1.What is STP & Redundant Links?
Spanning Tree Protocol (STP) is a protocol which prevents layer 2 loops. STP enables switches to
become aware of each other so that they can negotiate a Loop-Free path through network. In practical Scenario, Redundant links are created to avoid complete network failure in an event of failure of one link.
2.How STP works?
STP chooses a Reference point (Root Bridge) in the network and calculates all the redundant
paths to that reference point. Than it picks one path by which to forward frames and blocks other redundant paths.
3.What are the different port states?
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. The purpose of the
blocking state is to prevent the use of looped paths.
3. Listening - A port in listening state prepares to forward data frames without populating the MAC
address table. The port also sends and listens to BPDUs to make sure no loops occur on the network.
4. Learning - A port in 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 now can send and receive data frames, collect MAC addresses in its
address table, send and receive BPDUs. The port is now a fully functioning switch port within the
spanning-tree topology.
4.What is the default time a port takes to transition from the blocking state to the forwarding state?
The default time a port takes to transition from the blocking state to the forwarding state is 50
seconds: 20 seconds for Max Age, 15 seconds for listening, and 15 seconds for learning.
5.What are STP Timers and Explain different types of STP Timers?
STP uses three timers to make sure that a network converges properly before a bridging loop can
form.
1. Hello - The time interval between Configuration BPDUs sent by the root bridge. It is 2 seconds
by default.
2. Forward Delay - The time interval that a switch port spends in both the Listening and Learning
states. The default value is 15 seconds.
3. Max Age - Maximum length of time, a BPDU can be stored without receiving an update. It can
also be define as a time interval that a switch stores a BPDU before discarding it. It is 20 seconds by
default.
6.Explain types of STP Port Roles?
1. Root port - The root port is always the link directly connected to the root bridge, or the shortest
path to the root bridge. It is always on Non-Root Bridge.
2. Designated port - A designated port is one that has been determined as having the best (lowest)
cost. A designated port will be marked as a forwarding port. It can be on both Root Bridge & Non Root Bridge. All ports of Root Bridge are Designated Port.
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.
Any port other than Root port & Designated port is Block Port.
7.What is the STP blocking state?
When a switch starts, all ports are in the blocking state to prevent any loop in the network. If there is
a better path to the root bridge, the port remains in the blocked state. Ports in the blocked state cannot send or receive traffic, but they can receive BPDUs.
8.What is BPDU?
All the switches exchange information to select Root Bridge as well as for configuration of the network. This is done through Bridge Protocol Data Unit (BPDU). Each switch compares the
parameters in the BPDU that it sends to one neighbor with the one that it receives from another neighbor.
9.How often do Bridges send BPDUs on active ports?
The default time that bridges send BPDUs is 2 seconds.
10.What is the destination MAC address used by Bridge Protocol Data Units (BPDUs)?
Bridge Protocol Data Units (BPDUs) frames are sent out as at multicast destination MAC address
01:80:c2:00:00:00.
Read More Questions:
STP Interview Questions Part1
STP Interview Questions Part2
STP Interview Questions Part3
Spanning Tree Protocol (STP) is a protocol which prevents layer 2 loops. STP enables switches to
become aware of each other so that they can negotiate a Loop-Free path through network. In practical Scenario, Redundant links are created to avoid complete network failure in an event of failure of one link.
2.How STP works?
STP chooses a Reference point (Root Bridge) in the network and calculates all the redundant
paths to that reference point. Than it picks one path by which to forward frames and blocks other redundant paths.
3.What are the different port states?
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. The purpose of the
blocking state is to prevent the use of looped paths.
3. Listening - A port in listening state prepares to forward data frames without populating the MAC
address table. The port also sends and listens to BPDUs to make sure no loops occur on the network.
4. Learning - A port in 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 now can send and receive data frames, collect MAC addresses in its
address table, send and receive BPDUs. The port is now a fully functioning switch port within the
spanning-tree topology.
4.What is the default time a port takes to transition from the blocking state to the forwarding state?
The default time a port takes to transition from the blocking state to the forwarding state is 50
seconds: 20 seconds for Max Age, 15 seconds for listening, and 15 seconds for learning.
5.What are STP Timers and Explain different types of STP Timers?
STP uses three timers to make sure that a network converges properly before a bridging loop can
form.
1. Hello - The time interval between Configuration BPDUs sent by the root bridge. It is 2 seconds
by default.
2. Forward Delay - The time interval that a switch port spends in both the Listening and Learning
states. The default value is 15 seconds.
3. Max Age - Maximum length of time, a BPDU can be stored without receiving an update. It can
also be define as a time interval that a switch stores a BPDU before discarding it. It is 20 seconds by
default.
6.Explain types of STP Port Roles?
1. Root port - The root port is always the link directly connected to the root bridge, or the shortest
path to the root bridge. It is always on Non-Root Bridge.
2. Designated port - A designated port is one that has been determined as having the best (lowest)
cost. A designated port will be marked as a forwarding port. It can be on both Root Bridge & Non Root Bridge. All ports of Root Bridge are Designated Port.
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.
Any port other than Root port & Designated port is Block Port.
7.What is the STP blocking state?
When a switch starts, all ports are in the blocking state to prevent any loop in the network. If there is
a better path to the root bridge, the port remains in the blocked state. Ports in the blocked state cannot send or receive traffic, but they can receive BPDUs.
8.What is BPDU?
All the switches exchange information to select Root Bridge as well as for configuration of the network. This is done through Bridge Protocol Data Unit (BPDU). Each switch compares the
parameters in the BPDU that it sends to one neighbor with the one that it receives from another neighbor.
9.How often do Bridges send BPDUs on active ports?
The default time that bridges send BPDUs is 2 seconds.
10.What is the destination MAC address used by Bridge Protocol Data Units (BPDUs)?
Bridge Protocol Data Units (BPDUs) frames are sent out as at multicast destination MAC address
01:80:c2:00:00:00.
Read More Questions:
STP Interview Questions Part1
STP Interview Questions Part2
STP Interview Questions Part3
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