Runbook 02 — VLAN and Trunk Troubleshooting

Runbook Information

Item	Details
Runbook	02
Runbook Name	VLAN and Trunk Troubleshooting
Track	CompTIA Network+
Type	Network Operations Runbook
Primary Role	Network Technician / Network Administrator
Difficulty	Intermediate
Use Case	VLAN segmentation and trunk connectivity incidents
Primary Tools	Switch CLI, Ping, Traceroute, ARP, MAC Address Table, Wireshark, Syslog, SNMP
Primary Technologies	Ethernet, VLANs, IEEE 802.1Q, Access Ports, Trunks
Outcome	Identify, isolate, remediate, verify, escalate, and document Layer 2 connectivity faults

Runbook Objective: Provide a structured procedure for investigating VLAN and trunk connectivity incidents without making unnecessary configuration changes. The objective is to determine whether the failure exists at the endpoint, access port, VLAN database, trunk, allowed VLAN list, native VLAN configuration, MAC-learning process, or upstream network path.

When to Use This Runbook

Use this runbook when:

Users in one VLAN cannot communicate.

One department has lost network access.

Devices receive addresses from the wrong network.

A VLAN works on one switch but not another.

A VLAN cannot cross an inter-switch link.

Some VLANs work across a trunk while others fail.

Newly created VLANs do not work.

A switch replacement causes selective connectivity failures.

Wireless users connect but cannot reach their VLAN gateway.

A trunk is operational but selected users remain unreachable.

Typical incidents include:

incorrect access VLAN
missing VLAN
disabled VLAN
incorrect access-port configuration
trunk failure
incorrect trunk encapsulation
missing allowed VLAN
native VLAN mismatch
incorrect native VLAN
VLAN pruning
MAC-learning problems
inter-switch connectivity problems
Layer 2 loops
configuration drift

1. Understand the Layer 2 Path

Before troubleshooting, determine the expected Layer 2 path.

Example:

CLIENT01
   ↓
Access Port
   ↓
VLAN 10
   ↓
SW02
   ↓
802.1Q Trunk
   ↓
SW01
   ↓
Gateway

For communication to succeed:

Access Port
    +
VLAN
    +
Trunk
    +
Allowed VLAN
    +
MAC Learning
    +
Gateway Path

must all operate correctly.

2. Troubleshooting Principle

Follow:

Symptom
   ↓
Scope
   ↓
Access Port
   ↓
VLAN
   ↓
MAC Learning
   ↓
Trunk
   ↓
Allowed VLAN
   ↓
Upstream Path
   ↓
Root Cause
   ↓
Remediation
   ↓
Verification

Do not immediately:

Delete VLANs

Recreate Trunks

Reset Switches

Allow All VLANs

Reboot Switches

These actions can hide the original problem.

3. Record the Incident

Document:

Incident ID:

Date:

Time:

Reported By:

Affected Users:

Affected VLAN:

Affected Switch:

Affected Interfaces:

Business Impact:

Recent Changes:

Example:

Incident ID:
INC-VLAN-2041

Affected:
Finance Department

VLAN:
10

Switch:
SW02

Problem:
Finance users cannot reach their default gateway.

Impact:
25 users affected.

4. Record the Exact Symptom

Avoid:

VLAN is broken.

Prefer:

Clients connected to VLAN 10 on SW02 cannot reach
10.10.10.1.

VLAN 20 clients on the same switch remain operational.

This immediately suggests:

Selective VLAN Failure

rather than:

Complete Switch Failure

5. Determine Scope

Ask:

One Device?

One Port?

One VLAN?

One Switch?

Multiple Switches?

All VLANs?

One Trunk?

Entire Site?

Create:

Test	Result
Same VLAN / Same Switch
Same VLAN / Different Switch
Different VLAN / Same Switch
Different VLAN / Different Switch

6. Check Recent Changes

Review:

VLAN Creation

VLAN Deletion

Port Assignment Changes

Trunk Configuration

Allowed VLAN Changes

Native VLAN Changes

Switch Replacement

Firmware Upgrade

Configuration Restore

Record:

Change ID:

Engineer:

Time:

Affected Device:

7. Establish Expected VLAN Architecture

Example:

VLAN	Name	Network	Purpose
10	USERS	`10.10.10.0/24`	Corporate users
20	ENGINEERING	`10.10.20.0/24`	Engineering
30	SERVERS	`10.10.30.0/24`	Servers
40	MANAGEMENT	`10.10.40.0/24`	Infrastructure
50	CORP-WIFI	`10.10.50.0/24`	Wireless
60	GUEST	`10.10.60.0/24`	Guest

Do not troubleshoot without knowing:

What VLAN Should This Device Be In?

8. Identify the Endpoint

Collect:

Hostname:

IP Address:

Subnet Mask:

Default Gateway:

MAC Address:

Switch:

Switch Port:

Expected VLAN:

Windows:

ipconfig /all

Linux:

ip addr

9. Check Physical Connectivity

Verify:

NIC Enabled

Cable Connected

Link LED

Switch Port Up

Correct Speed

Correct Duplex

On the switch:

show interfaces status

If the port is:

DOWN

resolve Layer 1 before continuing with VLAN troubleshooting.

10. Verify the Access Port

Identify the endpoint interface.

Example:

CLIENT01
    ↓
SW02 Gi0/10

Run:

show interfaces Gi0/10 switchport

Verify:

Administrative Mode:
static access

Operational Mode:
static access

Access VLAN:
10

11. Check VLAN Membership

Run:

show vlan brief

Expected:

VLAN 10
Name: USERS
Status: active

Gi0/10
→ VLAN 10

Compare:

Expected VLAN
vs
Actual VLAN

12. Decision Point — Wrong Access VLAN

Suppose:

Expected:
VLAN 10

Actual:
VLAN 20

Likely root cause:

Incorrect Access-Port Assignment

Do not troubleshoot routing before correcting the Layer 2 configuration.

13. Verify VLAN Exists

Run:

show vlan brief

Confirm the VLAN exists on every required switch.

Example:

SW01:
VLAN 10 present

SW02:
VLAN 10 present

SW03:
VLAN 10 present

14. Decision Point — VLAN Missing

If:

SW01:
VLAN 10 exists

SW02:
VLAN 10 missing

devices connected to SW02 cannot correctly participate in VLAN 10.

Determine why the VLAN is missing before recreating it.

15. Verify VLAN Status

Confirm:

Status:
active

Do not assume that existence alone means the VLAN is operational.

16. Check MAC Address Learning

Run:

show mac address-table

Locate the endpoint MAC.

Expected:

MAC Address
    ↓
VLAN 10
    ↓
Gi0/10

Record:

MAC:

VLAN:

Interface:

Type:

17. Decision Point — MAC Not Learned

If the switch does not learn the endpoint MAC, investigate:

Cable

NIC

Port State

Access VLAN

Port Security

Endpoint Traffic

Generate controlled traffic:

ping <DEFAULT-GATEWAY>

Then check the MAC table again.

18. Check for MAC Learning on Wrong Port

If the MAC appears on an unexpected interface:

Expected:
Gi0/10

Actual:
Gi0/24

investigate:

Incorrect Cabling

Topology Documentation

Loop

Unexpected Intermediate Device

19. Test the Default Gateway

From the endpoint:

ping 10.10.10.1

If:

Gateway:
UNREACHABLE

but the endpoint configuration is correct, continue investigating the Layer 2 path.

20. Identify the Required Trunk

Example:

CLIENT01
   ↓
SW02
   ↓
Gi0/1
   ↓
802.1Q Trunk
   ↓
SW01

Determine every trunk the VLAN must traverse.

21. Check Trunk State

Run:

show interfaces trunk

Verify:

Port

Mode

Encapsulation

Status

Native VLAN

Expected:

Status:
trunking

22. Decision Point — Trunk Not Formed

If the expected link is not trunking, inspect both ends.

Check:

SW01 Interface Configuration

SW02 Interface Configuration

A trunk requires compatible configuration.

23. Compare Both Ends

Document:

Parameter	SW01	SW02
Interface
Mode
Trunk Status
Native VLAN
Allowed VLANs

Configuration mismatches are easier to identify when compared side-by-side.

24. Check Allowed VLANs

Run:

show interfaces trunk

Look for:

Vlans allowed on trunk

Example:

10,20,30,40

25. Decision Point — VLAN Missing from Allowed List

Suppose:

Required:
VLAN 10

Allowed:
20,30,40

The physical trunk may remain:

UP

while VLAN 10 traffic fails.

This produces:

Selective VLAN Outage

26. Compare Required and Allowed VLANs

Create:

VLAN	Required	SW01 Allowed	SW02 Allowed	Result
10	Yes
20	Yes
30	Yes
40	Yes
50	No/Yes

27. Do Not Solve by Allowing Everything

Avoid immediately configuring:

Allow All VLANs

Instead allow:

Only Required VLANs

This preserves segmentation and reduces unnecessary Layer 2 exposure.

28. Check Active VLANs on the Trunk

Review:

Vlans allowed and active in management domain

A VLAN may be allowed but unavailable because it does not exist locally.

Therefore:

Allowed
≠
Active

29. Check Forwarding VLANs

Review:

Vlans in spanning tree forwarding state and not pruned

This helps determine whether the VLAN is actually forwarding across the trunk.

30. Check Native VLAN

Run:

show interfaces trunk

Compare the native VLAN on both sides.

Example:

SW01:
Native VLAN 99

SW02:
Native VLAN 1

This is:

Native VLAN Mismatch

31. Understand Native VLAN Behavior

In standard 802.1Q operation:

Tagged Frames
→ VLAN Identifier Present

Native VLAN traffic may be transmitted untagged depending on platform configuration.

A mismatch can cause:

Unexpected Traffic Placement

Connectivity Problems

Security Risk

Protocol Warnings

32. Review Syslog for Native VLAN Warnings

Search for events such as:

Native VLAN mismatch

Record:

Timestamp:

Local Interface:

Local VLAN:

Neighbor:

Neighbor VLAN:

33. Check Spanning Tree

Where supported:

show spanning-tree

or:

show spanning-tree vlan 10

Verify the required interface state.

Possible states include:

Forwarding

Blocking / Discarding

depending on the implementation.

34. Do Not Disable STP to Restore Connectivity

Never treat:

Spanning Tree Blocking

as automatically incorrect.

STP may be preventing:

Layer 2 Loop

Understand the topology before making changes.

35. Look for Layer 2 Loops

Symptoms can include:

High Broadcast Traffic

MAC Address Flapping

High CPU

Network Instability

Packet Loss

Severe Performance Degradation

36. Check for MAC Flapping

Review switch logs and MAC tables.

A MAC repeatedly appearing on multiple ports may indicate:

Layer 2 Loop

Incorrect Cabling

Unexpected Switching Device

37. Check Interface Counters

Run:

show interfaces <interface>

Inspect:

Input Errors

Output Errors

CRC Errors

Drops

Runts

Giants

Collisions

Do not confuse physical degradation with VLAN misconfiguration.

38. Test Same-VLAN Connectivity

Test two devices:

CLIENT01
10.10.10.20

and:

CLIENT02
10.10.10.21

Both should be in VLAN 10.

Test:

ping 10.10.10.21

39. Interpret Same-VLAN Failure

If same-VLAN devices cannot communicate, investigate:

Access VLAN

Switch Port

Trunk

MAC Learning

Host Firewall

Physical Connectivity

Routing is generally not required for communication within the same subnet.

40. Test Across Switches

Place:

CLIENT01
→ SW02

and:

CLIENT02
→ SW03

in the same VLAN.

If they communicate on the same switch but fail across switches, investigate:

Inter-Switch Trunks

41. Use Controlled Comparison

Example:

CLIENT01 → CLIENT02
Same Switch
SUCCESS

CLIENT01 → CLIENT03
Different Switch
FAILURE

This strongly suggests the problem exists:

Between Switches

42. Check VLAN Tagging with Wireshark

Where capture location supports VLAN visibility, use:

vlan

or:

vlan.id == 10

Remember that endpoint captures may not display tags because access ports typically deliver untagged Ethernet frames.

43. Understand Where to Capture

For VLAN-tag analysis, capture near:

Trunk Links

SPAN / Mirror Ports

Virtual Switches

Hypervisor Interfaces

rather than relying only on endpoint captures.

44. Check ARP Behavior

From CLIENT01:

arp -a

Then:

ping 10.10.10.1

Inspect whether the gateway MAC is learned.

45. Decision Point — ARP Failure

If CLIENT01 repeatedly sends:

Who has 10.10.10.1?

without receiving a response, investigate:

VLAN Path

Gateway Interface

Trunk

Allowed VLAN

Layer 2 Connectivity

46. Capture ARP

Wireshark filter:

arp

Look for:

ARP Request

followed by:

ARP Reply

No reply can provide valuable Layer 2 evidence.

47. Check Gateway VLAN Interface

If Layer 2 appears healthy, verify the gateway interface.

Examples:

SVI

or:

Router Subinterface

Check:

show ip interface brief

48. Verify Router-on-a-Stick Configuration

Where used:

RTR01
   |
802.1Q Trunk
   |
SW01

Each routed subinterface should correspond to the correct VLAN.

Conceptually:

VLAN 10
→ 10.10.10.1

VLAN 20
→ 10.10.20.1

49. Verify VLAN Encapsulation Mapping

An incorrect mapping such as:

Gateway:
10.10.10.1

802.1Q VLAN:
20

can prevent VLAN 10 clients from reaching their gateway.

50. Distinguish Layer 2 from Layer 3

If:

Same VLAN Communication:
WORKING

Default Gateway:
UNREACHABLE

investigate:

Gateway Interface

Router Trunk

SVI

Router Subinterface

rather than access switching alone.

51. Investigate Wireless VLAN Mapping

For wireless incidents:

SSID
   ↓
VLAN Mapping
   ↓
AP Uplink
   ↓
Switch Trunk

Example:

GHC-CORP
→ VLAN 50

52. Check AP Switch Port

Determine whether AP01 requires:

Access Port

or:

Trunk

depending on the wireless architecture.

Multiple SSIDs mapped to multiple VLANs commonly require tagged VLAN transport.

53. Verify Wireless VLANs Across the Path

Check:

AP01
    ↓
Access Switch
    ↓
Core Switch
    ↓
Gateway

Verify VLAN 50 exists and is permitted everywhere required.

54. Investigate Management VLAN Problems

If network devices cannot be managed, verify:

Management VLAN

Management IP

Switch Management Interface

Default Gateway

Trunk Path

ACL

Do not confuse:

Data Plane Working

with:

Management Plane Working

55. Check Configuration Drift

Compare:

Running Configuration

against:

Known-Good Configuration

Focus on:

VLAN Definitions

Access Ports

Trunk Ports

Allowed VLANs

Native VLANs

SVIs

Management VLAN

56. Review Configuration Changes

Where available:

show running-config

Compare with backups or change-management records.

Look for:

interface configuration changes

switchport mode changes

switchport access vlan changes

switchport trunk allowed vlan changes

native VLAN changes

57. Build a Theory Matrix

Theory	Evidence For	Evidence Against	Test	Result
Wrong access VLAN
VLAN missing
Trunk down
VLAN not allowed
Native VLAN mismatch
STP issue
Gateway VLAN issue
Physical fault

58. Fault Isolation Example

Suppose:

CLIENT01
↓
Valid IP

Access Port
↓
VLAN 10

MAC Learned
↓
Correct

SW02 VLAN 10
↓
Present

SW02 → SW01 Trunk
↓
UP

but:

Allowed VLANs
↓
20,30,40

Root cause:

VLAN 10 Missing from Allowed VLAN List

59. Create the Root Cause Statement

Example:

Finance users connected to SW02 lost access to the
network because VLAN 10 was removed from the allowed
VLAN list on the SW02-to-SW01 trunk.

The trunk remained operational, but VLAN 10 frames
could no longer traverse the inter-switch link.

60. Document the Proposed Remediation

Before making changes:

Current Configuration:

Expected Configuration:

Proposed Change:

Affected Interface:

Affected VLAN:

Risk:

Rollback Plan:

61. Implement the Minimum Change

If the issue is:

VLAN 10 Missing

restore only the required VLAN according to the approved configuration.

Do not unnecessarily:

Reset Entire Trunk

Delete/Recreate VLAN Database

Reload Switch

62. Verify the Configuration

After remediation:

show vlan brief

show interfaces trunk

show mac address-table

Confirm expected Layer 2 state.

63. Retest the Original Symptom

From CLIENT01:

ping 10.10.10.1

Then test the required enterprise services.

64. Test Same-VLAN Communication

Verify:

CLIENT01
   ↓
CLIENT02

works as expected.

65. Test Inter-VLAN Communication

Where permitted:

CLIENT01
   ↓
Gateway
   ↓
SERVER01

Confirm routing still works after the Layer 2 remediation.

66. Verify Other VLANs

A trunk modification can affect multiple networks.

Test:

VLAN 10

VLAN 20

VLAN 30

VLAN 40

according to the network design.

67. Verify Monitoring

Check:

Switch Availability

Interface Status

Trunk Status

Packet Loss

Interface Errors

Ensure monitoring returns to baseline.

68. Review Syslog After Remediation

Look for:

Interface Changes

Trunk Changes

Native VLAN Warnings

STP Changes

MAC Flapping

Confirm no unexpected new events appear.

69. Verify Stability

Do not close the incident immediately after one successful ping.

Monitor:

Connectivity

Interface State

Errors

Packet Loss

MAC Stability

for an appropriate validation period.

70. Escalation Criteria

Escalate when:

multiple switches exhibit unexplained VLAN loss
STP instability exists
Layer 2 loops are suspected
MAC flapping continues
production core configuration requires senior approval
hardware failure is suspected
switch software defects are suspected
change authority is required
root cause cannot be isolated safely

71. Escalation Package

Provide:

Incident ID

Affected VLANs

Affected Switches

Affected Interfaces

Business Impact

Expected Topology

Actual Behavior

VLAN Output

Trunk Output

MAC Table

STP Output

Relevant Syslog

Tests Completed

Changes Made

72. Example Escalation

INC-VLAN-2041

VLAN 10 connectivity is unstable between SW02 and SW01.

Access ports and VLAN membership have been verified.

The inter-switch trunk remains operational.

VLAN 10 is allowed and active.

Syslog shows repeated spanning-tree topology changes.

MAC addresses are intermittently moving between Gi0/1
and Gi0/2.

No configuration changes have been made.

Suspected Layer 2 loop.

Requesting Network Engineering investigation.

73. VLAN Troubleshooting Decision Tree

VLAN Connectivity Failure
          ↓
Physical Port Up?
      ┌───┴───┐
     NO      YES
      ↓        ↓
  Layer 1   Correct Access VLAN?
             ┌───┴───┐
            NO      YES
             ↓        ↓
         Correct    VLAN Exists?
                     ┌───┴───┐
                    NO      YES
                     ↓        ↓
                  VLAN DB   MAC Learned?
                            ┌───┴───┐
                           NO      YES
                            ↓        ↓
                     Endpoint/L2   Cross-Switch?
                                  ┌───┴───┐
                                 NO      YES
                                  ↓        ↓
                              Local L2   Trunk Up?
                                         ┌───┴───┐
                                        NO      YES
                                         ↓        ↓
                                      Trunk    VLAN Allowed?
                                                ┌───┴───┐
                                               NO      YES
                                                ↓        ↓
                                             Allow    STP/Gateway

74. Selective VLAN Failure Decision Tree

Trunk Is UP
     ↓
Some VLANs Work?
     ↓
    YES
     ↓
Check Affected VLAN
     ↓
VLAN Exists?
     ↓
VLAN Allowed?
     ↓
VLAN Active?
     ↓
STP Forwarding?
     ↓
Gateway Available?

75. Quick Command Reference

VLANs

show vlan brief

Interface State

show interfaces status

Access Port

show interfaces <interface> switchport

Trunks

show interfaces trunk

MAC Table

show mac address-table

Interface Counters

show interfaces <interface>

Spanning Tree

show spanning-tree

show spanning-tree vlan <VLAN-ID>

Configuration

show running-config

Layer 3 Interfaces

show ip interface brief

76. Wireshark Quick Reference

Investigation	Filter
ARP	`arp`
VLAN tagged traffic	`vlan`
Specific VLAN	`vlan.id == 10`
ICMP	`icmp`
DHCP	`dhcp`

77. Common VLAN Problems

Problem	Typical Symptom
Wrong access VLAN	Device placed in wrong network
VLAN missing	Ports cannot use expected VLAN
Trunk down	Multiple VLANs fail across switches
VLAN not allowed	One or selected VLANs fail
Native VLAN mismatch	Unexpected connectivity / warnings
STP blocking	Path unavailable by design or topology issue
MAC-learning failure	Endpoint unreachable
Wrong AP VLAN mapping	Wireless connectivity failure
Wrong gateway VLAN mapping	VLAN cannot reach routed networks

78. Operational Checklist

Incident Intake

Incident recorded
Exact symptom documented
Business impact determined
Scope identified
Recent changes reviewed

Endpoint

Physical connectivity verified
IP configuration reviewed
MAC address recorded
Expected VLAN identified
Gateway tested

Access Port

Interface identified
Interface up
Speed/duplex checked
Access mode verified
Correct VLAN assigned
Errors reviewed

VLAN

VLAN exists
VLAN active
VLAN present on required switches
Correct ports assigned

MAC Learning

Endpoint MAC learned
Correct VLAN associated
Correct interface associated
MAC flapping checked

Trunk

Required trunk identified
Trunk operational
Both ends compared
Required VLAN allowed
VLAN active on trunk
Native VLAN compared

Spanning Tree

STP state reviewed
Unexpected blocking investigated
Topology changes reviewed
Layer 2 loops considered
MAC flapping investigated

Gateway

Gateway interface operational
Correct VLAN mapping verified
Router/SVI configuration checked
Gateway reachable

Wireless

SSID VLAN mapping checked
AP uplink configuration checked
Wireless VLAN allowed across trunks
Gateway reachable

Monitoring

SNMP checked
Syslog reviewed
Interface alerts reviewed
STP events reviewed

Remediation

Root cause identified
Current configuration documented
Proposed change documented
Risk considered
Rollback considered
Minimum required change implemented

Verification

VLAN configuration verified
Trunk configuration verified
MAC learning verified
Gateway connectivity verified
Same-VLAN connectivity verified
Cross-switch connectivity verified
Other VLANs tested
Monitoring healthy

Closure

Root cause documented
Corrective action documented
Preventive action documented
Users confirmed recovery
Incident closed

79. Incident Documentation Template

# VLAN and Trunk Incident

## Incident Information

Incident ID:

Date:

Start Time:

Resolution Time:

Severity:

## Business Impact

## Affected Users

## Affected VLANs

## Affected Switches

## Reported Symptoms

## Incident Scope

## Expected Network Path

## Recent Changes

## Initial Evidence

## Access Port Investigation

## VLAN Investigation

## MAC Address Investigation

## Trunk Investigation

## Allowed VLAN Investigation

## Native VLAN Investigation

## Spanning Tree Investigation

## Gateway Investigation

## Root Cause

## Corrective Action

## Verification

## Monitoring Status

## Preventive Recommendation

## Escalation

## Final Status

RESOLVED / ESCALATED

80. Preventive Recommendations

Depending on the root cause, consider:

Configuration Backups

Automated Configuration Comparison

Change Approval

Post-Change Testing

VLAN Standards

Trunk Standards

Allowed-VLAN Documentation

Native-VLAN Standards

STP Monitoring

MAC-Flapping Alerts

Configuration Templates

81. Example Preventive Action

Instead of:

Improve monitoring.

write:

Configure monitoring to alert when VLAN 10 disappears
from the allowed VLAN list on the SW01-to-SW02
production trunk.

Specific recommendations are operationally useful.

82. Runbook Success Criteria

The runbook is successfully completed when:

Affected VLAN
=
Operational

Access Ports
=
Correct

Required Trunks
=
Operational

Allowed VLANs
=
Correct

MAC Learning
=
Normal

Gateway Connectivity
=
Operational

and:

Root Cause
=
Documented

Runbook Review

VLAN troubleshooting should never begin with:

"The trunk is probably broken."

Instead determine:

Is the endpoint connected?
        ↓
Is the port correct?
        ↓
Is the VLAN correct?
        ↓
Does the VLAN exist?
        ↓
Is the MAC learned?
        ↓
Does traffic need a trunk?
        ↓
Is the trunk operational?
        ↓
Is the VLAN allowed?
        ↓
Is STP forwarding?
        ↓
Is the gateway available?

This transforms a vague Layer 2 problem into a sequence of testable questions.

A trunk can be:

UP

while one VLAN remains:

DOWN

from the user’s perspective.

Similarly:

VLAN Exists

does not automatically mean:

VLAN Can Traverse the Network

The professional troubleshooting approach is therefore:

Expected State
      ↓
Actual State
      ↓
Difference
      ↓
Evidence
      ↓
Root Cause
      ↓
Controlled Remediation
      ↓
Validation

The purpose of VLAN and trunk troubleshooting is not merely to restore connectivity. It is to identify exactly where Layer 2 forwarding differs from the intended network design, correct the minimum required configuration, and prove that segmentation and connectivity remain correct after remediation.

What’s Next?

Runbook 03 — DHCP and IP Addressing Troubleshooting

The next runbook moves from Layer 2 connectivity into endpoint network configuration and address-assignment failures.

You will troubleshoot:

missing IPv4 configuration
APIPA addresses
incorrect static addressing
incorrect subnet masks
incorrect default gateways
DHCP scope exhaustion
DHCP server failures
DHCP relay failures
incorrect DHCP options
duplicate IP addresses
lease problems
VLAN-to-DHCP relationships
DHCP packet analysis
DORA failures
remediation and verification

The operational progression becomes:

Layer 2 Connectivity
        ↓
VLAN / Trunk Validation
        ↓
IP Addressing
        ↓
DHCP
        ↓
Gateway Configuration
        ↓
Root Cause
        ↓
Remediation
        ↓
Validation

➡️ Next: Runbook 03 — DHCP and IP Addressing Troubleshooting