What problem does STP solve?

Ethernet Problems Without STP

Ethernet has no TTL and no loop detection, so loops cause:

1. Broadcast storms

2. MAC address table instability

3. Duplicate frames

CCNA Key Point

➡ STP prevents Layer 2 loops by blocking redundant paths

🔴 The problem: Network loops

If you connect switches with multiple paths, Ethernet doesn’t know when to stop sending frames → infinite loop → broadcast storm → network down.

🧠 Real-life analogy

Imagine 3 roads in a circle. A car enters and keeps driving forever because there’s no “STOP” sign.

🟢 What STP does

STP:

• Detects loops

• Blocks some paths

• Keeps one safe path

• Automatically unblocks if a link fails

👉 CCNA focuses mainly on 802.1D and 802.1w

Switch A ---- Switch B

\ /

\ /

Switch C

Without STP → 🔥 LOOPWith STP → STP blocks one link to form a tree (no circle).

🌲 How STP Works (Very Simple)

STP does 3 main things:

1. Elects a Root Switch

2. Chooses best paths to the root

3. Blocks extra paths

🏆 Root Bridge

• The boss switch

• Chosen by lowest Bridge ID

  • Bridge ID = Priority + MAC Address

Default priority = 32768

Lower = better

🧠 Real-life

Like a team leader everyone listens to.

📝 Exercise 1

Switch priorities:

• SW1 → 32768

• SW2 → 24576

• SW3 → 32768

👉 Which is Root Bridge?

✅ Answer: SW2 (lowest priority)

🧠 Real-life

• Root Port = fastest road to the city center

• Blocked Port = road closed with a barrier 🚧

Traditional STP states:

⏱ STP Timers (Very Important)

⏳ Total convergence time = 50 seconds 😴

📝 Exercise 2

A port goes from blocking to forwarding. How long does it take?

✅ Answer: 15s (Listening) + 15s (Learning) = 30 seconds

Bridge ID Components:

Bridge ID = Priority (16 bits) + MAC Address (48 bits)

Priority:

• Default = 32768

• Increments of 4096

• VLAN ID is added internally in PVST+

Root Bridge Rules:

1. Lowest priority wins

2. If tie → lowest MAC address

📌 Exam Tip

You never want access switches to become root.

📝 Exercise 1 (Exam Style)

Switches:

• SW1 → Priority 32768, MAC 0011.1111.1111

• SW2 → Priority 32768, MAC 0001.2222.2222

• SW3 → Priority 4096, MAC 0033.3333.3333

👉 Root Bridge?

✅ Answer: SW3 (lowest priority)

Path Cost = Speed-based

Root Path Cost:

➡ Sum of costs to reach root

🧠 Real Scenario

A switch with:

• FastEthernet path = cost 19

• Gigabit path = cost 4

👉 Gigabit is preferred

📝 Exercise 2

Which path is chosen?

• Path A: 100 Mbps + 100 Mbps = 38

• Path B: 1 Gbps = 4

✅ Answer: Path B

Rules:

• Each non-root switch has one Root Port

• Each segment has one Designated Port

STP Timers (Exam Critical)

Convergence Time:

20 + 15 + 15 = 50 seconds

📝 Exercise 3

How long until traffic flows after a link failure?

✅ Answer: 50 seconds

BPDU Purpose:

• Root election

• Path cost calculation

• Topology change detection

BPDU Fields (Know these):

• Root Bridge ID

• Sender Bridge ID

• Root Path Cost

• Port ID

• Timers

Who sends BPDUs?

• Root → every 2 seconds

• Others → relay BPDUs

What PortFast Does:

• Bypasses Listening & Learning

• Goes immediately to forwarding

Where to Use:

• End-device ports ONLY

Cisco Default:

spanning-tree portfast

⚠ PortFast does NOT disable STP

Exam Trap ❗

PortFast can cause loops if misused.

What BPDU Guard Does:

• Shuts port if BPDU is received

Why?

Protects PortFast ports from:

• Rogue switches

• Accidental loops

Result:

➡ Port goes to err-disabled

Configuration:

spanning-tree portfast bpduguard default

📝 Exercise 4

What happens if a switch is plugged into a BPDU Guard port?

✅ Answer: Port shuts down (err-disabled)