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

5.3 Given a scenario, troubleshoot common issues with network services

Switching Issues

📘CompTIA Network+ (N10-009)

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.

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.

b. Port Roles

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

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.
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.
Blocked Port
- Ports that do not forward traffic to prevent loops.
- They can become active if the network topology changes.

c. Port States

STP ports go through several states before forwarding traffic:

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

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

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.

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.

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.

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).

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.

4. Troubleshooting STP Issues

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

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

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.