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| 5.1 |  | Troubleshooting Network Layer Problems |
| 5.1.1 |  | What are network layer problems? |

Network layer problems include any problem that involves a Layer 3 protocol, including both routed and routing protocols. This module will focus primarily on IP routing protocols.

Problems at the network layer can have the following affects on a network:

- Network failure
- Failure to perform optimally

Network failure is when the network is nearly or completely nonfunctional, affecting all users and applications using the network. These failures are usually noticed quickly by users and network administrators, and are obviously critical to the productivity of a company.

Network optimization problems can be more difficult to discover and sometimes harder to diagnose. These problems usually involve a subset of users, applications, destinations, or a particular type of traffic. Optimization issues in general can be more difficult to detect and even harder to isolate and diagnose as they usually involve multiple layers or even the host computer itself. Determining that the problem is a network layer problem can take time. Aspects of network optimization are discussed in more detail in the module “Layer 1–7 Troubleshooting”.

A possible optimization problem on a link at the network layer can be identified because data will be transferred in a slow, sporadic, or unexpected manner compared to the data transfer rate documented in the baseline. Symptoms short of a complete failure to route packets may include misrouted packets that are lost or experience excessive transit delays.

Many of the problems discussed in this module can apply to both network failures and optimization problems, depending upon the scenario and configuration of the actual network.

Not all of the information in this module has to do with troubleshooting routing problems. This module also contains information regarding routing optimization, which includes some additional information on how the routing table process operates. Understanding the routing table process is helpful when troubleshooting certain situations, so that the cause of the problem is understood, not just how to solve it.

| Lab Activity Lab Exercise: Isolating Network Layer Problems After completing this lab, the student will be able to analyze user feedback and end-system data to decide in which OSI layer to begin isolating problems.