20 Spanning Tree Protocol Interview Questions and Answers

The Spanning Tree Protocol (STP) is a network protocol that helps ensure the physical integrity of a network by creating a loop-free topology. STP is typically used in Ethernet networks. When applying for a position that involves working with STP, it is likely that you will be asked questions about your knowledge and experience with the protocol. In this article, we review some of the most common STP interview questions and provide tips on how to answer them.

Spanning Tree Protocol Interview Questions and Answers

Here are 20 commonly asked Spanning Tree Protocol interview questions and answers to prepare you for your interview:

1. What is Spanning Tree Protocol?

Spanning Tree Protocol is a network protocol that helps to ensure that there is no loops in the network. It does this by creating a tree structure that spans all of the switches in the network.

2. What are the main characteristics of Spanning Tree Protocol?

Spanning Tree Protocol is a Layer 2 protocol that helps to prevent loops in a network by creating a logical tree that spans all of the switches in a network. STP works by electing a root switch, and then all other switches in the network calculate the shortest path to the root switch. STP then blocks any redundant paths to prevent loops.

3. What versions of Spanning Tree Protocol exist?

There are two versions of Spanning Tree Protocol: IEEE 802.1D and IEEE 802.1W.

4. Can you explain what a port state is in context with the Spanning Tree Protocol?

In short, the port state is the condition of the port with respect to the Spanning Tree Protocol. There are three possible states that a port can be in: blocking, listening, and learning. In the blocking state, the port cannot forward any traffic. In the listening state, the port can begin forwarding traffic, but it first needs to listen for a period of time to make sure that there are no other active paths. In the learning state, the port can forward traffic, and it is also learning the addresses of the devices on the network.

5. What happens to a switch’s ports when it receives a BPDU from another switch that has a lower priority or an equal priority?

When a switch receives a BPDU from another switch that has a lower priority, it will keep its current port configuration. However, if the BPDU has an equal priority, then the switch will compare the MAC addresses of the two switches. The switch with the lower MAC address will keep its current port configuration, while the switch with the higher MAC address will change its port configuration.

6. What are some common causes for STP problems?

There are a few common causes for STP problems:

– Incorrectly configured STP parameters
– Loops in the network
– Incorrectly placed STP bridges
– Incorrectly placed STP ports

7. What does “learning” mean in context with the Spanning Tree Protocol?

The Spanning Tree Protocol is a network protocol that helps to ensure that there are no loops in a network. In order to do this, it needs to be able to “learn” about the network, which means that it needs to be able to identify all of the devices and connections in the network.

8. When will a bridge send out BPDUs?

A bridge will send out BPDUs under the following circumstances:

-When the bridge first starts up
-When the bridge detects a topology change
-When the bridge receives a configuration BPDU from another bridge

9. How can you configure VLANs on a Cisco switch using STP?

You can configure VLANs on a Cisco switch using STP by creating a separate VLAN for each switch port. You will then need to configure the switch port to trunk mode so that it can carry traffic for all VLANs. Finally, you will need to enable STP on the switch port.

10. What is the difference between root and designated ports?

A root port is the port on a non-root switch that is closest to the root bridge. A designated port is a port that has been determined to be the best path to a particular segment.

11. What is the purpose of ‘portfast’ and how do you enable it?

Portfast is a Cisco-specific feature that is used to reduce the time it takes for a port to transition from the blocking state to the forwarding state. This can be useful in situations where you need to quickly connect a host to a network, such as when you are plugging in a laptop to a conference room network. To enable Portfast on a Cisco switch, you can use the following command:

switch(config)#spanning-tree portfast

12. What is UplinkFast?

UplinkFast is a Spanning Tree Protocol (STP) feature that helps to improve the convergence time after a topology change. It does this by pre-selecting a backup port to be used in the event of a failure on the primary port. This backup port is then put into a forwarding state as soon as the primary port fails, which helps to minimize the amount of time that is spent in the blocking state.

13. What is BackboneFast?

BackboneFast is a Cisco-proprietary protocol that is designed to speed up the convergence time of a Spanning Tree Protocol (STP) network. BackboneFast works by detecting indirect link failures and then rapidly reconverging the STP network around the failure.

14. What is PortFast BPDUGuard?

PortFast BPDUGuard is a security feature that can be enabled on Cisco switches. When PortFast is enabled on a port, the port is allowed to transition to the forwarding state immediately, without waiting for the Spanning Tree Protocol to converge. BPDUGuard then monitors the port for incoming BPDUs and shuts the port down if one is received. This prevents malicious devices from spoofing BPDUs and causing a Spanning Tree Protocol loop.

15. What is BPDUFilter? Is it different than PortFast BPDUGuard?

BPDUFilter is a Spanning Tree Protocol feature that is used to prevent loops in the network. It does this by filtering out BPDUs that are received on ports that have been configured for PortFast. BPDUGuard is a Spanning Tree Protocol feature that is used to protect ports that have been configured for PortFast. It does this by shutting down the port if a BPDU is received on it.

16. What is a loop guard? Why should you use one?

A loop guard is a feature of the Spanning Tree Protocol (STP) that helps to prevent network loops. When a loop is detected, the loop guard feature will automatically block the port that is causing the loop, which helps to prevent data from being sent in circles around the network.

17. What is Root Guard? Why would you want to use it?

Root Guard is a Spanning Tree Protocol (STP) feature that can be used to help ensure that a particular switch remains the root switch in the network. This can be useful in preventing configuration errors that could cause the network to become unstable.

18. What is UDLD aggressive mode?

UDLD aggressive mode is a mode of operation for the Spanning Tree Protocol (STP) that allows for faster detection of unidirectional links. In this mode, STP will immediately place any port that receives UDLD packets from only one direction into a blocking state. This can help to prevent issues caused by unidirectional links, such as data loss or network loops.

19. What is Bridge Assurance?

Bridge Assurance is a mechanism that helps to ensure the loop-free operation of a network. It does this by creating a logical link between two switches, which prevents loops from forming. Bridge Assurance also monitors the link for any failures, and can reroute traffic if necessary.

20. What is MST?

MST is a type of spanning tree protocol that is used in computer networking. It is an improvement over the older STP protocol, and allows for a more efficient use of network resources.