In the previous lesson on Spanning Tree Port Roles, we assigned Root Ports, Designated Ports, and Alternate (Blocked) Ports to keep our network loop-free.
However, Spanning Tree Protocol (STP) does more than just assign roles: it also controls how the network converges when a switch fails in the topology.Convergence refers to the process of ensuring all switches agree on a new Spanning Tree topology and can forward traffic effectively after a change, such as a link failure or the addition of a new device.
Figure 1 – STP Port Roles and Topology Change Scenario
This process relies on STP Timers and STP Port States, which together dictate how long it takes for the network to adapt to changes while preventing loops during the transition.
Convergence ensures the following:
Stable Topology: All switches agree on the Root Bridge and the port roles for each segment.
Efficient Traffic Flow: Traffic resumes efficiently after a topology change, avoiding loops.
Minimal Disruption: The network adapts without causing packet loss or instability.
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Before diving into the Port States, ensure you understand the role of STP Timers, as they govern the duration and transition between states.
Hello Timer
Purpose: Determines how often the Root Bridge sends BPDUs (Bridge Protocol Data Units) to communicate topology information.
Default Value: 2 seconds.
Significance: Once the Root Bridge is elected, it is the only switch that generates and sends BPDUs to the entire Spanning Tree topology. These BPDUs serve to announce the Root Bridge’s presence and maintain the topology.
Figure 2 – Hello Timer and BPDU Forwarding in STP
The non-root switches in the network do not generate their own BPDUs. Instead, they forward the BPDUs received from the Root Bridge to their neighbors, including their own sender Bridge ID, ensuring the topology remains synchronized.
Verifying STP Timers
We can confirm the timer values directly from the Root Bridge:
SW1# show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address aabb.ccdd.ee01 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address aabb.ccdd.ee01 Hello Time 2 sec Max Age 20 sec Forward Delay 15 secThis output confirms:
Hello Time = 2 sec
Max Age = 20 sec
Forward Delay = 15 sec
These are the default IEEE 802.1D timer values.
Forward Delay Timer
To fully grasp the concept of STP port states, it is important to understand this timer.
Purpose: Controls the time a port spends in the Listening and Learning states before transitioning to Forwarding.
Default Value: 15 seconds per state.
This means:
15 seconds in Listening
15 seconds in Learning
Total transition time = 30 seconds
This delay prevents loops during topology changes.
Max Age Timer
Purpose: Defines how long a switch retains BPDU information from the Root Bridge before considering it invalid.
Default Value: 20 seconds.
Significance: Ensures switches detect and react to topology changes (for example, when the Root Bridge crashes).
If a switch does not receive BPDUs for 20 seconds, it assumes the Root Bridge is no longer active and starts a new election process.
Answer the question below
What is the default Hello Timer in STP (seconds)?
To explain the port states, let’s use an example scenario where the Root Bridge fails, triggering a topology change.
Initial State: Stable Topology
The Root Bridge sends BPDUs every 2 seconds to maintain the topology.
Non-root switches forward these BPDUs while retaining their roles (Root Port, Designated Port, and Blocked Port).
Figure 3 – Stable Topology Before a Topology Change Event
Step 1: Root Bridge Failure
The Root Bridge stops sending BPDUs.
Figure 4 – Root Bridge Failure
Right after the failure, SW2 still believes SW1 is the Root Bridge.
That’s because STP keeps the last known Root information until the Max Age timer (20 seconds) expires.SW2# show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address aabb.ccdd.ee01 Cost 4 Port 1 (GigabitEthernet0/0) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address aabb.ccdd.ee02 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Gi0/0 Root FWD 4 128.1 P2p Gi0/1 Desg FWD 4 128.2 P2pSwitches wait for the Max Age Timer (20 seconds) to expire.
During this time, ports remain in their current states.
Figure 5 – Max Age Timer Expiry Delay in STP
Step 2: Transition for Root and Designated Ports
After the Max Age Timer expires, ports assigned as Root Ports and Designated Ports transition to the Listening State to reevaluate the topology.
Figure 6 – Spanning Tree Port State Transition Process
Ports in the Blocking State remain blocked unless a topology change requires them to become active.
Step 3: Listening State
In this state:
Ports exchange BPDUs to elect a new Root Bridge and reevaluate port roles.
No data traffic is forwarded to prevent loops.
Figure 7 – STP Listening State
Duration: 15 seconds (Forward Delay Timer).
SW2# show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address aabb.ccdd.ee02 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address aabb.ccdd.ee02 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Gi0/0 Desg LIS 4 128.1 P2p Gi0/1 Desg LIS 4 128.2 P2pStep 4: Learning State
Ports process incoming Ethernet frames to update the MAC address table.
No traffic is forwarded yet.
Figure 8 – STP Learning State
Duration: 15 seconds (Forward Delay Timer).
SW2# show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address aabb.ccdd.ee02 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address aabb.ccdd.ee02 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Gi0/0 Desg LRN 4 128.1 P2p Gi0/1 Desg LRN 4 128.2 P2pStep 5: Forwarding State
Root Ports and Designated Ports transition to the Forwarding State.
Figure 9 – Final Port States: Forwarding or Blocking
Ports actively forward traffic and participate in BPDU exchanges.
SW2# show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address aabb.ccdd.ee02 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address aabb.ccdd.ee02 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Gi0/0 Desg FWD 4 128.1 P2p Gi0/1 Desg FWD 4 128.2 P2pStep 6: Blocking State (if applicable)
Ports that are not selected as Root or Designated Ports remain in the Blocking State to prevent loops.
These ports continue to listen for BPDUs and may transition to active states if the topology changes again.
Answer the question below
In which STP state are MAC addresses learned but traffic not forwarded?
Answer the question below
After SW1 dies, the Listening-state output shows Root ID aabb.ccdd.ee02: which switch won the new election?
Answer the question below
Gi0/0 on SW2 was the Root port before the crash: which Role does it show once both ports are back in FWD?
Blocking: Prevents loops by dropping traffic while listening for BPDUs. No data traffic is forwarded, and the MAC address table is not updated.
Listening: Exchanges BPDUs to determine the Root Bridge and port roles. No data traffic is forwarded, and the MAC address table is not updated.
Learning: Learns MAC addresses to populate the table. No traffic is forwarded yet, but BPDUs are exchanged.
Forwarding: Actively forwards traffic and exchanges BPDUs. The MAC address table is updated in real time.
Disabled: The port is administratively down or not part of STP. It does not send, receive, or process any frames or BPDUs.
State
BPDU Handling
MAC Address Table Updates
Forwards Traffic?
Blocking
Listens for BPDUs
❌
❌
Listening
Exchanges BPDUs
❌
❌
Learning
Exchanges BPDUs
✅
❌
Forwarding
Exchanges BPDUs
✅
✅
Disabled
Does not process BPDUs
❌
❌
Table 1 – Overview of STP Port States
Answer the question below
Which STP state actively forwards traffic?
The convergence time of 50 seconds in STP is determined by the following timers:
Max Age Timer (20 seconds): A switch waits for this duration after losing BPDU information to ensure the Root Bridge is no longer active.
Forward Delay Timer (30 seconds): Composed of two 15-second intervals, during which ports transition through the Listening and Learning states to prevent forwarding loops.
Figure 10 – STP Convergence Timeline (50 Seconds)
While these timers ensure a stable topology, the lengthy process introduces significant delays in adapting to changes. For modern networks with real-time applications like VoIP, video streaming, or online gaming, such delays can result in service disruptions and performance degradation.
The Problem with Slow Convergence
In today’s networks, downtime and packet loss caused by STP’s slow convergence can lead to:
Increased Latency: Critical traffic is delayed, impacting time-sensitive applications.
Network Instability: Prolonged transition periods increase the risk of temporary loops and packet flooding.
User Dissatisfaction: End users experience service interruptions, leading to frustration and potential loss of trust.
How RSTP Solves the Issue
Rapid Spanning Tree Protocol (RSTP) addresses the limitations of STP by significantly reducing convergence times to just a few seconds.
In the following lesson, we’ll dive into RSTP, exploring its mechanisms and benefits for modern networks.
Answer the question below
What is the total convergence time in classic STP (seconds)?