Troubleshooting & Validating Configurations

Network Diagnostics

  • Uptime Robot
  • by Ookla
  • AWS Service Health Dashboard
  • – IP/DNS Detect – What is your IP, DNS, information sent to Websites.
  • NetWorx is a powerful tool to evaluate your bandwidth consumption. You can use it to collect bandwidth usage data, and measure the speed of your Internet or any other network connections.
    • NetWorx can help you identify possible sources of network problems, ensure that you do not exceed the bandwidth limits specified by your ISP, or track down suspicious network activity characteristic of Trojan horses and hacker attacks.
    • Right-click on the icon in the notification area, then Tools > Connection Monitor
      • In Connection Monitor, enable “Ping the above hosts periodically…”

Cisco IOS Diagnostic Commands

Make yourself familiar with the output from each of these commands.

  • show ip interface brief
  • show ip route
  • show ip protocol
  • show ip ospf neighbor
    • {If you’re using OSPF}
  • show controllers s0/0 or s0/1
    • {Determine if the serial interface is DCE or DTE}
  • show ip dhcp binding
    • {If the router is configured as a DHCP server}
  • show running-config
    • {Make sure to press the space bar to show additional pages if the CLI says “–More–”}
  • show ip arp

Router Configuration Checklist

  • Identify the interfaces to be used.
  • Identify the directly connected networks.
  • Determine the network address of each network.
  • Configure the IP address and subnet mask on only those interfaces to be used.
    • Double check you’re adding the correct IP address to the proper interface.
  • Enable the interface(s) : no shutdown
  • Add the routing protocol
    • Add only the network address of each “directly connected” network.

Validating Your Lab is Functional and 100% Accurate

Verifying you can successfully ping between hosts on every network, in both directions, and make sure to scrutinize the output of “show running-config” and other commands to make validate every IP address, mask, routing protocol, network address, etc., are 100% accurate, and all the required configurations have been made.

With simulation software, some functions may work, although the configuration is not 100% correct. Accuracy in the configuration must always be your goal.

Pay attention to every detail!!!

Troubleshooting the configuration and interfaces

  • Scrutinize the output from “show running-config” and make sure all the configured values are correct.
    • Are the IP addresses and subnet masks correct?
  • Did you enable all the required interfaces on all the routers? # no shutdown
  • Are both Status and Protocol up on all router interfaces in use?
    • # show ip interface brief
  • Did you check the clockrate on the DCE interface is set, and greater than zero?
    • # show controllers s0/0 or s0/1

Routing Protocols

  • Double check the network addresses and masks are correct on each router.
  • Did you add only the directly connected networks, on the router you’re on, to the routing protocol?
  • Does “show ip route” list the correct number of routes based on the network topology and instructions?
  • Did you add all the directly connected networks to OSPF?
    • Are the directly connected networks, type “C”, when you “show IP route”, missing from OSPF on that router?
    • Were the correct number of directly connected networks added to the routing protocol?
    • # show running-config | section include router
  • When you configured the routing protocol, did you use the network address or an interface’s IP address?
  • When you configured OSPF, did you use the wildcard mask or the subnet mask?


  • For each PC, is the Default Gateway the IP address of the router on the “local network”?
  • When you ping a host or router, is the ICMP request failing to get to its destination or is the target failing to respond?

Deciphering Ping/ICMP Responses

  • Destination host unreachable
    • The IP address returned by “Reply from” should be the destination IP address.
    • If it’s the IP of the Default Gateway, then the router doesn’t have the destination network in its routing table.
  • Request timed out
    • The packet was routed to the destination network, but the specific IP/host isn’t responding or doesn’t exist.
    • Does the destination host have the correct gateway address configured?


For a PC on the /24 network, with a Gateway of, and the destination network is missing from the routing table:

  • Reply from Destination host unreachable.

Still having problems? And you’re confident your configuration is correct? Then Save the lab, exit the simulation software, and then reload the lab.

Document the configuration of each router

How to save the output from commands on a router

  • Create a new TXT file with the same filename as the simulation file you’re documenting.
    syntax: CST3607 Lab-<##> LastName, FirstName v<version #>.txt
  • At the top of the TXT file, type your CST3607 Lab-<##> <YourLastName>, <YourFirstname>
  • Paste the output from all routers into the one TXT file:
    1. Make sure all routers have unique names: e.g. hostname 2621-A, 2621-B, etc.
    2. Close and reopen the CLI, to clear its history.
    3. enable
    4. terminal length 0
    5. show ip interface brief
    6. show ip route
    7. show ip protocol
    8. show ip ospf neighbor {If using OSPF}
    9. show controllers s0/0 or s0/1 {If the router is using a DCE interface}
    10. show ip dhcp binding {If the router is configured to be a DHCP server}
    11. show run
      • Make sure to press the space bar to complete the listing if the CLI says “More”
      • Your Interface and routing protocol configurations must be listed!
      • The last line of show run will say “End”
    12. Select the text in the CLI, then right-click, and choose copy.
    13. Switch to your documentation TXT file, and paste
    14. Save the TXT file.
    15. Repeat for each router in the lab. Putting all the data into one text file for the lab.
  • Send the one documentation TXT file with all router configurations to me:
    E-mail Subject: CST3607 Lab-<##> YourLastName, Firstname