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Module 12: Network Troubleshooting, image, image, image, image, image -…

- - - - The first step in troubleshooting involves confirming the problem through symptoms like alerts or user complaints and narrowing its scope to affected devices or networks. Organizations use trouble ticketing systems to assign and track resolutions efficiently.
    - - The "Gather Information" step involves identifying targets (hosts or devices), obtaining access, and collecting relevant data. If the issue lies beyond the organization's control, such as external internet connectivity, contact the external system administrator before proceeding.
    - - Possible causes must be identified. The gathered information is interpreted and analyzed using network documentation, network baselines, searching organizational knowledge bases, searching the internet, and talking with other technicians.
    - - If multiple causes are identified, then the list must be reduced by progressively eliminating possible causes to eventually identify the most probable cause. Troubleshooting experience is extremely valuable to quickly eliminate causes and identify the most probable cause.
    - - When the most probable cause has been identified, a solution must be formulated. At this stage, troubleshooting experience is very valuable when proposing a plan.
    - - The "Test Hypothesis" step involves evaluating the solution's impact and urgency, ensuring it won't adversely affect other systems, and preparing a rollback plan in case of failure. Implement the solution, verify its effectiveness, and document the results. If the solution fails, revert changes and return to the "Gather Information" step to reassess the issue.
    - - When the problem is solved, inform the users and anyone involved in the troubleshooting process that the problem has been resolved. Other IT team members should be informed of the solution. Appropriate documentation of the cause and the fix will assist other support technicians in preventing and solving similar problems in the future.
  - - - Follow-the-Path troubleshooting identifies the traffic path from source to destination, narrowing the focus to relevant links and devices in the forwarding path. It eliminates irrelevant components and often complements other troubleshooting methods.
    - - The substitution method involves replacing a suspected faulty device with a known working one. If the issue resolves, the removed device was the problem. This is ideal for quick fixes, especially in critical failures, but may not work when multiple devices are involved.
    - - Divide-and-conquer troubleshooting involves starting at a selected OSI layer, testing both upward and downward to identify issues. By documenting symptoms and user experiences, administrators make an informed guess on where to investigate. If a layer works properly, lower layers are assumed functional; if not, the focus shifts downward. For instance, inability to ping a server suggests issues below Layer 3, while successful pinging points to problems above it.
    - - Top-down troubleshooting begins with end-user applications and progresses down through the OSI model to locate the issue. It's suitable for simpler problems or software-related issues but requires checking every application, thorough documentation, and careful selection of where to start.
    - - The comparison approach, or spot-the-differences method, resolves issues by aligning nonoperational elements with working ones. This involves comparing configurations, software, hardware, or processes to identify discrepancies. While effective, it may solve the issue without exposing the root cause
    - - Bottom-up troubleshooting begins at the physical layer, progressing upward through the OSI model to locate the problem. It’s effective for physical issues, as most network problems occur at lower levels. However, it requires checking every device and interface, leading to extensive documentation and challenges in deciding where to start.
    - - The educated guess method, or shoot-from-the-hip approach, relies on identifying symptoms and making informed guesses to resolve issues. Its success depends on the technician's experience, with seasoned professionals achieving better results due to their knowledge and troubleshooting expertise. For less-experienced administrators, this method may lack structure and effectiveness
- - - - Digital Multimeters (DMMs): Measure voltage, current, and resistance, ensuring network devices receive proper power.
      - Cable Testers: Verify cable continuity, identifying broken or miswired connections.
      - Cable Analyzers: Provide detailed diagnostics on cable performance and signal quality.
      - Portable Network Analyzers: Capture and analyze network traffic on-site for troubleshooting.
      - Cisco Prime NAM: Monitors network performance, analyzing traffic patterns and identifying bottlenecks.
      - 12.3.4 Syslog Server as a Troubleshooting Tool:
        Syslog is a protocol (RFC 5424) that allows network devices to send log messages to a syslog server, aiding in security and troubleshooting. Cisco devices log events like configuration changes, ACL violations, and interface status, with messages sent to various facilities:
        
        Console Logging: Default logging method, visible via terminal emulation but not stored.
        
        Terminal Lines: Logs for active sessions, but not retained after logout.
        
        Buffered Logging: Temporarily stores logs in memory, cleared upon reboot.
        
        SNMP Traps: Sends alerts when predefined thresholds are exceeded, requiring SNMP setup.
        
        Syslog Server: Centralized logging for long-term storage and analysis.
- - - - 12.4.5 Transport Layer Troubleshooting - NAT for IPv4 There are several problems with NAT, such as not interacting with services like DHCP and tunneling. These can include misconfigured NAT inside, NAT outside, or ACLs. Other issues include interoperability with other network technologies, especially those that contain or derive information from host network addressing in the packet.
        The figure summarizes common interoperability areas with NAT
  - - - Step 1: Check the physical connectivity at the point where network communication stops. This includes cables and hardware. The issue could be related to a faulty cable, a defective interface, or misconfigured hardware components.
      - Step 2: Check for duplex mismatches.
      - Step 3: Review the addressing at the data link and network layers within the local network. This includes IPv4 ARP tables, IPv6 neighbor tables, MAC address tables, and VLAN assignments.
      - Step 4: Verify that the default gateway is correct.
      - Step 5: Ensure that devices determine the correct route from source to destination. If necessary, routing information should be adjusted.
      - Step 6: Verify that the transport layer is functioning correctly. Telnet can also be used from the command line to test transport layer connections.
      - Step 7: Check that there are no ACLs blocking traffic.
      - Step 8: Ensure that the DNS configuration is correct. There must be an accessible DNS server.
    - - Typically, the troubleshooting process begins with the detection of an issue with end-to-end connectivity. Two of the most common utilities used to verify an end-to-end connectivity problem are ping and traceroute.
      - Ping is likely the most popular connectivity testing utility in the field of networking and has always been part of Cisco's IOS software. This tool sends echo request messages to a specified host address. The ping command uses a Layer 3 protocol called ICMP (Internet Control Message Protocol), which is part of the TCP/IP suite.
        Ping utilizes ICMP Echo Request and ICMP Echo Reply packets. If the host at the specified address receives the ICMP Echo Request, it responds with an ICMP Echo Reply packet.
      - Traceroute generates a list of hops, router IP addresses, and the final destination IP address that are successfully reached along the path. This list provides valuable information for verification and troubleshooting.
        If the data reaches its destination, traceroute displays the interface of each router along the route. If the data fails at any hop, the last router that responded is identified. This address serves as an indication of where the issue or security restrictions might be located.
      - IPv6 Ping and Traceroute
        When using these utilities, Cisco IOS automatically recognizes whether the specified address is IPv4 or IPv6 and uses the appropriate protocol to test connectivity.