How to Troubleshoot a Network Connectivity Issue

by Tony DeSimone

There’s no one correct way to troubleshoot. It’s not an exact science. Having worked with your colleagues to address a networking issue, you probably have found that each engineer has his/her particular way of approaching troubleshooting. I personally have found that using the OSI (Open Systems Interconnection) model as a guide helps me immensely.

I tend to use the Divide-and-Conquer technique. The result of a ping indicates an upper or lower layer issue. If the ping across the network from source to destination fails, I will start analyzing the network layer and down. If the ping is successful, yet the application is still not working properly, I start looking above the network layer.

With an unsuccessful ping, here are a few items to verify:

Layer 3

  • The hosts’ IP addresses, subnet masks and default gateways are correct. Keep in mind that the routers may be configured with an FHRP (First Hop Redundancy Protocol) like HSRP (Hot Standby Router Protocol).
  • The addresses and subnet masks of the routers along the path are correct.
  • The switches have the hosts’ ports assigned to the proper VLAN (Virtual Local Area Network) and that VLAN is allowed along your trunked switch to switch connections.
  • The routers’ routing tables either have specific routes or default routes in the proper direction from source to destination and the return traffic from destination back to source. Keep in mind the routers may be performing NAT (Network Address Translation).
  • There are no packet filters on the hosts, routers or firewalls along the path.

Layers 1 and 2

  • The status of your router and switch ports along the path are up and line protocols are up.
  • The Layer 2 frame encapsulation types match between connected devices.
  • There is not an inordinate amount of input or output errors on the network devices’ interfaces such as CRC errors, collision errors, dropped packets.
  • The speed and duplex settings match between connected devices.
  • Interface link lights are green and the cables are fully seated and not damaged.

These verification items are some first steps into troubleshooting a network connectivity issue.  Some of these items raise more questions; a negative verification of any of these items may warrant further investigation. For example, what if I don’t find a route to the destination in a router’s routing table?  At this point, you now know you must start analyzing your routing further.

What are some of your favorite troubleshooting techniques?

Training Resources:
Implementing Cisco Enterprise Advanced Routing and Services (ENARSI)
Cisco Training

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