Blog

In which two port states does a switch learn MAC addresses and process BPDUs in a PVST network? (Choose two.)

• blocking
• disabled
• forwarding
• learning
• listening

In the context of a Per-VLAN Spanning Tree (PVST) network, the correct answers to the question of which two port states allow a switch to learn MAC addresses and process Bridge Protocol Data Units (BPDUs) are:

• Forwarding
• Learning

These are two crucial states in the Spanning Tree Protocol (STP) process that contribute significantly to network stability and efficiency. To provide a detailed breakdown of how these states function and their importance in a PVST network, let’s delve into each state in detail.

Understanding PVST and STP

Before diving into the specific port states, it’s important to understand PVST (Per-VLAN Spanning Tree). PVST is an enhancement of the original Spanning Tree Protocol (STP) and is used in Cisco switches. In a traditional STP environment, a single spanning tree instance would be created for the entire network. However, PVST creates a separate spanning tree instance for each VLAN, allowing for more granular control and flexibility.

STP operates to prevent network loops, which are harmful because they can cause broadcast storms, multiple frame transmission, and inconsistent frame forwarding. The algorithm in STP builds a loop-free topology by placing some switch ports into a blocking state and others into forwarding states. The different states a switch port undergoes in an STP environment are Blocking, Listening, Learning, Forwarding, and Disabled.

Port States in STP/PVST

1. Blocking: In this state, the switch port does not forward any data frames and only listens to BPDUs. The port does not learn MAC addresses and exists to prevent loops by ensuring redundant paths are blocked.
2. Disabled: This is an administratively disabled state where the port neither participates in the spanning tree process nor forwards traffic. It does not learn MAC addresses or process BPDUs.
3. Listening: When a port transitions to the listening state, it is preparing to forward traffic. It processes BPDUs but does not yet learn MAC addresses, nor does it forward any user data frames.
4. Learning: In the learning state, the switch starts to learn MAC addresses from the frames it receives but still does not forward them. Processing of BPDUs continues, as the switch must keep receiving topology information to decide whether it will eventually move the port into a forwarding state or keep it blocked.
5. Forwarding: In the forwarding state, the switch port fully participates in the network. It forwards user data frames, learns MAC addresses, and continues processing BPDUs. This is the operational state of a port that allows it to send and receive traffic.

The Learning State

The learning state is crucial for enabling the switch to build its MAC address table. When a port enters the learning state, the switch begins to inspect the source addresses of incoming frames and populates its MAC address table accordingly. The MAC address table helps the switch in determining which ports to forward frames to when they arrive.

In the learning state:

• MAC Address Learning: The switch records the source MAC addresses of incoming frames and maps them to the appropriate ports. This process reduces the number of broadcast frames in the network, as the switch can now directly forward unicast frames to the correct port without flooding all ports.
• No Forwarding of User Data: Although the switch is actively learning MAC addresses, it still does not forward any data traffic that is destined for other devices. The learning state is transitional, allowing the switch to gather information without fully forwarding traffic, ensuring that potential loops do not affect the network during topology changes.
• BPDU Processing: The switch continues to process BPDUs, which is essential for maintaining knowledge of the network topology. Processing BPDUs helps the switch identify root bridges and determine whether the port should eventually transition to a forwarding or blocking state.

The learning state is temporary, typically lasting for 15 seconds by default. Afterward, the port either moves to the forwarding state or back to blocking, depending on the network’s topology and the decisions made by the spanning tree algorithm.

The Forwarding State

The forwarding state is where a switch port becomes fully operational. Once a port enters the forwarding state, it:

• Forwards Data Frames: The port forwards user data traffic. At this point, the port is considered a fully functional part of the network and handles normal traffic for the VLAN or network it is part of.
• Continues to Learn MAC Addresses: The switch keeps updating its MAC address table by learning the source MAC addresses of incoming frames. This learning helps maintain accurate and efficient forwarding decisions, preventing the need for unnecessary broadcasts.
• Processes BPDUs: Even in the forwarding state, the switch continues to process BPDUs. This ensures that the spanning tree algorithm is always aware of changes in the network topology, such as the addition or removal of switches, or changes in path costs. This continuous processing of BPDUs allows the network to remain loop-free even if network topology changes occur.

BPDU Processing in Learning and Forwarding States

Both the learning and forwarding states process BPDUs, but they serve different purposes in each state:

• In the learning state, BPDU processing is important because the switch is still determining whether it should transition into forwarding or blocking. It collects information about the root bridge, designated ports, and path costs to ensure that it makes the right decision for maintaining a loop-free topology.
• In the forwarding state, BPDU processing continues so that the switch remains up-to-date with changes in the network. If another switch becomes the root bridge or if a topology change occurs, the forwarding switch can react accordingly by adjusting its port states.

Importance of These States in Network Stability

The transition between the learning and forwarding states is fundamental to network stability. A switch doesn’t immediately move from blocking or listening to forwarding; it must first pass through the learning state to ensure that when it finally begins forwarding traffic, it has the necessary information to prevent loops and optimize traffic flow.

Without the learning state, the switch wouldn’t know the correct port to forward traffic, leading to unnecessary flooding. On the other hand, without the forwarding state, the switch couldn’t contribute to normal network operations, as it would never forward traffic.

Together, the learning and forwarding states allow a PVST network to operate efficiently while maintaining loop prevention through STP. The careful balance between these states ensures that only the necessary ports forward traffic, while others are kept in a blocking state to prevent loops.

Conclusion

In summary, the learning and forwarding states in a PVST network are critical for enabling a switch to learn MAC addresses and process BPDUs. The learning state allows the switch to gather the necessary information about network topology without forwarding data frames, while the forwarding state fully activates the port, allowing it to forward traffic and continue processing BPDUs. These states ensure both efficiency and stability in the network, preventing loops and maintaining optimal traffic flow.

By passing through these states, the Spanning Tree Protocol ensures that only the appropriate ports forward traffic, while others remain in blocking to prevent network disruptions. This detailed and systematic approach is fundamental to the robustness of modern Ethernet networks, especially in complex environments with multiple VLANs and switches.