Useful Techniques for Troubleshooting Virtual Local Area Networks (VLANs) and Trunking Problems

Techniques that are most useful for Troubleshooting Virtual Local Area Networks (VLANs) and Trunking Problems

            VLANs are a grouping of ports that employ switches with logic inside to ensure that traffic from a group of ports is not send to other group of ports. On the other hand, trunking is a mechanism that allows multiple virtual area networks (VLANs) to function independently across several switches (Cisco, 2017). The most common VLANs include incorrect internet protocol (IP) addresses and missing VLANs, while the most common trunking problems are native VLAN mismatch, trunk mode mismatch and allowed VLANs on trunks.

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            Every VLAN should correspond to a unique IP subnet (Cisco Academy, 2017). When two devices in the same VLAN have different IP subnet, they cannot communicate. This is a common problem with the VLANs and can be solved through correct configuration and changing the subnet address to the correct one. Sometimes no connection is established even with correct IP addresses. This may signal missing VLANs. In such a case, the show vlancommand can be used to check whether the port belongs to an expected VLAN. If a port is found to be assigned to wrong VLAN, the command switchport access vlan can be used to assign the correct VLAN.

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            The common trunk problems include links incorrectly behaving as trunk links and trunk link information (Cisco Academy, 2017). For example, a switch port could behave like a trunk port though not configured as a trunk port. This could lead to the port accepting frames from VLANs other than those it is assigned. To trouble short trunk problems, the command “show interfaces trunk” are used to check the VLAN mismatch between local and peer VLANs and to check whether there is trunk establishment between switches.

Effective Troubleshooting Routine for Switches

There are many ways of troubleshooting switches. According to (Cisco, n.d), the increase in the features of a switch increases the possible number of things that can break. According to the author, developing an approach plan for troubleshooting a switch is critical in the long run than a hit-and-miss approach. The following are effective troubleshooting routine for switches.

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  • It is important for one to take to gain familiarity with the operation of the switch. The product information guides offers useful information on product configuration which offers information on how to solve any problems that may arise. An example is examination of the configuration process, which helps in cross-checking whether they were followed as outlined.
  • For problems that are more complex, it is important that an accurate physical and logical map of the network be obtained. The physical map provides the information on how the cables and devices are connected within the network. The logical map provides information on how segments in the VLANs exist in the network and the routers that provide routing services to the segments.
  • Ensure you have a map. The complexity of problems varies; while some can be simple others can be complex. Some problems may result from problems in other areas or layers. Having a structured way of verifying problems helping in not jumping into conclusions. For example, one can choose to start from the ports, and then proceed to check on switches. The choices to start with the ports ensures that ports are functional before proceeding to the switches thus eliminating the possibility of overlooking a possible problem in ports that could be the cause of the problem in switches.
  • Do not make assumptions, always check and verify that all components work. Make sure you check the basic things, for example starting with cable connections as this can derail the perfect working of network. Besides, checking basic things saves time that could be easily wasted.

Elements of a Switch Operation that Needs to be Checked to Implement Troubleshooting

  • Ports- ports are the foundation of operation of switches, if ports do not work, then switches will not function. For example, ports should be checked for hardware issues, configuration, and traffic issues. Hardware issues relates to whether the ports are connected well with working cables, while configuration and traffic issues include interfaces or internal processes.
  • Switch hardware and software failures- for example, sometimes faulty hardware due to electrostatic discharge damage on ports can cause switches not to function. Power-on-self-test should be checked on the switch to determine if any failures are indicated in the switches. Any strange behavior could be indicative of hardware and software problems. Software problems can be solved easily by reloading the software.

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Tools that Can Assist With Switch Troubleshooting

The tools which can be used to assist in switch troubleshooting include switch port mapper, STP tool and protocol analyzer.

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