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Module 12: Network Troubleshooting - Coggle Diagram
Module 12: Network Troubleshooting
12.4. Symptoms and causes of network problems.
Problems with performance indicate that there is a difference between the expected behavior and the observed behavior and that the system is not performing as expected.
Networks where these types of conditions are present are usually deactivated.
Data Link Layer Troubleshooting
The configuration and operation of these protocols are critical to creating a functional, well-tuned network.
Network Layer Troubleshooting
Network layer problems include any problem that involves a Layer 3 protocol, such as IPv4, IPv6, EIGRP, OSPF, etc.
Transport Layer Troubleshooting - ACLs
Network problems can arise from transport layer problems on the router, particularly at the edge of the network where traffic is examined and modified.
Application Layer Troubleshooting
Application layer protocols are typically used for network management, file transfer, distributed file services, terminal emulation, and email.
12.5. Troubleshooting IP Connectivity
Components of Troubleshooting End-to-End Connectivity
Employing a structured approach to the troubleshooting process, an administrator can reduce the time it takes to diagnose and solve a problem.
End-to-End Connectivity Problem Initiates Troubleshooting
Two of the most common utilities used to verify a problem with end-to-end connectivity are ping and traceroute, as shown in the figure.
Step 3 - Verify Addressing on the Local Network
When troubleshooting end-to-end connectivity, it is useful to verify mappings between destination IP addresses and Layer 2 Ethernet addresses on individual segments.
Step 2 - Check for Duplex Mismatches
In many Ethernet-based networks, point-to-point connections are now the norm, and the use of hubs and the associated half-duplex operation is becoming less common.
Step 5 - Verify Correct Path
When troubleshooting, it is often necessary to verify the path to the destination network.
Step 4 - Verify Default Gateway
If there is no detailed route on the router, or if the host is configured with the wrong default gateway, then communication between two endpoints in different networks does not work.
Step 8 - Verify DNS
The DNS protocol controls the DNS, a distributed database with which you can map hostnames to IP addresses.
Step 6 - Verify the Transport Layer
Two of the most common issues that affect transport layer connectivity include ACL configurations and NAT configurations. A common tool for testing transport layer functionality is the Telnet utility.
Step 1 - Verify the Physical Layer
When a network administrator determines that a problem exists on a given device, and that problem might be hardware-related, it is worthwhile to verify the operation of these generic components.
Step 7 - Verify ACLs
On routers, there may be ACLs that prohibit protocols from passing through the interface in the inbound or outbound direction.
12.1 Network Documentation
12.1.1 Documentation Overview
12.1.2 Network Topology Diagrams
Physical Topology
A physical network topology shows the physical layout of the devices connected to the network. You need to know how devices are physically connected to troubleshoot physical layer problems.
Logical IPv4 Topology
A logical network topology illustrates how devices are logically connected to the network. This refers to how devices transfer data across the network when communicating with other devices.
Logical IPv6 Topology
Although IPv6 addresses could also be displayed in the same IPv4 logical topology, for the sake of clarity, we have created a separate logical IPv6 network topology.
12.1.3 Network Device Documentation
12.1.4 Establish a Network Baseline
Measuring the initial performance and availability of critical network devices and links allows a network administrator to determine the difference between abnormal behavior and proper network performance, as the network grows, or traffic patterns change. The baseline also provides insight into whether the current network design can meet business requirements.
12.1.8 Data Measurement
When documenting the network, it is often necessary to gather information directly from routers and switches. Obvious useful network documentation commands include ping, traceroute, and telnet, as well as show commands.
12.2 Troubleshooting Process
12.2.1 General Troubleshooting Procedures
Troubleshooting can be time consuming because networks differ, problems differ, and troubleshooting experience varies.
12.2.2 Seven-Step Troubleshooting Process
The figure displays a more detailed seven-step troubleshooting process. Notice how some steps interconnect.
12.2.3 Question End Users
Many network problems are initially reported by an end user. However, the information provided is often vague or misleading.
12.2.5 Troubleshooting with Layered Models
The OSI and TCP/IP models can be applied to isolate network problems when troubleshooting.
12.2.6 Structured Troubleshooting Methods
There are several structured troubleshooting approaches that can be used.
Bottom-Up
Top-Down
Divide-and-Conquer
Follow-the-Path
Substitution
Comparison
Educated Guess
12.2.7 Guidelines for Selecting a Troubleshooting Method
To quickly resolve network problems, take the time to select the most effective network troubleshooting method.
12.3 Troubleshooting Tools
12.3.1 Software Troubleshooting Tools
As you know, networks are made up of software and hardware. Therefore, both software and hardware have their respective tools for troubleshooting.
Network Management System Tools
Knowledge Bases
Baselining Tools
12.3.3 Hardware Troubleshooting Tools
There are multiple types of hardware troubleshooting tools.
Digital Multimeters
Cable Testers
Cable Analyzers
Portable Network Analyzers
Cisco Prime Network Analysis Module
12.3.2 Protocol Analyzers
Protocol analyzers can investigate packet content while flowing through the network.
12.3.4 Syslog Server as a Troubleshooting Tool
Syslog is a simple protocol used by an IP device known as a syslog client, to send text-based log messages to another IP device, the syslog server. Syslog is currently defined in RFC 5424.
Console
Terminal lines
Buffered logging
SNMP traps
Syslog
