How to Resolve Spanning Tree Operation Issues

Certification: CCNA Routing and Switching - Cisco Certified Network Associate (CCNA) Routing and Switching

When a network is formed, there are various routes set from which data needs to travel. These roots ensure that the data has multiple paths to reach its destination. They also ensure that no congestion is caused due to data packets passing through the same route. However, in this process, there are many redundant routes created in the network which cause a lot of hindrance. These routes can cause many loops to be created where the data packets start to move around continuously. In order to get rid of this issue, the Spanning Tree Protocol (STP) has been introduced.

The primary purpose of a Spanning Tree Protocol is to ensure that all of the redundant links are shut down and only the required ones remain. This is achieved by getting a map of the whole network and then identifying the important routes for the flow of data. However, there are a number of issues faced by the network due to STP. Due to the automatic adjustment of settings, the STP might behave in an unusual manner. This could lead to a lot of issues including poor connections, loss of data packets, modification of data etc. It is extremely important to identify the root of these problems and troubleshoot them in order to let the network run properly. Following are the main issues faced during Spanning Tree operations and ways in which how they can be troubleshoot.

Issue of Forwarding Loops

When none of the ports in a redundant topology is blocked, the data packets flow in circles continuously. These indefinite loops are known as forwarding loops. The main issue associated with forwarding loops is the congestion of all the routes. Since this loop attracts all of the bandwidth towards itself, most of the links tend to be get congested. This leads to the loss of data packets and the network goes down. In order to solve this issue, there are a number of steps which need to be carried out to troubleshoot the network.

Firstly, you need to confirm and identify whether the loop has been created. In this regard, there are a number of symptoms which point towards this issue. The network will experience loss of connectivity if a forwarding loop has occurred. It will also show high link utilization and will experience an increased number of output drops on many interfaces. You will also get to know regarding it when most of the links on the network have been congested. So, once you confirm the problem, it is time to act.  The second step which you have to go through is the discovery of the ports which are involved in the link. In order to do this, you will have to use the “show interface” command. This command will provide you with all the information relating to the utilization of each interface. From this information, you will get to know which ports are experiencing high utilization. These ports will be the one involved in the forwarding loops.

Once you have identified the ports causing the issue, the third step stands to break this forwarding loop. In order to accomplish this, you have to disconnect all the ports which are involved in the loop. It is better to disconnect all of them at once. You should also check whether the utilization of traffic on ports is back to normal or not once the ports have been disconnected. Once you do this, the last step is to resolve the issue and find the root of the problem. In this regard, you will make use of a number of commands to check how the loop originated and repair the ports. Once done, your network will be back to normal.

Issues related to Convergence Time

This issue comes into being when the convergence time is of a higher duration than expected. This is caused due to the unusual behavior of STP which is different from the expected behavior. In addition to this, the topology of the network which has been created is different from the expected topology. These issues make the network act in an unusual manner and the users experience slow network performance. There could be a number of issues which lead to this problem, for example a software defect.

In order to troubleshoot this problem, one needs to look in depth at the performance of STP to determine why the convergence has been slow. In this regard, the debugging needs to be done on the switch processor on which all of the related processes take place. Once you examine the switch processor, you will get to know what problem is causing the convergence times to be high.

Topology change causing flooding

This is a rare problem, but is quite significant to be eliminated if it occurs. Topology changes occur after some of the ports are changed by STP and the new ports have to be identified to let the network run. However, these topology changes cause a good deal of problem when they occur continuously, leading to continuous changes in the topology of the network. This consequently leads to the issue of flooding on various ports and makes the network become unresponsive.

In order to troubleshoot this problem, you first need to figure out the source of this issue. You should make use of commands providing spanning tree information to get to the root of this issue. Once you have done this step, you then have to take steps to prevent any further topology changes. The main focus here remains to get rid of “excessive” topology changes which make the network unstable. In this regard, you first need to identify the number of topology changes which are occurring and then repair the source from which this problem is being originated.

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