VLAN Troubleshooting

Assignment 1:

VLAN Troubleshooting

CIS 436

Internetworking Troubleshooting

VLAN’s

A virtual local area network is a series of networks comprised of several network devices all communicating together across one singular network that was digitally created for communication. One benefit of VLAN’s is their simplicity in connecting multiple devices, and if there is ever trouble, it can be quite difficult to solve the issue.

Troubleshooting Examples/Methodology

With this assignment, we are presented with a network that has two different VLAN’s but several issues. It is our job to analyze these issues and provide a solution to resolving the issue. In many cases of VLAN problems, the issue is software based versus it being hardware based. It is still possible that there is a hardware based issue but for all of my examples, I will approach the issue as if it is not hardware until the software has been fully tested.

Switch 1 HR VLAN communication cannot take place with Switch 2 HR VLAN

This first example is one VLAN across two switches. I would start out by getting the configuration for both switches. I can issue “show running-configuration” to obtain information on how the switch is set up. From here I can examine the output, and issue “switchport mode access” and “switchport access vlan” to view the VLAN specifically. Next, we would want to verify that ports are up on both switches, if either port is down, issue the “shutdown/no shutdown” to bring online. If it is still not working at this point, I will try to manually set the speed and duplex at both ends. Once the VLAN is manually set up, the last possibility is a connection (hardware) issue. I would inspect the cabling and the ports at this point. Replace the cable if necessary and use a different port. The cause of this issue could be very simple like an update or patch changed the configuration of the router. If it is hardware, age or short could be to blame

Switch 2 Finance VLAN cannot communicate with Switch 4 Finance VLAN

In this example switch 2 cannot communicate with switch 4 across switch 3 and two different VLAN’s. This example would be quite challenging for any administrator. I would start by issuing a ping command from switch 2 to switch 4 to see if it is reaching at all. Next, I would issue “traceroute” to find out where is a communication breakdown in the chain. If the connection stops right at switch 3, I would need to pull the configuration for switch 3 to see why the switch is not allowing transmission from 2 to 4 and vise versa. Is switch 3 up or down? There is one other possibility, that switch 2 can not communicate with 3 and switch 4 cannot communicate with 3. If switch 2/4 can not communicate with 3 then we need to identify where the lapse is between them. Is switch 3 powered on? Is switch 3 plugged properly into 2 and 4? In solving this issue, it could easily be hardware or software related. Personally I would still start with the software and work towards the hardware. My premise for this is that most issues are software related.

Switch 4 has both trunk ports to switch 3

continuously going from blocking to forwarding

In our third example, we are posed with what appears to be a spanning tree protocol failure. The primary function of STP is to get rid of redundant links. STP will automatically set a port to block or forward traffic. To begin troubleshooting this issue, I would start with the “show system” command. This command will provide us with the current usage of the switch. Next, we want to investigate the blocking ports. We will issue the “show spanning-tree” and “show spantree statistics” commands. We will be able to determine with this information if blocked ports are receiving BPDUs. If everything is good here, we will move on to check for a mismatch. Issue the “show interfaces” command to check the status of the ports. We can also get port utilization from this step as well to see if a port is being over-loaded. This is possibly caused by a broadcast storm but based on the description; it sounds more like an STP failure. That is reason why I approached it as such. This is not a hardware issue.

Root bridge election is occurring every minute

In this example, a root bridge election is occurring every minute. At first, I thought that is may be a trick question before I started researching it. Root bridge elections should only occur in two instances: When switches are first booting up and connecting to one another or if a root bridge goes down and another is needed. My first though would be that the BDPU’s are not getting delivered and therefore the root bridge cannot be established, or that the switches have a communication issue and that would be another reason. I found that you can set a security feature on all switches that will prevent election of other root switches once one is set. According to Cisco, it is called a root-guard. This will enable an engineer to stop any further network disruption by stopping any further root bridge elections. In this last issue, this would also be a software-based issue. Either the configuration on the switch is incorrect or the load being placed upon them was not considered and would need adjustments.

References

Troubleshooting VLAN and Switch Problems. (2013, April 03). Retrieved from

https://www.petri.com/troubleshoot-vlan-switch

Spanning Tree Protocol Problems and Related Design Considerations. (2017, December 05).

Retrieved from https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-

protocol/10556-16.html

Network disruption due to Root bridge election. (n.d.). Retrieved from

https://www.experts-exchange.com/questions/27154473/Network-disruption-due-to-Root-bridge-election.html