STP (Loops, Root bridge, Port roles/states)

5.3 Given a scenario, troubleshoot common issues with network services

Switching Issues

📘CompTIA Network+ (N10-009)

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1. Why STP Exists

In networks, switches often have redundant paths. Redundancy is good for reliability, but without STP, these redundant paths can create loops:

• A loop happens when a frame keeps circulating between switches endlessly.
• Loops can flood the network, cause broadcast storms, and bring the network down.

STP solves this by logically blocking some paths while keeping a backup path ready.

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2. STP Terminology You Must Know

a. Root Bridge

• The root bridge is the most important switch in the STP topology.
• It acts as the reference point for all path calculations.
• All switches decide which ports to block or forward based on their distance to the root bridge.

How it is chosen:

• Each switch has a Bridge ID (BID), which is made of:
  • Bridge Priority (default is 32768)
  • MAC Address of the switch
• The switch with the lowest Bridge ID becomes the root bridge.
• If you want a specific switch to be the root, lower its priority.

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b. Port Roles

Each port on a switch in STP has a role based on how it connects to the network:

1. Root Port (RP)
   • The port on a non-root switch closest to the root bridge.
   • Always in forwarding state.
   • Each non-root switch has only one root port.
2. Designated Port (DP)
   • The port on a segment that has the lowest path cost to the root bridge.
   • It forwards traffic for that segment.
   • Every segment has one designated port.
3. Blocked Port
   • Ports that do not forward traffic to prevent loops.
   • They can become active if the network topology changes.

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c. Port States

STP ports go through several states before forwarding traffic:

1. Blocking – Port does not forward traffic. Receives BPDU messages only.
2. Listening – Port listens to BPDUs to determine if it should forward.
3. Learning – Port starts learning MAC addresses but does not forward frames.
4. Forwarding – Port forwards frames and continues learning MAC addresses.
5. Disabled – Port is administratively shut down and does nothing.

Tip: For the exam, remember: Blocking → Listening → Learning → Forwarding.

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d. BPDUs (Bridge Protocol Data Units)

• Switches exchange BPDUs to share STP information.
• BPDUs help elect the root bridge, determine port roles, and detect network changes.

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3. Common STP Issues

a. Loops

• A loop occurs when multiple active paths exist between switches without blocking.
• Symptoms:
  • High CPU load on switches
  • Network flooding
  • Slow network response or outages
• Cause: STP failed or misconfigured, or redundant links without STP.

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b. Incorrect Root Bridge

• If the wrong switch becomes the root bridge:
  • Traffic may take suboptimal paths.
  • Some links might block unnecessarily, reducing network performance.
• Solution:
  • Adjust bridge priorities to set the correct switch as root.

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c. Port Role/State Issues

• A port might be incorrectly blocked or forwarding due to misconfiguration.
• Example: Connecting a new switch can cause a port to temporarily block, which is normal as STP recalculates the topology.
• Solution:
  • Check STP status (show spanning-tree on Cisco devices).

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d. Convergence Delay

• STP takes some time to move ports from blocking to forwarding.
• Traditional STP can take up to 30-50 seconds to converge.
• Faster alternatives: RSTP (Rapid Spanning Tree Protocol), which converges in 1-5 seconds.

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4. Troubleshooting STP Issues

For the CompTIA Network+ exam, you should know how to identify and fix STP problems:

1. Identify loops
   • Symptoms: broadcast storms, high CPU, repeated frames.
   • Solution: Check STP topology, blocked ports, root bridge.
2. Check root bridge
   • Ensure the most appropriate switch is the root.
   • Use commands like: show spanning-tree
   • Adjust bridge priorities if needed.
3. Check port roles/states
   • Ensure proper ports are forwarding or blocked.
   • Verify the network segments have one designated port each.
4. Upgrade to RSTP if needed
   • Reduces downtime caused by STP recalculation.

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5. Key Exam Tips

• STP prevents loops in switched networks.
• Root bridge = reference point for the topology.
• Port roles: Root Port, Designated Port, Blocked Port.
• Port states: Blocking → Listening → Learning → Forwarding.
• Check BPDUs for troubleshooting.
• Know common problems: loops, wrong root bridge, port misconfigurations, convergence delays.

Focus on being able to explain the problem, identify the cause, and recommend a solution — CompTIA often asks scenario-based questions.