Professor Messer 5.5 | General Network Troubleshooting

Device configuration review

Before modifying device configurations, it’s crucial to review the existing setup to avoid troubleshooting blindly.

Start by viewing the current configuration through a native desktop, web-based console, or SSH/terminal access

It’s helpful to gather and analyze the configuration ahead of time.

Routing tables

Guide your device in deciding where to send traffic.

When traffic arrives at a layer 3 device, the destination IP address is checked against the table. Based on the table, the traffic is either forwarded to the next hop or dropped if no route is available. Helps your device find the path from point A to point B, much like asking for directions.

They often include default gateways, static routes, or dynamic routes generated automatically. Issues like routing loops and missing routes can arise, particularly with manual static route configurations.

Routing table example

If you're at your desk and need to communicate with a network at 10.10.10.1, but 10.10.10.0 is not in the routing table, the default route (0.0.0.0) will be used.

This default route directs your traffic to the next hop, 10.1.10.1. From there, you'd need to check the routing table on 10.1.10.1 to determine the next hop.

This process continues through each router until the traffic reaches its destination

Interface status

When troubleshooting network communication issues with a single device, checking the interface status on the switch or router is crucial.

Start by verifying basic settings such as physical connectivity—check if the link light is on and whether traffic is passing. Misconfigurations, such as duplex mismatches or incorrect speed settings, are common culprits.

These can cause issues like slow performance or disconnections. It's often one of the quickest fixes.

Interface Configuration Example

When reviewing an interface configuration, you can select any active interface to view its settings.

By moving to the "Edit" option, you can examine various configurations such as SNMP, auto-negotiation, auto-advertisement, and flow control. This helps ensure that the configuration on the switch aligns with the settings on the connected device

VLAN assignment

If a device is connected to a switch and receives an unexpected DHCP address or cannot communicate with the network.

Although the network link is active and the device has an IP address, its configuration could restrict access to resources.

To troubleshoot, review the switch configuration, verify the interface the device is connected to, and ensure the correct VLAN is assigned. This is often a simple but common fix.

Network performance baseline

Helps identify trends and potential issues that develop over time. Provides context for troubleshooting, especially for intermittent or all-day problems.

Metrics like utilization and device performance can reveal patterns or correlations, helping isolate issues. Organizations may already collect this data through a SIEM or management console, allowing you to analyze traffic patterns, protocols, and errors during specific times

May highlight areas where problems aren't occurring, refining your focus for troubleshooting.