Identify and Troubleshoot Errors with a Particular Physical Topology


Each of the physical network topologies requires its own troubleshooting strategies and methods. When troubleshooting a network, it is important to know which topology is used as it can greatly impact the procedures used to resolve any problems. This section lists each of the respective physical network topologies and some common troubleshooting strategies.

In one form or another, you can expect to be asked questions regarding troubleshooting the different topologies.


Star Topology

The most common topology used today is the star topology. The star topology uses a central connection point such as a hub in which all devices on the network connect. Each device on the network uses its own length of cable, thus allowing devices to be added or removed from the network without disruption to current network users. When troubleshooting a physical star network, consider the following:

  • The central device, hubs or switches, provides a single point of failure. When troubleshooting a loss of connectivity for several users, it might be a faulty hub. Try placing the cables in a known working hub to confirm.

  • Hubs and switches provide light-emitting diodes (LEDs) that provide information regarding the port status. For instance, by using the LEDs, you can determine whether there is a jabbering network card, whether there is a proper connection to the network device, and whether there are too many collisions on the network.

  • Each device, printer, or computer connects to a central device using its own length of cable. When troubleshooting a connectivity error in a star network, it might be necessary to verify that the cable works. This can be done by swapping the cable with a known working one or using a cable tester.

  • Ensure that the patch cables and cables have the correct specifications.

Figure 11.1 shows how a single cable break would affect other client systems on the network.

Figure 11.1. Identifying cable breaks in a star network.


Ring Topology

Although not as commonly used as it once was, you might find yourself troubleshooting a ring network. Most ring networks are logical rings, meaning that each computer is logically connected to each other. A physical ring topology is a rare find but a Fiber Distributed Data Interface (FDDI) is often configured in a physical ring topology. A logical ring topology uses a central connecting device as with a star network called a multistation access unit (MSAU). When troubleshooting either a logical or physical ring topology, consider the following:

  • A physical ring topology uses a single length of cable interconnecting all computers and forming a loop. If there is a break in the cable, all systems on the network will be unable to access the network.

  • The MSAU on a logical ring topology represents a single point of failure. If all devices are unable to access the network, it might be that the MSAU is faulty.

  • Verify that the cabling and connectors have the correct specifications.

  • All Network Interface Cards (NICs) on the ring network must operate at the same speed.

  • When connecting MSAUs in a ring network, ensure that the ring in and ring out configuration is properly set.

Figure 11.2 shows how a single cable break would affect other client systems on a physical ring network.

Figure 11.2. Identifying cable breaks in a physical ring network.


Bus Network Errors

Although the bus topology is rarely implemented anymore, enough of them are out there for it to be included in the CompTIA Network+ exam objectives. So if you do not encounter a bus network in the real world, you will most certainly be faced with one on the exam.

Troubleshooting a bus network can be a difficult and frustrating task. The following list contains a few hotspots to be aware of when troubleshooting a bus network:

  • A bus topology must be continuous. A break in the cable at any point will render the entire segment unusable. If the location of the break in the cable is not apparent, you can check each length of cable systematically from one end to the other to identify the location of the break, or you can use a tool such as a time domain reflectometer, which can be used to locate a break in a cable.

  • The cable used on a bus network has two distinct physical endpoints. Each of these cable ends requires a terminator. Terminators are used to absorb electronic signals so that they are not reflected back on the media, compromising data integrity. A failed or missing terminator will render the entire network segment unusable.

  • The addition, removal, or failure of a device on the network might prevent the entire network from functioning. Also, the coaxial cable used in a bus network can be damaged very easily. Moving cables in order to add or remove devices can cause cable problems. The T connectors used on bus networks do allow devices to be added and removed without necessarily affecting the network, but care must be taken when doing this.

  • One end of the bus network should be grounded. Intermittent problems or a high occurrence of errors can indicate poor or insufficient grounding.

Figure 11.3 shows how a single cable break would affect other client systems on a bus network.

Figure 11.3. Identifying cable breaks in a bus network.


Mesh Network Errors

A mesh topology offers high redundancy by providing several paths for data to reach its destination. In a true mesh network, each device on the network is connected to every other device, and if one cable fails, there is another to provide an alternative data path. Although a mesh topology is resilient to failure, the number of connections involved can make a mesh network somewhat tricky to troubleshoot.

When troubleshooting a mesh network, consider the following points:

  • A mesh topology interconnects all devices on the network, offering the highest level of redundancy of all the topologies. In a pure mesh environment, all devices are directly connected to all other devices. In a hybrid mesh environment, some devices are connected only to certain others in the topology.

  • Although a mesh topology can accommodate failed links, mechanisms should still be in place so that failed links are detected and reported.

  • Design and implementation of a true mesh network can be complex and often requires specialized hardware devices.

Mesh networks are so rare that it's unlikely you will be faced with troubleshooting one, but there will likely be questions on the Network+ Exam that focus on mesh networks.

Most mesh networks are used to connect multiple networks, such as in a WAN scenario, rather than to connect computers in a LAN.




    Network+ Exam Cram 2
    Network+ Exam Cram 2
    ISBN: 078974905X
    EAN: N/A
    Year: 2003
    Pages: 194

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