Using a structured approach to troubleshooting

Using a structured approach to troubleshooting 
9.2.2 Troubleshooting is a process that allows a user to find problems on a network. This page explains why an orderly process should be used to troubleshoot a network. This process should be based on the networking standards set in place by a network administrator. Documentation is a very important part of the troubleshooting process. 
The steps in this model are as follows:
Step 1 Collect all available information and analyze the symptoms of the failure.
Step 2 Localize the problem to a particular network segment, module, unit, or user.
Step 3 Isolate the trouble to specific hardware or software within the unit, module, or user network account.
Step 4 Locate and correct the problem.
Step 5 Verify that the problem has been solved.
Step 6 Document the problem and the solution.
Another approach to troubleshooting. These are not the only ways to troubleshoot a network. However, an orderly process is important to keep a network running smoothly and efficiently.
When a structured approach is used, every member of a network support team knows which steps the other team members have completed to troubleshoot the network. If a variety of troubleshooting ideas are tried with no organization or documentation, problem solving is not efficient. Even if a problem is solved in the non-structured environment, it will be difficult to replicate the solution for similar problems.
The Interactive Media Activity will help students become familiar with the troubleshooting process.
The next page will teach students the types of errors that occur at the first three layers of the OSI model.

Network Testing / Introduction to network testing

Network Testing
>Introduction to network testing
9.2.1 This page will give students an overview of how to test a network.
Basic testing of a network should proceed in sequence from one OSI reference model layer to the next. Begin with Layer 1 and work up to Layer 7, if necessary. At Layer 1, look for simple problems such as power cords plugged in the wall and other physical connections. The most common problems that occur on IP networks result from errors in the addressing scheme. It is important to test the address configuration before continuing with further configuration steps.
Each test presented in this lesson focuses on network operations at a specific layer of the OSI model. At Layer 3, the commands telnet and ping are used to test the network.
The next page will discuss the troubleshooting process.

Observing multiple paths to destination

Observing multiple paths to destination 
9.1.9 Multi-path algorithms permit traffic over multiple lines, provide better throughput, and are more reliable than single path algorithms.
IGRP supports unequal cost path load balancing, which is known as variance. The variance command instructs the router to include routes with a metric less than n times the minimum metric route for that destination, where n is the number specified by the variance command. The variable n can take a value between 1 and 128, with the default being 1, which means equal cost load balancing.
rt1 has two routes to network 192.168.30.0. The variance command will be set on rt1 to ensure that both paths to network 192.168.30.0 are utilized.
Figure shows the output from show ip route from rt1 before the variance is configured. FastEthernet 0/0 is the only route to 192.168.30.0. This route has an Administrative Distance of 100 and a metric of 8986.
Figure shows the output from show ip route from rt1 after the variance is configured. The preferred route is interface FastEthernet 0/0, but Serial 0/0 will also be used. After the variance command is executed, IGRP will use load balancing between the two links. 
This page concludes this lesson. The next lesson will discuss network testing. The lesson begins with an overview.

Determining the route next hop / Determining the last routing update

Determining the route next hop
9.1.7 Routing algorithms fill routing tables with a variety of information. Destination next hop associations determine the best path and which router to forward the packet to next. This router represents the next hop on the way to the final destination. 
When a router receives an incoming packet, it checks the destination address and attempts to associate this address with a next hop. 
Determining the last routing update 
9.1.8 

• show ip route 
• show ip route address 
• show ip protocols 
• show ip rip database 

Determining the route metric

Determining the route metric 
9.1.6 Routing protocols use metrics to determine the best route to a destination. The metric is a value that measures the desirability of a route. Some routing protocols use only one factor to calculate a metric. For example, RIP v1 uses hop count as the only factor to determine the metric of a route. Other protocols base their metric on hop count, bandwidth, delay, load, reliability, and cost. 
Each routing algorithm interprets what is best in its own way. The algorithm generates a number, called the metric value, for each path through the network. A lower metric number generally indicates a better path.
Factors such as bandwidth and delay are static because they remain the same for each interface until the router is reconfigured or the network is redesigned. Factors such as load and reliability are dynamic because they are calculated for each interface in real-time by the router.  
The more factors that make up a metric, the greater the flexibility to tailor network operations to meet specific needs. By default, IGRP uses the static factors bandwidth and delay to calculate a metric value. These two factors can be configured manually to control which routes a router chooses. IGRP may also be configured to include the dynamic factors of load and reliability in the metric calculation. By using dynamic factors, IGRP routers can make decisions based on current conditions. If a link becomes heavily loaded or unreliable, IGRP will increase the metric of routes using that link. An alternate route with a lower metric would be used instead.
IGRP calculates the metric by adding the weighted values of different characteristics of the link to the network in question. Here is the formula for calculating the composite metric for IGRP:
Metric = [K1 * Bandwidth + (K2 * Bandwidth)/(256-load) + K3*Delay] * [K5/(reliability + K4)]
The default constant values are K1 = K3 = 1 and K2 = K4 = K5 = 0.
If K5 = 0, the [K5/(reliability + K4)] term is not used. Given the default values for K1 through K5, the composite metric calculation used by IGRP reduces to Metric = Bandwidth + Delay.
The Interactive Media Activity will help students understand route metrics.
The next page explains how a next hop is chosen.

Determining the route administrative distance

Determining the route administrative distance 
9.1.5 A router can discover routes through dynamic routing protocols or routes can be configured manually. After the routes are discovered or configured, the router must choose the best routes to other networks.
The router uses the administrative distance of each route to determine the best path to a particular destination. The administrative distance is a number that measures the trustworthiness of the source of the route information. The lower the administrative distance, the more trustworthy the source.
Different routing protocols have different default administrative distances. The path with the lowest administrative distance is installed in the routing table.
In the Lab Activity, students will analyze two routing protocols.
The next page will discuss route metrics.

Determining L2 and L3 addresses

Determining L2 and L3 addresses 
9.1.4 This page will explain how Layer 2 and Layer 3 addresses are used to route a packet.
For a packet to get from the source to the destination, both Layer 2 and Layer 3 addresses are used. Figure explains the process that occurs as a packet moves through a network.
The Layer 3 address is used to route the packet from the source network to the destination network. The source and destination IP addresses remain the same. The MAC address changes at each hop or router. A data link layer address is necessary because delivery within the network is determined by the address in the Layer 2 frame header.
The Interactive Media Activity will help students recognize physical and logical addresses.
The next page will describe the administrative distance of a route.