Ticket#3 – VLAN Mismatch: End Users Unable to Reach Gateway – A Real Cisco Fix [CCNP ENTERPRISE]

Problem Summary

Multiple end-users on the 3rd floor raised a complaint that they were unable to reach the default gateway. They couldn’t access internal resources or the internet. The issue was isolated to a single switch block, while other floors remained unaffected.

The NOC team initially suspected DHCP or routing issues, but the problem turned out to be a classic VLAN mismatch—a silent killer in Layer 2 environments.

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Symptoms Observed

• Users couldn’t ping the gateway (e.g., 192.168.10.1)
• No DHCP address received; APIPA (169.x.x.x) was assigned
• show cdp neighbors revealed correct uplink to the distribution switch
• The trunk was up/up, but no MACs learned on user ports
• Other floors using the same VLAN ID were unaffected

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Root Cause Analysis

After Layer 1/2 checks, the following was found:

• Access switch ports were assigned to VLAN 10 (user VLAN)
• But the uplink trunk to distribution switch did not allow VLAN 10
• On the distribution switch, VLAN 10 was mapped to a different subnet
• There was a mismatch in VLAN configuration, i.e.,
  • VLAN 10 on Access Switch ≠ VLAN 10 on Distribution Switch

In short, same VLAN ID but different purpose, resulting in broken L2 forwarding.

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The Fix

1. Verify VLAN Status
   • Used show vlan brief and show interfaces trunk on both switches
2. Validate Allowed VLANs
   • Confirmed VLAN 10 was not in the allowed vlan list on the trunk port
3. Match VLAN Naming Convention
   • Found VLAN 10 was named “Voice” on the distribution switch
     while access switch used it for “Users”
4. Reconfigure VLAN and Trunk
   • Changed user ports to correct VLAN (VLAN 20)
   • Updated trunk ports to carry correct VLANs across the link
5. Add Missing VLAN if Needed
   • Created VLAN on upstream switch: vlan 20 → name Users

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EVE-NG Lab Topology

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CLI Fix Simulation

Access Switch:

interface range fa0/1 - 24
 switchport mode access
 switchport access vlan 20

Trunk Check:

show interfaces trunk

Trunk Allow Fix:

interface gi0/1
 switchport trunk allowed vlan add 20

Create VLAN:

vlan 20
 name Users

Verify:

show vlan brief
show mac address-table

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Verification

Step	Command	Expected Output
VLAN existence check	show vlan brief	VLAN 20 listed on both switches
Trunk port status	show interfaces trunk	VLAN 20 allowed
Ping from user PC	ping 192.168.20.1	Successful
MAC learning check	show mac address-table	MACs learned in VLAN 20
DHCP allocation	show ip dhcp binding	IPs assigned from correct subnet

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Key Takeaways

• VLAN mismatch is often a silent issue with no interface errors.
• Trunk ports must carry all necessary VLANs.
• Never assume same VLAN ID means same purpose — always use naming conventions.
• Check both access and distribution layers for alignment.
• VLANs not being passed = broken L2 domain = no gateway reachability.

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Best Practices & Design Tips

• Always use unique VLAN IDs per function (Data, Voice, etc.)
• Apply naming conventions to VLANs (Users, Voice, Server) to avoid confusion
• Enable VTP in transparent mode or use VLAN databases carefully
• Use port-based VLAN monitoring tools (NetFlow, SPAN)
• Regularly audit trunk port configurations

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FAQs – VLAN Mismatch Troubleshooting

1. What is a VLAN mismatch?

Answer:
It occurs when two ends of a trunk or port carry different VLANs or VLANs with the same ID but different purposes, breaking Layer 2 forwarding.

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2. Why didn’t CDP or port status indicate any issue?

Answer:
Because physical link and trunking (802.1Q) was operational, but Layer 2 was misaligned, so no errors were generated.

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3. Can same VLAN ID be used for different functions?

Answer:
Yes, but it’s not recommended. Use consistent ID and name conventions across the network.

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4. How do I know if a VLAN is missing on a switch?

Answer:
Use:

show vlan brief

If the VLAN isn’t listed, it’s missing.

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5. What happens if a VLAN is not allowed on a trunk?

Answer:
Traffic for that VLAN is not forwarded between switches, causing end-users to lose access to upstream resources.

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6. How do I allow a VLAN on a trunk?

Answer:

interface gi0/1  
switchport trunk allowed vlan add 20

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7. Will mismatched native VLAN cause this issue?

Answer:
Not exactly. Native VLAN mismatches cause CDP warnings and potential tagging issues, but not silent breakage like this.

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8. Why did users get APIPA addresses?

Answer:
Because DHCP packets could not reach the server due to the VLAN isolation, so clients self-assigned 169.x.x.x.

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9. How to prevent VLAN mismatches in future?

Answer:

• Document VLAN ID vs purpose
• Use consistent VLAN names
• Run regular audits with show interfaces trunk

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10. Should I use VTP to avoid such issues?

Answer:
Not necessarily. VTP has its own risks (VTP bombs). Better to manually manage VLANs or use automation tools.

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11. Can this issue affect VOIP phones?

Answer:
Yes. If voice VLANs are misaligned or not passed, phones won’t register with call manager.

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12. Is this common in new switch deployments?

Answer:
Yes. It often happens when switches are cloned, copied, or not properly documented.

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YouTube Video

Watch the Complete CCNP Enterprise: VLAN Mismatch: End Users Unable to Reach Gateway – A Real Cisco Fix Demo & Explanation on our channel:

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Final Note

Understanding how to differentiate and implement VLAN Mismatch: End Users Unable to Reach Gateway – A Real Cisco Fix 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.

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