Most Frequently Asked STP Interview Questions & Its concepts

In the networking field, you need to know a lot about the Spanning Tree protocol. “STP Interview Questions” is related to this article. Here are some of the most important questions about STP and their answers (Spanning Tree protocol). And these will help you a lot in job interviews that have to do with networking.

What does STP do?

STP stands for “Spanning Tree Protocol” It is kind of a Layer-2 protocol that works on bridges and switches. STP helps you avoid loops in networks and makes sure you don’t make them when you set up redundant paths. If there is a loop in a network, it will stop people from being able to talk to each other.

Why is Spanning Tree Protocol important?

The STP is a very useful protocol that is used to get rid of loops in Ethernet switches in LANs. By blocking redundant links across the network, STP stops loops that can cause a network to go down. Also, it makes the backup links work if the main link fails or goes down.

What is Spanning Tree Protocol’s Root Port?

If a Switch has numerous paths to the Root-Bridge, it must designate one path and the port associated with it as the Root-Port.

What are the fundamental rules for Spanning Tree Protocol?

  • However, the switch with the lowest bridge ID will become the Root-Bridge (Priority: MAC Address)
  • Each non-root Switch must have one root-port, which is the port with the lowest path cost to the Root-Bridge.
  • Each port on the Root Bridge is labeled.
  • Each segment must contain one Designated-Port.
  • All root and Designated Ports will be in the forwarding state, while all other ports will be in the blocking state.

What is the Primary difference between STP & RSTP?

RSTP (Rapid Spanning Tree Protocol) is significantly faster than STP. In just seconds, it can complete a convergence. In big networks, STP convergence can take up to a minute or more to finish. During the convergence phase, this can lead to the loss of connectivity between different regions of the network and the subsequent loss of data packets. In a matter of seconds, RSTP enables STP Root Ports and STP Designated Ports to go from blocking to forwarding port state. While STP follows 5 states to prevent network loops, they are:

  • Disabled State
  • Learning state 
  • Listening state
  • Blocking state
  • Forwarding state


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What are the different types Bridge Protocol Data Unit BPDUs in STP?

BPDUs (Bridge Protocol Data Units) are the messages that each switch in the network sends containing data based on this assumption. Because every switch in an Ethernet network initially believes it is the root-bridge, switches send spanning tree protocol configuration messages in the form of BPDUs. These BPDU-messages contain the subsequent information:

  • Root-bridge Identification:- Every switch assumes the role of root bridge at startup, and the initial root bridge ID is the switch’s local bridge ID.
  • Root path Cost:- The optimal cost or path to root-bridge. As each STP switch believes it is the root-bridge, this cost will initially be $0.
  • Max age:- The length of time a BPDU is deemed valid. Standard value is 20 seconds.
  • Sender Bridge ID:- The sender bridge id is the bridge ID of the local switch.
  • Port ID
  • Hello time:- By default, IEEE 802.1D uses 2s.
  • Forwards delay:- The amount of time spent in the listening and learning states, which is 15 seconds by default.

What are the STP Path-Cost values for various bandwidths?

The STP Cost-Value is inversely proportional to the link’s bandwidth, indicating a low-cost path. Various values for various bandwidths are as follows:

Bandwidth STP Cost
10 Mbps 100
45 Mbps 39
100 Mbps 19
622 Mbps 6
1 Gbps 4
10 Gbps 2

Explain STP Port Role Types?

The root port is the port that always has a direct link to the root bridge, or the shortest path to the root bridge. Always resides on Non-Root Bridge.

A port that forwards frames is referred to as a forwarding port.

All of Root Bridge’s ports are Designated Ports, and a designated-port is a port with the lowest cost. A port will be specified as a forwarding port. It can exist on both the Root Bridge and the Non-Root Bridge.

A blocked port is used to prevent loops and does not forward information. It just monitors BPDUs. A Block Port is any port other than the Root port and the designated port.

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