Common Use Cases and Troubleshooting

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A common use case for Cisco switches is network segmentation and quality control through VLANs. By creating separate VLANs for wireless, phones, cameras and printers, you segment and can prioritize different traffic types (e.g., prioritizing voice over printing).

Cisco routers also enable network segmentation using standard or extended access control lists. These can restrict traffic from specific sources or limit certain traffic types entering or exiting router segments.

Here is a step-by-step guide for configuring a Cisco router for a network segment:

  1. Configure the router hostname and enable a secret password.

  2. Assign IP addresses to the router interfaces.

  3. Either configure static routing information or configure a routing protocol to route traffic for the attached network segments.

  4. Configure DHCP to distribute IP address and other DHCP options to clients that connect to the network.

  5. Configure ACLs to shore up security and restrict incoming and outgoing traffic.

  6. Configure NAT for internet access.

  7. Enable logging.

  8. Save the configuration.

Common Problem Scenarios and Troubleshooting Tips

Here are some common issues for Cisco routers and switches and tips for resolving them:

Scenario: Devices unable to communicate across VLANs or subnets

  • Check VLAN configurations and trunking on switches.
  • Verify the IP addresses and subnet masks.
  • Check the routing table and ensure routes exist.
  • Test connectivity with ping and traceroute.
  • Verify that ACLs are not blocking traffic.

Scenario: Interface is down or flapping

  • Check physical connections and cable integrity.
  • Verify the interface configuration with show interface.
  • Shut down and re-enable the interface.
  • Test different speed and duplex settings.

Scenario: Unauthorized access attempts or suspicious traffic

  • Review logs and use show logging.
  • Check ACL configurations and hit counts.
  • Verify AAA and TACACS+ settings.
  • Implement port security on switches.

Bonus Tips

The following tips can help you manage your Cisco devices more efficiently:

  • If you aren’t familiar with Cisco IOS, be sure to take advantage of the context-sensitive help — simply type ? at any point in a command to get suggestions and available

  • Save time with command shortcuts and tab completion. For example, type sh run instead of show running-config.

  • Use the up and down arrows (or Ctrl+P and Ctrl+N) to quickly access recent commands.

  • If you don’t have access to a Cisco router, there are multiple Cisco device simulators available for download that will allow you to practice and become familiar with the commands.

Always make security a top priority:

  • Limit who can access your Cisco devices.

  • Ensure that all accounts use encrypted long passwords.

  • Enforce the principle of least privilege when assigning roles.

  • Create access control lists to restrict different traffic origins and types.

  • Enable logging and use a third-party monitoring solution to get alerted if your configurations are altered, either accidentally or maliciously.