers
Cable testers are specialized, handheld devices designed for testing the various types of data communication cabling. Cable testers can be used to detect broken wires, crossed-over wiring, shorted connections, and improperly paired connections. More sophisticated testers, such as a time-domain reflectometer (TDR), can pinpoint the distance to a break in a cable. Cable testers can also determine the length of a cable.
Digital Multimeters
Digital multimeters (DMMs) are test instruments that directly measure electrical values of voltage, current, and resistance. In network troubleshooting, most of the multimeter tests involve checking power-supply voltage levels and verifying that network devices are receiving power.
Portable Network Analyzers
By plugging a network analyzer into a switch anywhere on the network, a network engineer can see the average and peak utilization of the segment. The analyzer can also be used to identify the devices producing the most network traffic, analyze network traffic by protocol, and view interface details. Network analyzers are useful when troubleshooting problems caused by malware or denial-of-service attacks.
Knowledge, Skills and Abilities
The Cisco Certified Entry Networking Technician (CCENT) certification validates the skills required for entry-level network support positions, the starting point for many successful careers in networking. CCENT certification is the first step toward achieving CCNA certification (Cisco Certified Network Associate), which covers medium-size enterprise branch networks that have more complex connections. To obtain CCENT certification, a candidate must pass the ICND1 examination at a Cisco Certified Testing Center.
The ICND1 exam (640-822) tests the ability to install, operate, and troubleshoot a small branch office network. The exam includes topics on networking fundamentals:
Mastering a Cisco certification exam is not an easy task. Cisco has maintained the difficulty of the CCNA exam series by changing the exam requirements regularly. Some candidates pass the exam the first time; many pass it after multiple attempts, while some do not pass it. Good preparation is the best way to ensure that you pass the exam the first time.
Before preparing for any certification examination, it is important to understand the purpose of the exam. Cisco certification examinations are designed to measure the knowledge, skills, and abilities of an individual in a defined area of expertise. The exams use a combination of techniques to enable a candidate to demonstrate readiness to perform various networking tasks. The exam can contain multiple choice questions, various exercises, and simulated network configuration tasks. Each question or task is designed to address a specific objective.