📘 CCNA 200-301 v1.1
2.5 Interpret basic operations of Rapid PVST+ Spanning Tree Protocol
2.5.b Port states and roles
What is Rapid PVST+ (Rapid Per VLAN Spanning Tree Plus)
Rapid PVST+ is Cisco’s version of Rapid Spanning Tree Protocol (RSTP – IEEE 802.1w).
It is an improvement over the older STP (IEEE 802.1D) and provides faster convergence when there is a change in the network topology (for example, a switch or link fails).
Key points:
- Each VLAN runs its own instance of Spanning Tree (that’s why it’s called Per VLAN).
- It prevents loops in Layer 2 networks.
- It allows faster transition of ports to the forwarding state compared to the original STP.
🧩 Why Port States and Roles Matter
In Rapid PVST+, each port on a switch plays a specific role and can be in a specific state depending on:
- Where the Root Bridge is located.
- The path cost to reach the Root Bridge.
- The connection type (point-to-point or shared).
This ensures only the best paths are active and loops are avoided.
🔹 PORT ROLES in Rapid PVST+
A port role defines the function of the port in the spanning tree.
There are four main port roles in Rapid PVST+:
| Port Role | Description | Active or Blocking Traffic |
|---|---|---|
| Root Port (RP) | The port that provides the best path to the Root Bridge. | Forwarding |
| Designated Port (DP) | The port on a network segment that sends frames toward the Root Bridge. It is the best port on that segment. | Forwarding |
| Alternate Port (AP) | A backup port that provides an alternate path to the Root Bridge if the current Root Port fails. | Discarding |
| Backup Port (BP) | A redundant port on the same segment as the Designated Port. It is a backup to the Designated Port on the same switch. | Discarding |
Let’s break them down more clearly:
1. Root Port (RP)
- Found only on non-root bridges (because the Root Bridge does not have a Root Port).
- It is the port with the lowest cost path to reach the Root Bridge.
- There is only one Root Port per switch.
- State: Forwarding (sending and receiving user traffic).
Example in IT environment:
In an access switch connected to the distribution switch (which is the Root Bridge), the uplink port toward that distribution switch will be the Root Port.
2. Designated Port (DP)
- The best port on a segment to send traffic toward the Root Bridge.
- Every network segment must have one Designated Port.
- State: Forwarding.
- There can be multiple Designated Ports on a switch (one per segment it connects to).
Example in IT environment:
If a distribution switch connects to multiple access switches, each interface going toward those access switches is likely a Designated Port because it provides the best path toward the Root Bridge.
3. Alternate Port (AP)
- Provides a backup path to the Root Bridge in case the Root Port fails.
- It listens to network traffic but does not forward it.
- State: Discarding (not sending user traffic).
- Can immediately move to Forwarding if the Root Port goes down — this gives fast convergence.
Example in IT environment:
If an access switch has two uplinks — one active (Root Port) and one backup — the backup link is an Alternate Port.
4. Backup Port (BP)
- A redundant port on the same segment as another Designated Port (usually within the same switch).
- Rarely seen in modern networks because most modern switch connections are point-to-point, not shared segments.
- State: Discarding.
Example in IT environment:
If two ports of the same switch are connected to the same hub or shared segment, one becomes Designated and the other becomes Backup.
🔸 PORT STATES in Rapid PVST+
A port state defines what the port is doing at any moment — whether it’s blocking, learning, or forwarding traffic.
In Rapid PVST+, there are three main port states:
| Port State | Description | Does it forward user traffic? | Does it learn MAC addresses? |
|---|---|---|---|
| Discarding | Port does not forward user traffic or learn MAC addresses. It only listens to BPDUs. | ❌ No | ❌ No |
| Learning | Port learns MAC addresses but does not yet forward user traffic. | ❌ No | ✅ Yes |
| Forwarding | Port forwards user traffic and learns MAC addresses. | ✅ Yes | ✅ Yes |
Let’s explain them more clearly:
1. Discarding State
- The port is blocked to prevent network loops.
- The port can receive and process BPDUs (Bridge Protocol Data Units).
- It does not learn or forward user data.
- Used by Alternate and Backup ports.
2. Learning State
- The port begins to learn MAC addresses from incoming frames.
- This builds the switch’s MAC address table before forwarding starts.
- It still does not forward user traffic.
- It helps the switch prepare to forward efficiently once it becomes active.
3. Forwarding State
- The port is now active in the network.
- It sends and receives user data frames.
- It also learns MAC addresses from the traffic.
🔹 Relationship Between Roles and States
| Port Role | Typical Port State | Description |
|---|---|---|
| Root Port | Forwarding | Active path toward the Root Bridge |
| Designated Port | Forwarding | Active path for segment |
| Alternate Port | Discarding | Backup path to Root Bridge |
| Backup Port | Discarding | Backup for Designated Port |
⚡ How Rapid PVST+ Improves Over Classic STP
| Feature | STP (802.1D) | Rapid PVST+ (802.1w) |
|---|---|---|
| Convergence time | ~30-50 seconds | ~1-2 seconds |
| Port states | Blocking, Listening, Learning, Forwarding | Discarding, Learning, Forwarding |
| Port roles | Root, Designated, Non-designated | Root, Designated, Alternate, Backup |
| BPDU handling | Switches rely on Root sending BPDUs | All switches send BPDUs (faster detection) |
🧠 Summary Table (For Quick Exam Revision)
| Concept | Explanation |
|---|---|
| Root Port | Best path toward Root Bridge, forwarding traffic |
| Designated Port | Best port on each segment to reach Root Bridge |
| Alternate Port | Backup port to Root Port, discarding |
| Backup Port | Backup port to Designated Port, discarding |
| Discarding State | Not learning or forwarding, only receiving BPDUs |
| Learning State | Learns MACs, no user traffic yet |
| Forwarding State | Fully active, sends and receives data |
| Convergence Speed | Rapid PVST+ converges much faster than STP |
| Per VLAN Instance | Each VLAN has its own Rapid Spanning Tree instance |
✅ In short for the CCNA exam:
Be clear that Root and Designated ports forward, while Alternate and Backup ports discard.
Remember the 4 port roles and 3 port states.
Understand which role corresponds to which state.
Know that Rapid PVST+ = Cisco’s RSTP per VLAN and gives fast convergence.
