Professor Messer 5.5 | Common Network Issues 2.0

Rogue DHCP server

Can assign unauthorized IP addresses due to the lack of inherent security in the DHCP protocol. This could lead to clients receiving invalid or duplicate IP addresses, causing intermittent or no connectivity. Devices may show error messages and disable their network connections

To prevent this, enable DHCP snooping on switches and configure authorized DHCP servers in Active Directory.

Once identified, disable it and renew the IP leases to reassign addresses from the legitimate DHCP server.

Exhausted DHCP scope

Occurs when there are no more available IP addresses in the DHCP server's pool. Devices unable to get an IP will be assigned an Automatic Private IP Address (APIPA), limiting them to local subnet communication only.

To resolve this, check the DHCP server, add more IP addresses if possible, or use IP address management (IPAM) tools to monitor shortages. You can also lower the lease time, especially in environments with many transient users.

IP configuration issues

Can cause devices to communicate with local IP addresses but not with subnets outside the local network.

In some cases, devices may not communicate with any other device, even within the same subnet, or only be able to reach certain IP addresses but not others.

Troubleshooting IP configurations

Start by checking the basic settings like IP address, subnet mask, gateway, and DNS to ensure they are correct. Monitor the traffic and examine local broadcasts, especially if the subnet mask is unclear.

Compare your device's settings with nearby devices that have correct configurations. Use tools like traceroute and ping to test communication with local IPs, the default gateway, and external addresses to identify where the issue might lie.

Low optical link budget

Can disrupt communication, as blocking light blocks the network.

Attenuation, common over long distances or with dirty connectors, may require cleaning during troubleshooting.

Use a light meter to measure the light on both ends of the connection and compare the results with your network device's documentation to ensure it's receiving enough light for proper operation.

Certificate issues

Often trigger security alerts and warnings of invalid certificates, indicating something is wrong. You may see messages like "the site’s security certificate is not trusted.

Check the certificate details by clicking the lock icon, as it may be expired, issued to the wrong domain, or signed by an untrusted authority. Ensure the correct time and date, as incorrect values can prevent proper validation and cause error messages.

Hardware failure

Can be tough to diagnose due to a lack of response from the device.

If an application or device doesn’t respond, confirm connectivity by pinging it—without a ping, there's no connection on. Running a traceroute can reveal if traffic is being filtered.

If issues persist, you may need to physically inspect the device to check if it's powered on, observe any lights or error messages, and continue troubleshooting from there.

Incorrect firewall setting

Can cause an application to stop working, often due to blocked protocols or ports.

Start by checking the host-based firewall settings, which may require administrator access or be managed centrally. Confirm the network-based firewall configuration by reviewing policy lists and logs.

Using a packet capture can help identify if information was sent but not received, indicating a firewall issue. The operating system might also be dropping packets due to the misconfiguration.

Incorrect VLAN configurations

Often occur due to improper VLAN assignments on the switch. This is a common issue, especially with data and voice VLANs in VoIP setups.

Validate the physical interface with the VLAN number.

Usually, a single configuration adjustment can reassign the VLAN, allowing the user to communicate on the network again.

DNS issues

Can manifest as web browsing failures, leading users to feel that "the Internet is broken."

While you may be able to ping an IP address from a workstation, browsing to fully qualified domain names will fail.

This can also affect applications that rely on domain names instead of IP addresses for communication.

Troubleshooting DNS issues

Start by checking the IP configuration to ensure the DNS server IP address is correct. Verify the IP address, subnet mask, default gateway, and DNS server settings.

Use tools like nslookup or dig to test if DNS resolution works, allowing you to query the Domain Name System.

If you suspect the problem lies with a specific DNS server, consider changing the device's DNS configuration to point to a different server, such as Google’s 8.8.8.8 or Quad9’s 9.9.9.9.

NTP issues

Essential for synchronizing date and time across all devices on a network, particularly for time-sensitive cryptography.

Active Directory requires that clocks be set within five minutes of each other for proper operation, as Kerberos communication relies on time stamps. If the authentication ticket is too old, it becomes invalid, preventing client logins.

Check the timestamps of both the client and server, ensure it is configured on all devices, and automate clock setting to maintain synchronization.

Licensed feature issues

Arise when features are individually licensed, requiring payment or a license agreement.

Certain features may be unavailable if the necessary license key isn’t present, leading to complications during upgrades or configuration updates.

For example, if a configuration is created in a lab based on specific licensed materials, it may fail in production if those licensed features are not enabled on the production devices.

To avoid this, ensure that all configurations in the lab align with the licensing of the production equipment.

Network performance issues

Can lead to applications performing poorly from the user's perspective, but the underlying problems could stem from various factors.

There isn’t just one performance metric; issues with the I/O bus, CPU speed, storage access speed, or network throughput can collectively degrade performance.

Although it can be challenging to track all relevant metrics, effective monitoring will make it easier to pinpoint problems. Once the issue is resolved, improvements should be evident in application responsiveness.