Network Troubleshooting

Once a network is operational, administrators have to monitor its performance for the sake of the organization's productivity. From time-to-time network outages can occur. Sometimes they are planned and their impact on the organization easily managed. Sometimes they are not planned and their impact on the organization can be severe. In the event of an unexpected network outage administrators must be able to troubleshoot and bring the network back into full production. In this chapter you will learn a systematic process for troubleshooting network outages.

ocumenting Your Network
To efficiently diagnose and correct network problems, a network engineer needs to know how a network has been designed and what the expected performance for this network should be under normal operating conditions. This information is called the network baseline and is captured in documentation such as configuration tables and topology diagrams.
Network configuration documentation provides a logical diagram of the network and detailed information about each component. This information should be kept in a single location, either as hard copy or on the network on a protected website. Network documentation should include these components:
Network configuration table
End-system configuration table
Network topology diagram

Network Configuration Table
Contains accurate, up-to-date records of the hardware and software used in a network. The network configuration table should provide the network engineer with all the information necessary to identify and correct the network fault. The table contain the information about
Type of device, model designation
IOS image name
Device network hostname
Location of the device (building, floor, room, rack, panel)
If it is a modular device, include all module types and in which module slot they are located
Data link layer addresses
Network layer addresses
Any additional important information about physical aspects of the device
End-system Configuration Table
Contains baseline records of the hardware and software used in end-system devices such as servers, network management consoles, and desktop workstations. An incorrectly configured end system can have a negative impact on the overall performance of a network.
For troubleshooting purposes, the following information should be documented:
Device name (purpose)
Operating system and version
IP address
Subnet mask
Default gateway, DNS server, and WINS server addresses
Any high-bandwidth network applications that the end-system runs
Network Topology Diagram
Graphical representation of a network, which illustrates how each device in a network is connected and its logical architecture. A topology diagram shares many of the same components as the network configuration table. Each network device should be represented on the diagram with consistent notation or a graphical symbol. Also, each logical and physical connection should be represented using a simple line or other appropriate symbol. Routing protocols can also be shown.
At a minimum, the topology diagram should include:
Symbols for all devices and how they are connected
Interface types and numbers
IP addresses
Subnet masks

etwork Documentation Process

When you document your network, you may have to gather information directly from routers and switches. Commands that are useful to the network documentation process include:
The ping command is used to test connectivity with neighboring devices before logging in to them. Pinging to other PCs in the network also initiates the MAC address auto-discovery process.
The telnet command is used to log in remotely to a device for accessing configuration information.
The show ip interface brief command is used to display the up or down status and IP address of all interfaces on a device.
The show ip route command is used to display the routing table in a router to learn the directly connected neighbors, more remote devices (through learned routes), and the routing protocols that have been configured.
The show cdp neighbor detail command is used to obtain detailed information about directly connected Cisco neighbor devices.
This activity covers the steps to take to discover a network using primarily the telnet, show cdp neighbors detail, and show ip route commands. This is Part I of a two-part activity.
The topology you see when you open the Packet Tracer activity does not reveal all of the details of the network. The details have been hidden using the cluster function of Packet Tracer. The network infrastructure has been collapsed, and the topology in the file shows only the end devices. Your task is to use your knowledge of networking and discovery commands to learn about the full network topology and document it.


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