STP Interview Questions 1

Question 1

What is STP?

Answer 1

STP is a protocol that prevents layer 2 loops by electing a Root Bridge, Root Ports, Designated ports, and Blocking ports.

Question 2

How STP works?

Answer 2

STP chooses a Root Bridge. Then it picks one path by which to forward frames and blocks other redundant paths.

Question 3

What are the different port states?

Answer 3

1. Disabled - A port in the disabled state does not participate in the STP.
2. Blocking - A blocked port does not forward frames. It only listens to BPDUs.
3. Listening - The port sends and listens to BPDUs to ensure no loops occur on the network.
4. Learning - A port in the learning state populates the MAC address table but doesn’t forward data frames. The port still sends and receives BPDUs as before.
5. Forwarding - The port can now send and receive data frames, collect MAC addresses in its address table, and send and receive BPDUs.

Question 4

Explain types of STP Port Roles.

Answer 4

1. Root port - The root port is always the link directly connected to the root bridge or the shortest path to the root bridge.
2. Designated port - A designated port will be marked as a forwarding port. It can be on both the Root Bridge & Non-Root Bridge. All ports of Root Bridge are Designated ports.
3. Forwarding port - A forwarding port forwards frames.
4. Blocked port - A blocked port is the port that is used to prevent loops. It only listens to frames.

Question 5

What is the STP blocking state?

Answer 5

When a switch starts, all ports are in the blocking state to prevent any loop in the network.