The Lazy Networker

STP operates by transitioning ports through different states to avoid loops while still allowing network connectivity. Each port in STP can be in one of the following states:

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1️⃣ Disabled State

🔴 Not part of STP operation

• The port is administratively shut down or physically disconnected.
• No BPDU (Bridge Protocol Data Unit) frames are sent or received.

🔹 Command to Disable a Port:

Switch(config)# interface GigabitEthernet0/1
Switch(config-if)# shutdown

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2️⃣ Blocking State

🟡 Prevents network loops

• The port listens for BPDUs but does NOT forward traffic.
• The switch does not learn MAC addresses on this port.
• Used for backup links that will only be activated if needed.

🔹 When does a port stay in Blocking?

• When a redundant link exists and a better path is available.
• If the port receives a superior BPDU from another switch.

🔹 Verification Command:

Switch# show spanning-tree

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3️⃣ Listening State (15 sec, STP only)

🟠 Preparing to forward

• The port is evaluating BPDUs to determine its role (Root, Designated, or Blocking).
• The port is NOT forwarding frames yet.

🔹 Why does it exist?

• To prevent temporary loops before full STP convergence.

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4️⃣ Learning State (15 sec, STP only)

🔵 Learning MAC addresses

• The switch starts learning MAC addresses but does NOT forward traffic yet.
• The port continues to process BPDUs.

🔹 Why does it exist?

• Helps build the MAC address table before forwarding starts.

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5️⃣ Forwarding State

🟢 Port is fully operational

• The port forwards traffic normally.
• It learns MAC addresses and processes BPDUs.

🔹 Conditions for Forwarding:

• The port is a Root Port (RP) or Designated Port (DP).
• No superior BPDUs are received.

🔹 Verification Command:

Switch# show spanning-tree interface GigabitEthernet0/1

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Port State Transition Diagram (STP – 802.1D)

   Blocking → Listening → Learning → Forwarding
         |                                  ↑
         |——> (If better path found) ——→ |

🚨 Total Convergence Time in Classic STP: ~50 sec

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STP vs RSTP Port States

RSTP (802.1w) removes Listening State and introduces new roles:

STP (802.1D)	RSTP (802.1w)	Function
Blocking	Discarding	Blocks traffic, prevents loops
Listening	(Removed)	Not needed in RSTP
Learning	Learning	Learns MAC addresses
Forwarding	Forwarding	Normal data forwarding

🚀 RSTP Converges in <6 seconds!