Lab 08 — Spanning Tree and Layer 2 Redundancy

Mission Information

Item	Details
Lab	08
Lab Name	Spanning Tree and Layer 2 Redundancy
Track	CompTIA Network+
Difficulty	Intermediate
Estimated Time	120–150 minutes
Primary Role	Network Technician / Junior Network Administrator
Environment	Network+ Redundant Switching Lab
Primary Systems	NETPLUS-ADMIN, NETPLUS-CLIENT01, NETPLUS-SERVER01, NETPLUS-SW01, NETPLUS-SW02, NETPLUS-SW03
Primary Tools	Managed Switch CLI, Wireshark, ping, MAC-table inspection
Skills	STP, RSTP Concepts, Root Bridge, Root Port, Designated Port, Alternate/Blocked Port, Path Cost, Redundancy, Loop Prevention, Reconvergence

Mission Objective: Build a redundant Layer 2 topology and use Spanning Tree Protocol to maintain resiliency without creating Ethernet loops or broadcast storms.

Mission Scenario

GHC Enterprise wants to improve network availability.

The existing topology contains a single switch path.

That creates a risk:

Switch Failure
      ↓
Network Outage

The network team wants redundant links.

You build:

                 NETPLUS-SW01
                    /     \
                   /       \
                  /         \
          NETPLUS-SW02-----NETPLUS-SW03

Endpoints connect to the switches:

CLIENT01
   |
SW02

SERVER01
   |
SW03

ADMIN
   |
SW01

This improves redundancy, but creates another problem.

The three switch links form a:

Layer 2 Loop

Without loop prevention, Ethernet frames could circulate indefinitely.

Your mission is to configure, observe, and troubleshoot:

Redundant Links
      ↓
Spanning Tree
      ↓
Logical Blocking
      ↓
Loop-Free Topology
      ↓
Automatic Failover

Mission Objectives

By completing this lab, you will be able to:

explain why Layer 2 loops are dangerous
understand broadcast storms
understand MAC-table instability
explain Spanning Tree Protocol
identify the root bridge
understand bridge ID
understand bridge priority
identify root ports
identify designated ports
identify alternate or blocked paths
understand STP path cost
verify the active forwarding topology
observe STP convergence
test redundant-link failure
observe reconvergence
understand Rapid Spanning Tree concepts
troubleshoot incorrect root-bridge selection
troubleshoot blocked and forwarding ports
document a loop-free redundant topology

1. Build the Redundant Switching Topology

Create:

                     NETPLUS-SW01
                       /       \
                      /         \
                     /           \
              NETPLUS-SW02-----NETPLUS-SW03

Connect endpoints:

NETPLUS-ADMIN
      |
      |
NETPLUS-SW01

NETPLUS-CLIENT01
      |
      |
NETPLUS-SW02

NETPLUS-SERVER01
      |
      |
NETPLUS-SW03

2. Use the Existing VLAN

For simplicity, place the lab devices in the same test VLAN.

Example:

VLAN 10 — USERS/LAB

Use:

NETPLUS-ADMIN
10.10.10.10/24

NETPLUS-CLIENT01
10.10.10.20/24

NETPLUS-SERVER01
10.10.10.30/24

All switch-to-switch links should carry VLAN 10.

3. Configure Inter-Switch Links

Configure the switch-to-switch interfaces as trunks where required.

Cisco-like example:

interface <uplink>
 switchport mode trunk
 switchport trunk allowed vlan 10
 no shutdown

Repeat for each inter-switch connection.

4. Verify the Physical Topology

Check:

show interfaces status

Confirm all three inter-switch links are:

Up

At this point, the physical network contains a loop.

Do not disable STP.

5. Understand the Layer 2 Loop

The topology contains:

SW01
 ↓
SW02
 ↓
SW03
 ↓
SW01

An Ethernet frame could theoretically circulate:

Frame
 ↓
SW01
 ↓
SW02
 ↓
SW03
 ↓
SW01
 ↓
SW02
 ↓
...

Ethernet frames do not have a Layer 2 TTL field equivalent to IP TTL.

This is why loops are dangerous.

6. Understand Broadcast Storms

Consider an ARP broadcast:

ff:ff:ff:ff:ff:ff

A switch floods broadcasts.

With a loop:

Broadcast
   ↓
SW01
  ↙  ↘
SW02  SW03
  ↘  ↙
Broadcast returns

Copies can multiply rapidly.

This can cause:

High Bandwidth Usage

High Switch CPU

Packet Loss

Network Instability

Complete Layer 2 Outage

7. Understand MAC-Table Instability

Suppose the same source frame appears through multiple loop paths.

A switch may observe:

MAC-A → Port 1

then:

MAC-A → Port 2

then:

MAC-A → Port 1

repeatedly.

This can create:

MAC Flapping

or unstable forwarding behavior.

8. Why STP Exists

Spanning Tree Protocol creates a:

Loop-Free Logical Topology

while preserving:

Physical Redundancy

Conceptually:

Physical:
Triangle

Logical:
Tree

One redundant path is placed into a non-forwarding role.

9. STP High-Level Process

STP performs:

Elect Root Bridge
      ↓
Calculate Best Paths to Root
      ↓
Select Root Ports
      ↓
Select Designated Ports
      ↓
Block Redundant Paths

10. Inspect Spanning Tree

On each switch, run a platform-appropriate command.

Cisco-like:

show spanning-tree

Record:

Root ID

Bridge ID

Priority

Root Port

Port Roles

Port States

Path Cost

11. Understand the Bridge ID

STP identifies switches using a:

Bridge ID

Conceptually based on:

Bridge Priority
      +
MAC Address

Modern implementations may include VLAN-related extended system ID values, but for Network+ focus on:

Priority
+
MAC

12. Understand Root Bridge Election

The switch with the:

Lowest Bridge ID

wins root-bridge election.

The comparison is:

Lowest Priority
      ↓
If Tie
      ↓
Lowest MAC Address

13. Determine the Current Root Bridge

Run:

show spanning-tree vlan 10

or equivalent on each switch.

Identify:

This bridge is the root

or compare:

Root ID

with:

Bridge ID

Record:

Current Root Bridge:
<switch>

14. Build the Bridge Table

Create:

Switch	Priority	Bridge MAC	Root?
SW01	`<value>`	`<MAC>`	Yes/No
SW02	`<value>`	`<MAC>`	Yes/No
SW03	`<value>`	`<MAC>`	Yes/No

15. Understand the Root Bridge

The root bridge becomes the logical reference point for STP.

Think of it as:

Root Bridge
    ↓
All other switches calculate
their best path toward it

16. Understand Root Ports

Every non-root switch selects one:

Root Port

The root port is the switch’s best path toward the root bridge.

Conceptually:

Non-Root Switch
      ↓
Best Path to Root
      ↓
Root Port

17. Root Bridge Has No Root Port

The root bridge is already:

The Root

Therefore it does not need a root port.

Its relevant active ports generally become designated for their respective segments.

18. Identify SW02 Root Port

On SW02:

show spanning-tree vlan 10

Find the port role:

Root

Record:

SW02 Root Port:
<interface>

19. Identify SW03 Root Port

Repeat on SW03.

Record:

SW03 Root Port:
<interface>

20. Understand Designated Ports

Each network segment needs one forwarding port selected as:

Designated Port

Conceptually:

Segment
   ↓
Best Path Toward Root
   ↓
Designated Port

On the root bridge, ports are usually designated ports.

21. Understand Redundant Port Blocking

On the link between SW02 and SW03, one side may become:

Alternate

or a classic STP:

Blocking

role/state.

That prevents the loop.

22. Build the Logical Topology

Suppose SW01 is root.

STP may logically create:

                 SW01
                /    \
               /      \
             SW02     SW03
               \      /
                \ X  /
             Redundant Path
                Blocked

Physical link:

Still Connected

Logical forwarding:

Disabled on One Side

23. Identify Port Roles

Create:

Switch	Interface	Connected To	STP Role	State
SW01	`<port>`	SW02	Designated	Forwarding
SW01	`<port>`	SW03	Designated	Forwarding
SW02	`<port>`	SW01	Root	Forwarding
SW03	`<port>`	SW01	Root	Forwarding
SW02/SW03	`<port>`	Peer	Designated/Alternate	Forwarding/Discarding

Use your actual results.

24. Understand Port States

Classic STP uses states including:

Blocking

Listening

Learning

Forwarding

Disabled

Rapid STP simplifies these operational states into:

Discarding

Learning

Forwarding

Network+ expects familiarity with both concepts.

25. Understand Forwarding

A forwarding port can:

Receive User Frames

Send User Frames

Learn MAC Addresses

Process STP Control Traffic

26. Understand Blocking / Discarding

A blocked/discarding redundant port does not normally forward user data.

But it continues participating in:

Spanning Tree

so it can become active if topology changes.

27. Understand Learning State

During transition, a port may learn:

Source MAC Addresses

without yet forwarding normal user traffic.

This helps prepare the forwarding table before the port fully transitions.

28. Understand BPDU

STP switches exchange:

Bridge Protocol Data Units

or:

BPDUs

These carry information about:

Root Bridge

Path Cost

Bridge Identity

Topology

29. Capture STP Traffic

If your environment permits Wireshark access to the switch link, apply:

stp

You may observe BPDUs.

Inspect fields such as:

Root Identifier

Root Path Cost

Bridge Identifier

Port Identifier

30. Understand BPDU Purpose

BPDUs allow switches to agree on:

Who Is Root?

What Is the Best Path?

Which Links Must Be Blocked?

Without this information exchange, STP cannot create a consistent loop-free topology.

31. Understand STP Path Cost

STP uses:

Path Cost

to determine the preferred path toward the root.

Generally:

Higher-Speed Link
      ↓
Lower Cost

and:

Lower-Speed Link
      ↓
Higher Cost

Exact cost values depend on STP implementation/version.

32. Root Port Selection

A non-root switch prefers the path with the:

Lowest Total Root Path Cost

Conceptually:

Path A
Cost 4

Path B
Cost 19

The switch chooses:

Path A

33. Understand STP Tie-Breaking

When path costs tie, STP uses additional tie-breakers.

At Network+ level, remember factors can include:

Lower Sender Bridge ID

Lower Sender Port ID

Lower Local Port ID

depending on where the tie occurs.

34. Verify Connectivity While a Link Is Blocked

From ADMIN:

ping -c 3 10.10.10.20

Then:

ping -c 3 10.10.10.30

Both should work even though one redundant inter-switch path is not forwarding.

This proves:

Blocked Redundant Path
≠
Broken Network

35. Inspect MAC Tables

On each switch:

show mac address-table

Generate traffic between:

ADMIN ↔ CLIENT01

ADMIN ↔ SERVER01

CLIENT01 ↔ SERVER01

Observe which ports are used.

36. Trace the Active Forwarding Path

If the SW02–SW03 link is blocked, CLIENT01 to SERVER01 may travel:

CLIENT01
   ↓
SW02
   ↓
SW01
   ↓
SW03
   ↓
SERVER01

instead of directly:

SW02
 ↓
SW03

because STP has logically blocked that redundant link.

37. Test Redundant Link Failure

Now safely simulate failure of one active uplink.

Example:

SW01 ↔ SW02

Disable one side of the link.

Cisco-like:

interface <uplink>
 shutdown

38. Observe Connectivity During Failure

Start a continuous ping before disabling the link.

Linux:

ping 10.10.10.20

Windows:

ping -t 10.10.10.20

Observe:

Temporary Packet Loss

followed by:

Connectivity Restoration

depending on STP mode and timers.

39. Observe Reconvergence

After the active path fails:

Old Forwarding Link
      ↓
Failure
      ↓
STP Recalculates
      ↓
Previously Blocked Path
      ↓
Transitions to Forwarding

This is:

Reconvergence

40. Inspect STP After Failure

Run:

show spanning-tree vlan 10

on all switches.

Compare:

Before Failure

versus:

After Failure

Identify which port changed role/state.

41. Document the Failover

Create:

Item	Before	After
Root Bridge	`<switch>`	`<switch>`
SW02 Root Port	`<port>`	`<port>`
SW03 Root Port	`<port>`	`<port>`
Blocked Port	`<port>`	`<port>`
Active Path	`<path>`	`<path>`

42. Restore the Failed Link

Re-enable:

interface <uplink>
 no shutdown

Observe STP reconverge again.

The previously restored link may return to forwarding or a redundant role depending on topology.

43. Understand Convergence

Convergence means:

All Switches Agree
on
Loop-Free Topology

During topology changes:

Old Topology
      ↓
Change Detected
      ↓
STP Calculation
      ↓
New Stable Topology

44. Understand Rapid Spanning Tree

Traditional STP convergence can be relatively slow.

Rapid Spanning Tree Protocol:

RSTP

was designed for faster convergence.

Common standard:

IEEE 802.1w

Classic STP:

IEEE 802.1D

45. Compare STP and RSTP

Feature	STP	RSTP
Standard	802.1D	802.1w
Convergence	Slower	Faster
Port states	5 major states	3 operational states
Alternate role	Less explicit	Explicit alternate role
Modern preference	Legacy	Common modern approach

46. Understand RSTP Port Roles

Common RSTP roles include:

Root

Designated

Alternate

Backup

For Network+, understand especially:

Root
Designated
Alternate

47. Identify the STP Mode

Run a platform-specific command such as:

show spanning-tree summary

Determine whether your lab uses:

STP

RSTP

Rapid-PVST-like implementation

MST

For this lab, focus on STP/RSTP concepts.

48. Understand Per-VLAN Spanning Tree Concept

Some switch platforms maintain a separate STP instance per VLAN.

Conceptually:

VLAN 10
   ↓
Spanning Tree A

VLAN 20
   ↓
Spanning Tree B

This can allow different VLANs to use different root bridges or active paths.

49. Root Bridge Design Best Practice

The root bridge should normally be:

Intentionally Selected

not:

Whichever Switch Has Lowest MAC

by accident.

Core/distribution switches are commonly preferred.

50. Configure SW01 as Preferred Root

Inside the lab, lower SW01’s STP priority for VLAN 10.

Cisco-like example:

spanning-tree vlan 10 priority 24576

or a platform shortcut such as:

spanning-tree vlan 10 root primary

Verify command syntax for your lab platform.

51. Verify the Root Change

Run:

show spanning-tree vlan 10

on all switches.

Confirm:

SW01
=
Root Bridge

52. Understand Priority Values

Many classic STP implementations use priority increments such as:

The exact displayed value may also incorporate extended system ID information.

Lower priority is preferred.

53. Configure a Secondary Root Concept

In enterprise design, another switch may be configured with the second-lowest priority.

Example:

SW01
Primary Root

SW02
Secondary Root

If SW01 fails:

SW02
likely becomes Root

This creates predictable topology behavior.

54. Troubleshooting Scenario 1 — Unexpected Root Bridge

Suppose SW03 unexpectedly becomes root.

Investigate:

show spanning-tree

Check:

Bridge Priorities

MAC Addresses

Configuration

Possible cause:

SW03 has lower Bridge ID

55. Correct Unexpected Root Selection

Configure the intended root with a lower priority.

Then verify:

Root ID
=
Intended Core Switch

56. Troubleshooting Scenario 2 — Unexpected Blocked Port

A port is blocking, and an administrator believes it should forward.

Do not immediately enable or force forwarding.

First determine:

Is This Link Redundant?

What Is Root Bridge?

What Is Path Cost?

Which Port Is Root?

Which Port Is Designated?

The blocked port may be functioning exactly as designed.

57. Troubleshooting Scenario 3 — High-Cost Path Selected

Suppose traffic is taking an unexpected slower path.

Review:

STP Cost

Interface Speed

Configured Cost

Root Bridge Position

A lower path cost should normally be preferred.

58. Troubleshooting Scenario 4 — Link Up but Not Forwarding

You see:

Interface:
Up

but:

STP State:
Discarding / Blocking

This means:

Layer 1:
Working

STP:
Intentionally Preventing Forwarding

Do not misdiagnose this as a failed physical link.

59. Troubleshooting Scenario 5 — Redundant Link Fails to Activate

After an active link fails, connectivity does not recover.

Check:

Alternate Link Physical State

VLAN Allowed on Link

Trunk Configuration

STP State

Port Configuration

Topology

A backup path cannot help if the VLAN is not actually carried across it.

60. Troubleshooting Scenario 6 — VLAN Missing on Redundant Trunk

Remove VLAN 10 from the backup trunk’s allowed list.

Then fail the primary path.

Expected:

STP Path May Activate

but:

VLAN 10 Traffic Still Fails

because the trunk itself does not carry VLAN 10.

This demonstrates:

Redundancy requires both Layer 2 topology and correct VLAN configuration.

61. Restore VLAN 10

Re-add:

VLAN 10

to the trunk.

Verify:

show interfaces trunk

and:

show spanning-tree vlan 10

62. Understand Edge Ports

Ports connected directly to endpoints are sometimes configured as:

Edge Ports

or platform-specific:

PortFast

This allows them to transition to forwarding more quickly.

Use only on ports that should not connect to another switch.

63. Understand Why Edge Configuration Matters

A user workstation does not normally create a Layer 2 switching loop.

Therefore waiting through full STP convergence may unnecessarily delay:

DHCP

Login

Network Access

Edge-port features address this.

64. Understand the Risk

Never configure an inter-switch link as an edge/PortFast port without understanding the consequences.

Doing so can weaken loop-prevention behavior.

65. Understand BPDU Guard Concept

BPDU Guard is commonly used on edge ports.

Conceptually:

Edge Port
   ↓
Unexpected BPDU Received
   ↓
Port Disabled / Protected

This can help protect against accidental switch connections.

66. Understand Root Guard Concept

Root Guard can help prevent an unexpected downstream switch from becoming STP root.

Conceptually:

Unexpected Better BPDU
      ↓
Do Not Allow Root Change

These are important Layer 2 protection concepts.

67. Understand Loop Guard Concept

Loop Guard helps protect against certain situations where STP BPDUs unexpectedly stop arriving and a blocked link might otherwise become forwarding.

At Network+ level, understand it as an additional STP loop-protection mechanism.

68. STP Protection Summary

Feature	Purpose
PortFast / Edge	Faster endpoint forwarding
BPDU Guard	Protect edge ports from switch BPDUs
Root Guard	Prevent unexpected root bridge
Loop Guard	Help prevent forwarding during BPDU-loss conditions

69. Build the STP Decision Summary

1. Elect Root Bridge
      ↓
2. Each Non-Root Selects Root Port
      ↓
3. Each Segment Selects Designated Port
      ↓
4. Redundant Ports Become Alternate/Blocked
      ↓
5. Forwarding Topology Becomes Loop-Free

70. Mission Challenge — Root Election

Switches have:

SW01
Priority 32768
MAC 00:00:00:00:00:30

SW02
Priority 32768
MAC 00:00:00:00:00:20

SW03
Priority 32768
MAC 00:00:00:00:00:10

Which becomes root?

Answer:

SW03

because all priorities tie and SW03 has the lowest MAC address.

71. Mission Challenge — Priority Election

Now:

SW01
Priority 24576

SW02
Priority 32768

SW03
Priority 32768

Which becomes root?

Answer:

SW01

because it has the lowest priority.

72. Mission Challenge — Root Port

SW02 has two possible paths:

Direct to Root
Cost 4

Through SW03
Cost 8

Which port becomes root port?

Answer:

Direct path
Cost 4

73. Mission Challenge — Identify the Loop-Free Topology

Given:

SW01 = Root

SW02 → SW01 = Root Port

SW03 → SW01 = Root Port

The SW02–SW03 link must have:

One Designated Side

One Alternate/Blocked Side

to prevent the loop.

74. Mission Challenge — Failover

If:

SW01 ↔ SW02

fails, explain how CLIENT01 can still reach the network.

Expected path:

CLIENT01
   ↓
SW02
   ↓
Previously Blocked SW02–SW03 Link
   ↓
SW03
   ↓
SW01

after STP reconvergence.

75. Mission Challenge — Diagnose the Problem

Symptoms:

All interfaces show UP

CLIENT01 cannot reach SERVER01

One trunk shows VLAN 10 missing

STP shows alternate path forwarding after primary link failure

Likely cause:

Backup topology exists,
but VLAN 10 is not allowed across the active alternate trunk.

76. Create the Lab Workspace

On NETPLUS-ADMIN:

mkdir -p ~/NetworkPlus-Labs/LAB08/{Captures,Screenshots,Configs,Notes}

Create:

touch ~/NetworkPlus-Labs/LAB08/Notes/lab08-notes.md

77. Save Switch Evidence

Capture outputs from all switches:

show spanning-tree

show spanning-tree vlan 10

show mac address-table

show interfaces status

show interfaces trunk

Store them under:

~/NetworkPlus-Labs/LAB08/Configs/

78. Save STP Packet Capture

If available, save:

LAB08-STP-BPDU.pcapng

If you capture topology-change behavior, save:

LAB08-STP-FAILOVER.pcapng

79. Document the STP Baseline

Use:

# LAB08 — Spanning Tree and Layer 2 Redundancy

## Topology

SW01 connections:

SW02 connections:

SW03 connections:

## VLAN

VLAN:
10

## Bridge Information

### SW01

Priority:
MAC:
Bridge ID:
Root:
Yes / No

### SW02

Priority:
MAC:
Bridge ID:
Root:
Yes / No

### SW03

Priority:
MAC:
Bridge ID:
Root:
Yes / No

## Root Bridge

Switch:

Reason:

## Port Roles

### SW01

Port:
Role:
State:
Cost:

### SW02

Port:
Role:
State:
Cost:

### SW03

Port:
Role:
State:
Cost:

## Blocked / Alternate Path

Interface:

Reason:

## Initial Forwarding Path

CLIENT01 → SERVER01:

## Failover Test

Failed Link:

Connectivity Impact:

New Root Port:

New Forwarding Path:

Reconvergence Result:

## Root Bridge Configuration

Original Root:

Configured Root:

Priority:

## Troubleshooting

### Unexpected Root

Problem:

Root Cause:

Fix:

### Missing VLAN on Backup Path

Problem:

Root Cause:

Fix:

## Final Assessment

Summarize how STP maintained a loop-free redundant topology.

80. Evidence to Capture

Capture:

01-redundant-switch-topology.png

02-switch-interface-status.png

03-switch-trunks.png

04-initial-stp-sw01.png

05-initial-stp-sw02.png

06-initial-stp-sw03.png

07-root-bridge.png

08-root-port-sw02.png

09-root-port-sw03.png

10-alternate-blocked-port.png

11-port-role-table.png

12-stp-bpdu.png

13-mac-address-tables.png

14-connectivity-before-failure.png

15-primary-link-shutdown.png

16-stp-reconvergence.png

17-new-forwarding-port.png

18-connectivity-after-failover.png

19-link-restored.png

20-root-priority-config.png

21-root-election-verified.png

22-missing-vlan-backup-trunk.png

23-redundancy-restored.png

24-final-stp-topology.png

81. Validation Checklist

Topology

SW01 created/configured
SW02 created/configured
SW03 created/configured
Redundant triangle topology built
Endpoint connectivity established
Inter-switch links verified
VLAN 10 allowed on required trunks

Loop Concepts

Layer 2 loop understood
Broadcast storm understood
MAC-table instability understood
Reason Ethernet loops are dangerous understood

STP Fundamentals

STP purpose understood
BPDU concept understood
Bridge ID understood
Bridge priority understood
Root bridge identified
Root election logic understood
Root ports identified
Designated ports identified
Alternate/blocked port identified

Path Selection

STP path cost understood
Lowest-cost path concept understood
Tie-breaking concept reviewed
Active forwarding topology documented

Port States

Forwarding understood
Learning understood
Blocking/discarding understood
Classic STP states understood
RSTP states understood

Redundancy

Connectivity verified with redundant link blocked
Active link failure simulated
Temporary convergence impact observed
Alternate path transitioned correctly
New forwarding path documented
Failed link restored
STP reconvergence observed

Root Design

Original root identified
Preferred root configured
Root election verified
Secondary-root concept understood

Protection Concepts

Edge/PortFast concept understood
BPDU Guard understood
Root Guard understood
Loop Guard understood

Troubleshooting

Unexpected root scenario understood
Blocked port correctly interpreted
High-cost path considered
Link-up-but-discarding state understood
Backup-link failure investigated
VLAN-on-backup-trunk problem completed

Documentation

Bridge table completed
Port role table completed
Initial forwarding path documented
Failover path documented
STP packet capture saved where available
Evidence screenshots captured
Lab notes completed

82. Mission Review

In this mission, you learned that redundancy introduces both:

Resilience

and:

Loop Risk

Without STP:

Redundant Ethernet Links
        ↓
Layer 2 Loop
        ↓
Broadcast Storm
        ↓
MAC Instability
        ↓
Network Failure

With STP:

Redundant Ethernet Links
        ↓
Root Bridge Election
        ↓
Best Paths Calculated
        ↓
Redundant Path Blocked
        ↓
Loop-Free Forwarding

When an active path fails:

Link Failure
      ↓
Topology Change
      ↓
STP Reconvergence
      ↓
Alternate Path Activates
      ↓
Connectivity Restored

The key lesson is:

Spanning Tree allows Ethernet networks to keep redundant physical links while maintaining only one loop-free logical forwarding path at a time.

Skills Developed

After completing this mission, you should be able to:

explain Layer 2 loops
explain broadcast storms
explain MAC flapping
describe STP operation
identify bridge IDs
determine root-bridge election
identify root ports
identify designated ports
recognize alternate and blocked paths
interpret STP path cost
understand STP port states
explain BPDU operation
verify spanning-tree state
design a preferred root bridge
understand RSTP
test link redundancy
observe reconvergence
troubleshoot STP behavior
understand common STP protection mechanisms

What’s Next?

Lab 09 — Static Routing Fundamentals

You now have resilient Layer 2 networking.

The next mission moves into Layer 3 routing.

You will build multiple IP networks and configure routers to move packets between them using static routes.

You will work with:

routing tables
directly connected networks
static routes
default routes
next-hop addresses
exit interfaces
longest prefix match
administrative distance concepts
packet forwarding
router ARP behavior
multi-router paths
traceroute
route failure
incorrect next-hop troubleshooting
missing route troubleshooting

The progression becomes:

Layer 2 Switching
      ↓
VLANs
      ↓
Redundancy
      ↓
Multiple IP Networks
      ↓
Routers
      ↓
Routing Tables
      ↓
Static Routes
      ↓
End-to-End Connectivity

➡️ Next: Lab 09 — Static Routing Fundamentals