Quản trị mạng - Chapter 8: Network troubleshooting

The difficulty in troubleshooting Layer 2 technologies, such as PPP and Frame Relay, is the unavailability of common Layer 3 troubleshooting tools, such as ping, to assist with anything but the identification that the network is down. • Most of the problems that occur with PPP involve link negotiation.

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1Chapter 8 – Network Troubleshooting CCNA Exploration 4.0 Học viện mạng Bach Khoa - Website: www.bkacad.com 2 Notation of the CCNA Voucher • Testking CCNA 640-802 • Pass4sure • Testinside • Actualtest • Vnpro.org • Sadikhov.com • • 9tut.net • Examcollection.com Học viện mạng Bach Khoa - Website: www.bkacad.com 3 Học viện mạng Bach Khoa - Website: www.bkacad.com 4 Introduction Học viện mạng Bach Khoa - Website: www.bkacad.com 5 Establishing the Network Performance Baseline Học viện mạng Bach Khoa - Website: www.bkacad.com 6 Documenting Your Network • To efficiently diagnose and correct network problems, a network engineer needs to know how a network has been designed and what the expected performance for this network should be under normal operating conditions. This information is called the network baseline and is captured in documentation such as configuration tables and topology diagrams. • Network documentation should include these components: 1.Network configuration table 2.End-system configuration table 3.Network topology diagram Học viện mạng Bach Khoa - Website: www.bkacad.com 7 Documenting Your Network 1. Network Configuration Table The data set that should be included for all components: – Type of device, model designation – IOS image name – Device network hostname – Location of the device (building, floor, room, rack, panel) – If it is a modular device, include all module types and in which module slot they are located – Data link layer addresses – Network layer addresses – Any additional important information about physical aspects of the device Học viện mạng Bach Khoa - Website: www.bkacad.com 8 Documenting Your Network 2. End-System Documentation The following information should be documented: – Device name (purpose) – Operating system and version – IP address – Subnet mask – Default gateway, DNS server, and WINS server addresses – Any high-bandwidth network applications that the end-system runs Học viện mạng Bach Khoa - Website: www.bkacad.com 9 Documenting Your Network 3. Network Topology Diagram At a minimum, the topology diagram should include: – Symbols for all devices and how they are connected – Interface types and numbers – IP addresses – Subnet masks Học viện mạng Bach Khoa - Website: www.bkacad.com 10 Network Documentation Process • Commands that are useful to the network documentation process include: – ping – telnet – show ip interface brief – show ip route – show cdp neighbor detail Học viện mạng Bach Khoa - Website: www.bkacad.com 11 Why is Establishing a Network Baseline Important ? • Establishing a network performance baseline requires collecting key performance data from the ports and devices that are essential to network operation. Học viện mạng Bach Khoa - Website: www.bkacad.com 12 Steps for Establishing a Network Baseline • Animation 8.1.4.1 • Step 1. Determine what types of data to collect Học viện mạng Bach Khoa - Website: www.bkacad.com 13 Steps for Establishing a Network Baseline • Step 2. Identify devices and ports of interest Devices and ports of interest include: – Network device ports that connect to other network devices – Servers – Key users – Anything else considered critical to operations. Học viện mạng Bach Khoa - Website: www.bkacad.com 14 Steps for Establishing a Network Baseline • Step 3. Determine the baseline duration It is important that the length of time and the baseline information being gathered are sufficient to establish a typical picture of the network. This period should be at least seven days to capture any daily or weekly trends. Weekly trends are just as important as daily or hourly trends. Học viện mạng Bach Khoa - Website: www.bkacad.com 15 Measuring Network Performance Data • Sophisticated network management software is often used to baseline large and complex networks. • For example, the Fluke Network SuperAgent module enables administrators to automatically create and review reports using its Intelligent Baselines feature. Học viện mạng Bach Khoa - Website: www.bkacad.com 16 Measuring Network Performance Data • In simpler networks, the baseline tasks may require a combination of manual data collection and simple network protocol inspectors. Học viện mạng Bach Khoa - Website: www.bkacad.com 17 Troubleshooting Methodologies and Tools Học viện mạng Bach Khoa - Website: www.bkacad.com 18 A General Approach to Troubleshooting • Two extreme approaches to troubleshooting almost always result in disappointment, delay, or failure. At one extreme is the theorist, or rocket scientist, approach. At the other extreme is the impractical, or caveman, approach. – The rocket scientist analyzes and reanalyzes the situation until the exact cause at the root of the problem has been identified and corrected with surgical precision. – The caveman's first instinct is to start swapping cards, cables, hardware, and software until miraculously the network begins operating again. Học viện mạng Bach Khoa - Website: www.bkacad.com 19 • Using Layered Models for Troubleshooting Học viện mạng Bach Khoa - Website: www.bkacad.com 20 General Troubleshooting Procedures • Stage 1 Gather symptoms - Troubleshooting begins with the process of gathering and documenting symptoms from the network, end systems, and users. • Stage 2 Isolate the problem - The problem is not truly isolated until a single problem, or a set of related problems, is identified. • Stage 3 Correct the problem - Having isolated and identified the cause of the problem, the network administrator works to correct the problem by implementing, testing, and documenting a solution. Học viện mạng Bach Khoa - Website: www.bkacad.com 21 Troubleshooting Methods • There are three main methods for troubleshooting networks: 1. Bottom up 2. Top down 3. Divide and conquer • Bottom-Up Troubleshooting Method – In bottom-up troubleshooting you start with the physical components of the network and move up through the layers of the OSI model until the cause of the problem is identified. Học viện mạng Bach Khoa - Website: www.bkacad.com 22 Troubleshooting Methods • Top-Down Troubleshooting Method – In top-down troubleshooting your start with the end-user applications and move down through the layers of the OSI model until the cause of the problem has been identified. Học viện mạng Bach Khoa - Website: www.bkacad.com 23 Troubleshooting Methods • Divide-and-Conquer Troubleshooting Method – When you apply the divide-and-conquer approach toward troubleshooting a networking problem, you select a layer and test in both directions from the starting layer. Học viện mạng Bach Khoa - Website: www.bkacad.com 24 Guidelines for Selecting a Troubleshooting Method Học viện mạng Bach Khoa - Website: www.bkacad.com 25 Gathering Symptoms Học viện mạng Bach Khoa - Website: www.bkacad.com 26 Gathering Symptoms Học viện mạng Bach Khoa - Website: www.bkacad.com 27 Software Troubleshooting Tools • NMS Tools – Network management system (NMS) tools include device-level monitoring, configuration, and fault management tools. – Examples of commonly used network management tools are CiscoView, HP Openview, Solar Winds, and What's Up Gold. Học viện mạng Bach Khoa - Website: www.bkacad.com 28 Software Troubleshooting Tools • Knowledge Bases – On-line network device vendor knowledge bases have become indispensable sources of information. – The figure shows the Cisco Tools & Resources page found at This is a free tool providing information on Cisco- related hardware and software. Học viện mạng Bach Khoa - Website: www.bkacad.com 29 Software Troubleshooting Tools • Baselining Tools – Many tools for automating the network documentation and baselining process are available. These tools are available for Windows, Linux, AUX operating systems. – The figure shows a screen chapter of the SolarWinds LANsurveyor and CyberGauge software. Học viện mạng Bach Khoa - Website: www.bkacad.com 30 Software Troubleshooting Tools • 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. – The figure shows a screen capture of the Wireshark protocol analyzer. Học viện mạng Bach Khoa - Website: www.bkacad.com 31 Hardware Troubleshooting Tools • Network Analysis Module – A network analysis module (NAM) can be installed in Cisco Catalyst 6500 series switches and Cisco 7600 series routers to provide a graphical representation of traffic from local and remote switches and routers. Học viện mạng Bach Khoa - Website: www.bkacad.com 32 Hardware Troubleshooting Tools • Digital Multimeters – Digital multimeters (DMMs) are test instruments that are used to directly measure electrical values of voltage, current, and resistance. • Cable Testers – Cable testers are specialized, handheld devices designed for testing the various types of data communication cabling. Học viện mạng Bach Khoa - Website: www.bkacad.com 33 Hardware Troubleshooting Tools • Cable Analyzers – Cable analyzers are multifunctional handheld devices that are used to test and certify copper and fiber cables for different services and standards. • Portable Network Analyzers – Portable devices that are used for troubleshooting switched networks and VLANs. Học viện mạng Bach Khoa - Website: www.bkacad.com 34 Troubleshooting Tools Refer to 8.2.6.3 Học viện mạng Bach Khoa - Website: www.bkacad.com 35 Common WAN Implementation Issues Học viện mạng Bach Khoa - Website: www.bkacad.com 36 WAN Communications • A communications provider or a common carrier normally owns the data links that make up a WAN. The links are made available to subscribers for a fee and are used to interconnect LANs or connect to remote networks. • WAN data transfer speed (bandwidth) is considerably slower than the common LAN bandwidth. • The charges for link provision are the major cost element, therefore the WAN implementation must aim to provide maximum bandwidth at acceptable cost. Học viện mạng Bach Khoa - Website: www.bkacad.com 37 Steps in WAN Design • Businesses install WAN connectivity to meet the strategic business requirement of moving data between external branches. • Because WAN connectivity is important to the business and expensive, you need to design the WAN in a systematic manner. Học viện mạng Bach Khoa - Website: www.bkacad.com 38 WAN Traffic Considerations • To determine traffic flow conditions and timing of a WAN link, you need to analyze the traffic characteristics specific to each LAN that is connected to the WAN. Học viện mạng Bach Khoa - Website: www.bkacad.com 39 WAN Topology Considerations • Many WANs use a star topology. As the enterprise grows and new branches are added, the branches are connected back to the head office, producing a traditional star topology. • Star endpoints are sometimes cross-connected, creating a mesh or partial mesh topology. This provides for many possible combinations for interconnections. Học viện mạng Bach Khoa - Website: www.bkacad.com 40 WAN Topology Considerations Học viện mạng Bach Khoa - Website: www.bkacad.com 41 WAN Topology Considerations • A three-layer hierarchy is often useful when the network traffic mirrors the enterprise branch structure and is divided into regions, areas, and branches. • It is also useful when there is a central service to which all branches must have access but traffic levels are insufficient to justify direct connection of a branch to the service. Học viện mạng Bach Khoa - Website: www.bkacad.com 42 WAN Connection Technologies Học viện mạng Bach Khoa - Website: www.bkacad.com 43 Using the Internet as a WAN • Although the Internet may pose a security problem it does provides an alternative for inter-branch traffic. • Part of the traffic that must be considered during design is going to or coming from the Internet. Học viện mạng Bach Khoa - Website: www.bkacad.com 44 WAN Bandwidth Considerations • Many companies rely on the high-speed transfer of data between remote locations. Consequently, higher bandwidth is crucial because it allows more data to be transmitted in a given time. • When bandwidth is inadequate, competition between various types of traffic causes response times to increase, which reduces employee productivity and slows down critical web-based business processes. Học viện mạng Bach Khoa - Website: www.bkacad.com 45 Common WAN Implementation Issues Học viện mạng Bach Khoa - Website: www.bkacad.com 46 Case Study: WAN Troubleshooting from an ISP’s Perspective • A significant proportion of the support calls received by an ISP refer to slowness of the Network. • To troubleshoot this effectively, you have to isolate the individual components and test each one as follows: – Individual PC host – LAN – Link from the edge of the user network to the edge of the ISP – Backbone of the ISP – Server being accessed Học viện mạng Bach Khoa - Website: www.bkacad.com 47 Network Troubleshooting Học viện mạng Bach Khoa - Website: www.bkacad.com 48 Interpreting Network Diagrams to Identify Problems • Information recorded on the physical network diagram typically includes: – Device type – Model and manufacturer – Operating system version – Cable type and identifier – Cable specification – Connector type – Cabling endpoints Học viện mạng Bach Khoa - Website: www.bkacad.com 49 Interpreting Network Diagrams to Identify Problems • Information recorded on a logical network diagram may include: – Device identifiers – IP address and subnet – Interface identifiers – Connection type – DLCI for virtual circuits – Site-to-site VPNs – Routing protocols – Static routes – Data-link protocols – WAN technologies used Học viện mạng Bach Khoa - Website: www.bkacad.com 50 Physical Layer Troubleshooting • A physical layer problem occurs when the physical properties of the connection are substandard, causing data to be transferred at a rate that is consistently less than the rate of data flow established in the baseline. Học viện mạng Bach Khoa - Website: www.bkacad.com 51 Physical Layer Troubleshooting • Examples of configuration errors that affect the physical layer include: – Serial links reconfigured as asynchronous instead of synchronous – Incorrect clock rate – Incorrect clock source – Interface not turned on Học viện mạng Bach Khoa - Website: www.bkacad.com 52 Physical Layer Troubleshooting • Check operational statistics and data error rates – Use Cisco show commands to check for statistics such as collisions and input and output errors. The characteristics of these statistics vary depending on the protocols used on the network. Học viện mạng Bach Khoa - Website: www.bkacad.com 53 Data Link Layer Troubleshooting Học viện mạng Bach Khoa - Website: www.bkacad.com 54 Data Link Layer Troubleshooting Học viện mạng Bach Khoa - Website: www.bkacad.com 55 Troubleshooting Layer 2 - PPP • The difficulty in troubleshooting Layer 2 technologies, such as PPP and Frame Relay, is the unavailability of common Layer 3 troubleshooting tools, such as ping, to assist with anything but the identification that the network is down. • Most of the problems that occur with PPP involve link negotiation. Học viện mạng Bach Khoa - Website: www.bkacad.com 56 Troubleshooting Layer 2 - PPP Học viện mạng Bach Khoa - Website: www.bkacad.com 57 Troubleshooting Layer 2 - Frame Relay Học viện mạng Bach Khoa - Website: www.bkacad.com 58 Troubleshooting Layer 2 - Frame Relay Học viện mạng Bach Khoa - Website: www.bkacad.com 59 Troubleshooting Layer 2 - Frame Relay Học viện mạng Bach Khoa - Website: www.bkacad.com 60 Troubleshooting Layer 2 - Frame Relay Học viện mạng Bach Khoa - Website: www.bkacad.com 61 Troubleshooting Layer 2 - STP Loops • To verify STP operation, use the show spanning-tree command on each switch. • If you discover that STP is not operating, you can enable it using the spanning-tree vlan ID command. Học viện mạng Bach Khoa - Website: www.bkacad.com 62 Network Layer Troubleshooting • Network layer problems include any problem that involves a Layer 3 protocol, both routed protocols and routing protocols. Học viện mạng Bach Khoa - Website: www.bkacad.com 63 Network Layer Troubleshooting • In most networks, static routes are used in combination with dynamic routing protocols. • Improper configuration of static routes can lead to less than optimal routing and, in some cases, create routing loops or parts of the network to become unreachable. • Troubleshooting dynamic routing protocols requires a thorough understanding of how the specific routing protocol functions. Học viện mạng Bach Khoa - Website: www.bkacad.com 64 Transport Layer Troubleshooting Học viện mạng Bach Khoa - Website: www.bkacad.com 65 Transport Layer Troubleshooting • The biggest problem with all NAT technologies is interoperability with other network technologies, especially those that contain or derive information from host network addressing in the packet. • Some of these technologies include: – BOOTP and DHCP – DNS and WINS – SNMP – Tunneling and encryption protocols Học viện mạng Bach Khoa - Website: www.bkacad.com 66 Application Layer Troubleshooting Học viện mạng Bach Khoa - Website: www.bkacad.com 67 Application Layer Troubleshooting Học viện mạng Bach Khoa - Website: www.bkacad.com 68 Application Layer Troubleshooting • A problem at the application layer can result in unreachable or unusable resources when the physical, data link, network, and transport layers are functional. • Another type of problem at the application layer occurs when the physical, data link, network, and transport layers are functional, but the data transfer and requests for network services from a single network service or application do not meet the normal expectations of a user. Học viện mạng Bach Khoa - Website: www.bkacad.com 69 Application Layer Troubleshooting • The same general troubleshooting process that is used to isolate problems at the lower layers can also be used to isolate problems at the application layer. • The concepts are the same, but the technological focus has shifted to involve things such as refused or timed out connections, access lists, and DNS issues. Học viện mạng Bach Khoa - Website: www.bkacad.com 70 Application Layer Troubleshooting • Step 1: Make a backup • Step 2: Make an initial hardware or software configuration change. • Step 3: Evaluate and document each change and its results. • Step 4: Determine if the change solves the problem. • Step 5: Stop when the problem is solved. • Step 6: If necessary, get assistance from outside resources. • Step 7: Document. Học viện mạng Bach Khoa - Website: www.bkacad.com 71 Labs Học viện mạng Bach Khoa - Website: www.bkacad.com 72 Summary Học viện mạng Bach Khoa - Website: www.bkacad.com 73

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