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| 2.2 |  | Troubleshooting Approaches |
| 2.2.2 |  | Bottom-up |

When applying a bottom-up approach towards troubleshooting a networking problem, the examination starts with the physical components of the network and then is worked up through the layers of the OSI model until the cause of the problem is identified. It is a good approach for a troubleshooter to use when the problem is suspected to be physical. Most networking problems reside at the lower levels, so implementing the bottom-up approach will often result in effective results.

The downside to selecting this approach is that it requires checking of every device and interface on the network until the possible cause of the problem is found. It is a requirement to document each conclusion and possibility. The challenge is to determine which devices to start with.

In many cases, problems within the first four layers can be determined by entering a ping or traceroute command. If the connection is successful, then the cause is likely at the application level. Otherwise, a closer look at the lower levels will be needed to locate the problem.

Verify that Internet control message protocol (ICMP) echo request and echo reply are enabled on the network in order for commands such as ping and traceroute to work. This action should include authorization from the network administrator and documentation of that authorization. If ping has been disabled on the network, it is a result of the implementation of policy. Document in a station log or your personal work log that ping, or any command that was initially disabled, was enabled for network testing and subsequently disabled. This is important should there be an unauthorized intrustion into the network while you are troubleshooting the network. If disabled, the failure of a ping or traceroute command can easily be mistaken for a loss of connectivity.