Troubleshoot and resolve spanning tree operation issues

Exam: Cisco 200-120 - Interconnecting Cisco Networking Devices: Accelerated (CCNAX)

In this chapter we will take up the topic of “Troubleshoot and resolve spanning tree operation issues”. This is a very important topic from the CCNA exam point of view and you must try to spend good time preparing this topic. We have tried to cover all the points that are important from the exam point of view. Do keep your preparations on these lines in order to get a good score.

We have already explained in the previous chapters that troubleshooting is basically a way to solve the many problems that may arise in a system or in a network. In this chapter we will concentrate on the problems that can happen in a spanning tree operation. The spanning tree is commonly reffered to as the STP in a network. As a network administrator you will often come across problems that frequently occur in the STP. The STP is a network protocol that ensures a loop free technology for a bridged Ethernet local network. The STP has the basic function of preventing bridge loops and broadcast the radiation that results from the STP. You will notice the STP when two bridges are used to connect the same computer to two network segments. This bridge that is created using the STP will allow the protocols to exchange information. This STP ensures that here is no bridge loop. The spanning tree algorithm is basically a program that allows each bridge to understand how the protocol must be used. This algorithm plays the most important role in avoiding the bridge loops.

As you can see that the STP plays a very crucial role in the network. You will often come across problems that occur usually n the STP area. We will now discuss the common problems that you may face in the STP as the network administrator. Since 1990 STP is being used but as the organisations do not pay much attention to the STP the errors in this are very common. Some of these errors keep coming again and again. These problems are:

1. The root bridge is not configured – most of the places accept the default configuration of the STP which may not be a good idea as different STP will have different requirements. You must configure a main switch with lower STP priorities. This is ensures that there is one single root bridge. If there is any other core bridge it will have a little higher value. Using the command Core-Sw1(config)# spanning-tree vlan 1-4096 root primary the root switch can be configured. You must also create a secondary root switch.
2. Another common problem is the se of the IEEE 802.1d and not the rapid STP – the timer of IEEE 802.1d is not adequate for the network that we have today. It is an old network that is actually obsolete today. It is very slow. Most of the switches today can easily use the rapid spanning tree protocol. As a network administrator you can try to implement this. This is called the RSTP and it improves the performance of the bridge significantly. The RSTP can be used whenever possible.
3. Blocked uplinks – to understand this point you will have to have a clear idea of what the STP does. It basically tries to ensure that there are no loops formed. How it does this is to understand the sub optimal paths and ensure that these less desirable links are blocked. When there are many parallel paths between the switches one of them will be selected and blocked so that there is no loop between the switches. If you want to use both the uplinks for a better bandwidth then you will have to use Cisco nexus switches along with the virtual port channel. You can also use stackable switches and configure each uplink port that can connect to a different switch in the stack.
4. Exceeding STP maximum dimensions – the networks will keep growing like a city and new devices are constantly added to the network. Similarly as new switches are added to the LAN the STP will also grow. The organization must be planned for this growth and know how to deal with it. These organizations will often be in trouble if the network expands to a large extent. This will ensure that the topology exceeds the maximum dimension of the STP. A spanning tree may not have more than seven bridge hops. If you are looking into the network of a college or a hospital such problems are common. You must ensure that the dimensions of the spanning tree is regularly checked and documented. You must also keep looking for the topology change notification.
5. VTP domains – in a STP there can be possibilities of developing problems with VTP (VLAN trucking protocol). Some of the organizations do the mistake of using the same VTP domain name across all the switches. The VTP should be used carefully. As a network administrator you must know which are the VTP servers and clients. You must use a VTP password. The VTP must be disabled if there is a need to do so.
6. Those organizations that are using the first hop redundancy protocol must ensure that there is alignment between the STP root and the active default gateway.
7. Failure to control the STP is a common problem that you will face. This mainly happens because the organizations generally do not configure the spanning tree accordingly as a result controlling them becomes difficult. You must ensure that the STP is configured correctly and not that only default settings are implemented in them.
8. Another common problem with the spanning tree is the inconsistent spanning tree metrics. The traditional STP used 16-bit value. Today the speed of the spanning tree can be as high as 32-bit. In order to assign a long path on the switch you can enter the global configuration mode and use the command - Core-Sw1(config)# spanning-tree pathcost method long.

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