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Chapter 20 — Spanning Tree and Layer 2 Redundancy

A complete networking lesson based on the course chapter menu.

20 topicsPractical examplesTroubleshooting checksReview Q&A

Estimated reading time0% read

Chapter 20: Spanning Tree and Layer 2 Redundancy

This chapter follows the topics shown in the Networking chapter menu. Work through each section in order, then use the review questions to check recall and troubleshooting reasoning.

20.1 Why Switching Loops Occur

Why Switching Loops Occur is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Why Switching Loops Occur in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Why Switching Loops Occur without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.2 Broadcast Storms

Broadcast Storms concerns traffic delivered to every member of a Layer 2 or IP broadcast scope. Broadcast behavior matters because excessive or unintended broadcasts can consume shared capacity and reveal segmentation problems.

Example: place Broadcast Storms in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Broadcast Storms without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.3 MAC Table Instability

MAC Table Instability is a Layer 2 concept involving Ethernet hardware addressing or switch forwarding state. Switches learn source MAC addresses and use destination MAC information to choose whether to forward, flood, or filter frames.

Example: place MAC Table Instability in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain MAC Table Instability without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.4 STP Overview

STP prevents persistent Ethernet loops by placing selected redundant paths into non-forwarding states and recalculating after topology changes.

Example: place STP Overview in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain STP Overview without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.5 Root Bridge

Root Bridge is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Root Bridge in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Root Bridge without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.6 Bridge ID

Bridge ID is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Bridge ID in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Bridge ID without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.7 Path Cost

PAT lets many inside hosts share one or a small number of public IPv4 addresses by tracking transport protocol and port mappings.

Example: place Path Cost in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Path Cost without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.8 Root Port

Root Port is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Root Port in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Root Port without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.9 Designated Port

NAT changes IP address information as traffic crosses a translation boundary. PAT is a many-to-one form that also distinguishes conversations by transport-layer port numbers.

Example: place Designated Port in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Designated Port without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.10 Blocking/Discarding

Blocking/Discarding is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Blocking/Discarding in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Blocking/Discarding without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.11 BPDUs

BPDUs is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place BPDUs in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain BPDUs without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.12 Topology Changes

Topology Changes is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Topology Changes in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Topology Changes without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.13 RSTP Concepts

STP prevents persistent Ethernet loops by placing selected redundant paths into non-forwarding states and recalculating after topology changes.

Example: place RSTP Concepts in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain RSTP Concepts without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.14 PortFast/Edge Port Concepts

PortFast/Edge Port Concepts is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place PortFast/Edge Port Concepts in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain PortFast/Edge Port Concepts without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.15 BPDU Guard

BPDU Guard is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place BPDU Guard in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain BPDU Guard without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.16 Loop Protection

Loop Protection is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Loop Protection in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Loop Protection without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.17 STP Troubleshooting

STP prevents persistent Ethernet loops by placing selected redundant paths into non-forwarding states and recalculating after topology changes.

Example: place STP Troubleshooting in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
Compare actual configuration and measurements with the intended design, baseline, or documentation.
Change one variable at a time, verify the result, and document both the cause and the final fix.

Practice: Practice: explain STP Troubleshooting without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.18 Redundant Links

Redundant Links is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Redundant Links in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Identify which OSI/TCP-IP layer and device type are primarily responsible.
Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.

Practice: Practice: explain Redundant Links without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.19 Layer 2 Failure Scenarios

Layer 2 Failure Scenarios is a troubleshooting condition. The useful approach is to confirm symptoms, determine scope, identify the relevant layer, compare actual values with the intended design, test one theory at a time, and verify service after the fix.

Example: place Layer 2 Failure Scenarios in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
Compare actual configuration and measurements with the intended design, baseline, or documentation.
Change one variable at a time, verify the result, and document both the cause and the final fix.

Practice: Practice: explain Layer 2 Failure Scenarios without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

20.20 Broadcast Storm Troubleshooting

Broadcast Storm Troubleshooting concerns traffic delivered to every member of a Layer 2 or IP broadcast scope. Broadcast behavior matters because excessive or unintended broadcasts can consume shared capacity and reveal segmentation problems.

Example: place Broadcast Storm Troubleshooting in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
Compare actual configuration and measurements with the intended design, baseline, or documentation.
Change one variable at a time, verify the result, and document both the cause and the final fix.

Practice: Practice: explain Broadcast Storm Troubleshooting without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

Chapter 20 Review Questions

1. What should you remember about Why Switching Loops Occur?

Answer: Why Switching Loops Occur is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

2. What should you remember about MAC Table Instability?

Answer: MAC Table Instability is a Layer 2 concept involving Ethernet hardware addressing or switch forwarding state. Switches learn source MAC addresses and use destination MAC information to choose whether to forward, flood, or filter frames.

3. What should you remember about Root Bridge?

Answer: Root Bridge is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

4. What should you remember about Path Cost?

Answer: PAT lets many inside hosts share one or a small number of public IPv4 addresses by tracking transport protocol and port mappings.

5. What should you remember about Designated Port?

Answer: NAT changes IP address information as traffic crosses a translation boundary. PAT is a many-to-one form that also distinguishes conversations by transport-layer port numbers.

6. What should you remember about Topology Changes?

Answer: Topology Changes is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

7. What should you remember about PortFast/Edge Port Concepts?

Answer: PortFast/Edge Port Concepts is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

8. What should you remember about Loop Protection?

Answer: Loop Protection is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

9. What should you remember about Redundant Links?

Answer: Redundant Links is one of the core topics in Spanning Tree and Layer 2 Redundancy. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

10. What should you remember about Broadcast Storm Troubleshooting?

Answer: Broadcast Storm Troubleshooting concerns traffic delivered to every member of a Layer 2 or IP broadcast scope. Broadcast behavior matters because excessive or unintended broadcasts can consume shared capacity and reveal segmentation problems.