Mastering Layer 2 Troubleshooting Commands: A Complete Guide with CLI & EVE-NG Labs [CCNP Enterprise]

Today we’re diving into something every network engineer must master—Layer 2 Troubleshooting. Whether you’re preparing for CCNP or resolving critical enterprise network issues, knowing the right CLI commands can make all the difference. In this post, we’ll walk through key Layer 2 concepts, explore essential commands, test real-world use cases, and build a compact EVE-NG lab to practice everything hands-on.

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Theory in Brief

What is Layer 2 in Networking?

Layer 2, also known as the Data Link Layer, handles frame transmission within the same network segment. It involves MAC addressing, switching, VLANs, and protocols like STP (Spanning Tree Protocol).

Why Troubleshoot Layer 2?

Most network issues—like loops, broadcast storms, or VLAN mismatches—originate at Layer 2. Quick diagnostics can save hours of downtime.

What You Need to Know

To troubleshoot effectively, you must understand:

• MAC Address Table
• Spanning Tree States
• Port Security
• VLAN Configuration
• Interface and Trunking Modes

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Comparison: Common Layer 2 Issues

Issue Type	Symptoms	CLI for Detection	Pros	Cons
VLAN Mismatch	No connectivity	show vlan, show int trunk	Quick to fix	Can break inter-VLAN routing
STP Loop	Broadcast storm, slow net	show spanning-tree	Protects from loops	Misconfig = blocked ports
Port Security	Port shut down unexpectedly	show port-security	Prevents MAC flooding	Can block legit users
Duplex Mismatch	Packet drops, low throughput	show interface	Easy to detect	Performance issues
MAC Table Issues	Flapping or stale entries	show mac address-table	Pinpoints MAC movement	Doesn’t show physical path

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Essential CLI Commands

Purpose	CLI Command
Check interface status	show interfaces status
Verify VLAN configuration	show vlan brief
Inspect port mode	show interfaces switchport
Review MAC address table	show mac address-table
Check trunking interfaces	show interfaces trunk
Display STP details	show spanning-tree
Examine port security	show port-security interface
Debug Layer 2 issues	debug spanning-tree events
View BPDU traffic	debug spanning-tree bpdu receive
Clear MAC table	clear mac address-table dynamic

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Real-World Use Cases

Scenario	Common Commands Used
Access port not forwarding traffic	show interfaces switchport, show vlan
User intermittently disconnected	show port-security, show mac address
Slow network during file transfer	show interface, show spanning-tree
No ping between devices on same VLAN	show vlan, show int trunk, show mac
Loop detected in network	show spanning-tree, debug spanning-tree

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EVE-NG LAB Setup

Topology Diagram:

Configuration Samples:

Access Switch:

hostname SW1
interface range fa0/1 - 2
 switchport mode access
 switchport access vlan 10
 spanning-tree portfast
 no shutdown

interface vlan 10
 ip address 192.168.10.1 255.255.255.0
 no shutdown

Distribution Switch:

hostname SW2
interface fa0/1
 switchport mode trunk

interface fa0/2
 switchport mode trunk

vlan 10
 name Users

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Troubleshooting Tips

Problem	Likely Cause	Suggested Action
Port in err-disabled state	Port security violation	Use errdisable recovery or re-enable
STP blocking legitimate port	Wrong root bridge config	Change STP priority
No MAC entry for device	VLAN not assigned	Check show vlan, assign correct VLAN
Trunk link not forwarding VLANs	VLAN not allowed	Add VLANs to trunk with switchport trunk allowed vlan
End device can’t ping gateway	SVI down or VLAN issue	Check interface vlan <id>, VLAN status

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FAQs (Frequently Asked Questions)

1. What are the most commonly used Layer 2 troubleshooting commands on a Cisco switch?

Answer:
The essential Layer 2 troubleshooting commands include:

• show mac address-table – View MAC learning
• show vlan brief – Verify VLAN existence and assignment
• show interfaces status – Check interface link and mode
• show spanning-tree – Check STP state and port roles
• show cdp neighbors – Discover directly connected Cisco devices
  These commands provide a quick snapshot of switch health and Layer 2 status.

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2. How do I identify VLAN misconfigurations on a switch?

Answer:
Use:

show vlan brief

This command shows:

• All configured VLANs
• Ports assigned to each VLAN
  If a port is not in the correct VLAN or a VLAN is missing, it may cause devices in the same subnet to fail to communicate.

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3. How can I verify MAC address learning on a switch port?

Answer:
Run:

show mac address-table interface <interface>

Example:

show mac address-table interface Gig0/1

This displays all MACs learned on that port. If none are listed and the port is up, it may indicate a connectivity issue or wrong VLAN.

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4. What command shows if a switch port is blocking due to STP?

Answer:
Use:

show spanning-tree interface <interface>

It shows the STP state of the port—forwarding, blocking, or learning. If a port is blocking, it may be due to a loop or incorrect STP priority settings.

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5. How do I find physical layer issues on a switch port?

Answer:
Use:

show interfaces <interface>

Look for:

• CRC errors
• Input/output errors
• Duplex mismatches
  These indicators help pinpoint physical cabling or speed/duplex negotiation issues.

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6. How can I detect Layer 2 loops in a switched network?

Answer:
Look for:

• High CPU usage on switches
• MAC flapping (same MAC appearing on multiple ports)

Use:

show mac address-table | include <MAC>
show logging | include LOOP

Also check STP status using:

show spanning-tree

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7. What does the “err-disabled” port status mean and how do I recover it?

Answer:
A port in err-disabled state has been shut down automatically due to a violation (e.g., BPDU guard, port-security, loopback).
Check with:

show interfaces status err-disabled
show errdisable recovery

To recover manually:

shutdown
no shutdown

Or enable automatic recovery:

errdisable recovery cause all

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8. How do I verify trunk link configurations?

Answer:
Use:

show interfaces trunk

It displays:

• Trunking interfaces
• Allowed VLANs
• Native VLAN

Mismatched trunk settings can cause VLAN leakage or dropped frames.

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9. How can I verify Layer 2 connectivity between switches?

Answer:
Use:

show cdp neighbors

It shows directly connected Cisco devices and their interfaces. To test Layer 2 reachability, also verify matching VLANs and STP status on trunk links.

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10. What are some Layer 2 issues that can break inter-VLAN routing or gateway reachability?

Answer:
Common causes include:

• Host port assigned to wrong VLAN
• Trunk link missing allowed VLANs
• Native VLAN mismatch
• STP blocking key uplink ports
• MAC address not being learned due to port-security
  To troubleshoot:
• Use show vlan, show interfaces trunk, and show spanning-tree
• Confirm the gateway IP is reachable using ping from different VLANs

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YouTube Link

Watch the Complete CCNP Enterprise: Mastering Layer 2 Troubleshooting Commands: A Complete Guide with CLI & EVE-NG Lab Demo & Explanation on our channel:

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Final Note

Understanding how to differentiate and implement Mastering Layer 2 Troubleshooting Commands: A Complete Guide with CLI is critical for anyone pursuing CCNP Enterprise (ENCOR) certification or working in enterprise network roles. Use this guide in your practice labs, real-world projects, and interviews to show a solid grasp of architectural planning and CLI-level configuration skills.

If you found this article helpful and want to take your skills to the next level, I invite you to join my Instructor-Led Weekend Batch for:

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