Chapter 8 – Sections & Objectives
- 8.1 Troubleshooting Methodology
- Explain troubleshooting approaches for various network problems.
- 8.2 Troubleshooting Scenarios
- Troubleshoot end-to-end connectivity in a small to medium-sized business network, using a systematic approach.
8.1 Troubleshooting Methodology
Network Documentation
-
- Documentation is critical to being able to monitor and troubleshoot a network.
- Documentation includes:
- Configuration files, including network configuration files and end-system configuration files
- Physical and logical topology diagrams
- A baseline performance levels
-
- To establish and capture an initial network baseline, perform the following steps:
- Step 1. Determine what types of data to collect.
- Step 2. Identify devices and ports of interest.
- Step 3. Determine the baseline duration.
- To establish and capture an initial network baseline, perform the following steps:
-
- Common Cisco IOS commands used for data collection.
- Manual collection of data should be reserved for smaller networks or limited to mission-critical network devices.
- Sophisticated network management software is typically used to baseline large and complex networks.
Troubleshooting Process
Common Cisco IOS commands used to gather the symptoms of a network problem.
Isolating the Issue Using Layered Models
After gathering symptoms, the network administrator compares the characteristics of the problem to the logical layers of the network to isolate and solve the issue.
Using the layered models, there are three primary methods for troubleshooting networks:
- Bottom-up
- Top-down
- Divide-and-conquer
8.2 Troubleshooting Scenarios
Using IP SLA
Cisco IP Service Level Agreements (SLA) generate traffic to measure network performance.
Additional benefits include:
- SLA monitoring, measurement, and verification
- Measure the jitter, latency, or packet loss in the network
- IP service network health assessment
- Edge-to-edge network availability
IP SLA ICMP Echo Configuration
Verifying IP SLA Configuration
Troubleshooting Tools
Common software troubleshooting tools include:
- Network Management System Tools include device-level monitoring, configuration, and fault-management tools. These tools can be used to investigate and correct network problems.
- Knowledge Bases from device vendors, combined with Internet search engines like Google, are used by network administrators to access a vast pool of experience-based information.
- Tools & Resources at http://www.cisco.com provide information on Cisco-related hardware and software.
- Baselining Tools can draw network diagrams, help keep network software and hardware documentation up-to-date, and help to cost-effectively measure baseline network bandwidth use.
- Protocol Analyzers are useful to investigate packet content while flowing through the network.
Common hardware troubleshooting tools include:
- Digital Multimeters are test instruments that are used to directly measure electrical values of voltage, current, and resistance.
- Cable Testers are specialized, handheld devices designed for testing the various types of data communication cabling. These devices send signals along the cable and wait for them to be reflected. The time between sending the signal and receiving it back is converted into a distance measurement.
- 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 can be plugged in anywhere in the network and used for troubleshooting.
- Network Analysis Module can capture and decode packets and track response times to pinpoint an application problem to a particular network or server.
Symptoms and Causes of Network Troubleshooting
Physical Layer Troubleshooting
Data Link Layer Troubleshooting
Network Layer Troubleshooting
Transport Layer Troubleshooting
Application Layer Troubleshooting
Troubleshooting IP Connectivity
Common bottom-up troubleshooting steps for end-to-end connectivity:
- Step 1. Check physical connectivity
- Step 2. Check for duplex mismatches.
- Step 3. Check data link and network layer addressing.
- Step 4. Verify that the default gateway is correct.
- Step 5. Ensure that devices are determining the correct path from the source to the destination.
- Step 6. Verify the transport layer is functioning properly.
- Step 7. Verify that there are no ACLs blocking traffic.
- Step 8. Ensure that DNS settings are correct.
8.3 Chapter Summary
Summary
- For network administrators to be able to monitor and troubleshoot a network, they must have a complete set of accurate and current network documentation, including configuration files, physical and logical topology diagrams, and a baseline performance level.
- The three major stages to troubleshooting problems are gather symptoms, isolate the problem, then correct the problem.
- The OSI model or the TCP/IP model can be applied to a network problem. A network administrator can use the bottom-up method, the top-down method, or the divide-and-conquer method.
- Common software tools that can help with troubleshooting include network management system tools, knowledge bases, baselining tools, host-based protocol analyzers, and Cisco IOS EPC.
- Hardware troubleshooting tools include a NAM, digital multimeters, cable testers, cable analyzers, and portable network analyzers. Cisco IOS log information can also be used to identify potential problems.
- There are characteristic physical layer, data link layer, network layer, transport layer, and application layer symptoms and problems of which the network administrator should be aware. The administrator may need to pay particular attention to physical connectivity, default gateways, MAC address tables, NAT, and routing information.
