Preparing for Network Failure
You can always recover from a network failure easier if you are prepared ahead of time. To see if you are prepared for a network failure, answer the following questions:
Internetworks come in a variety of topologies and levels of complexity; from single-protocol, point-to-point links connecting cross-town campuses, to highly-meshed, large-scale WANs traversing multiple time zones and international boundaries. The industry trend is toward increasingly complex environments, involving multiple types of media, multiple protocols, and often providing interconnection to unknown networks.
Consequently, the potential for connectivity and performance problems in internetworks is high, and the sources of such problems are often elusive. The goal of this section is to help you isolate and resolve the most common connectivity and performance problems within your OSPF Network.
Symptoms, Problems, and Solutions
Failures in internetworks are characterized by certain symptoms. These symptoms might be general (such as clients being unable to access specific servers) or more specific (routes not in the routing table). Each symptom can be traced to one or more problems or causes by using specific troubleshooting tools and techniques. After they are identified, each problem can be remedied by implementing a solution consisting of a series of actions.
The section that follows describes how to define symptoms, identify problems, and implement solutions as apply to a OSPF network but the basics can also apply to any generic network environment. Always apply the specific context in which you are troubleshooting to determine how to detect symptoms and diagnose problems for your specific environment.
General Problem-Solving Model
When troubleshooting problems within an OSPF networked environment, a systematic approach works best. Define the specific symptoms, identify all potential problems that could be causing the symptoms, and then systematically eliminate each potential problem (from most likely to least likely) until the symptoms disappear.
This process is not a rigid outline for troubleshooting an internetwork. Rather, it is a solid foundation from which you can build a problem-solving process to suit the particular needs of your OSPF environment.
The following seven steps detail the problem-solving process:
Step 1: Clearly Define the Problem
When analyzing a network problem, make a clear problem statement. You should define the problem in terms of a set of symptoms and potential causes. To do this, identify the general symptoms and then ascertain what kinds of problems (causes) could result in these symptoms.
For example, hosts might not be responding to service requests from clients (a symptom). Possible causes might be a misconfigured host, bad interface cards, or missing router configuration commands.
Step 2: Gather Facts
Gather the facts you need to help isolate possible causes by asking questions of your peers and others such as: