zNetLaB › Guided labs › Enterprise
Troubleshoot — native VLAN mismatch on a trunk
Enterprise Intermediate 4 devices 3 checked tasks 100 points Plan lab
Two switches are trunked and the PCs in VLAN 10 work — but the switches log a native VLAN mismatch every minute and untagged traffic leaks between VLAN 1 and VLAN 99. Find the two ends that disagree and make them match. You build it in a browser, type real commands at each device, and every task below is
checked against the running configuration — so you find out it is wrong before anybody else does.
Open this lab in the simulator
The simulator runs in your browser with nothing to install. This lab is part of the full simulator: open it free for 7 days with a new account, then with a plan. It needs an account, which keeps your progress.
The scenario
Two switches are trunked and the PCs in VLAN 10 work — but the switches log a native VLAN mismatch every minute and untagged traffic leaks between VLAN 1 and VLAN 99. Find the two ends that disagree and make them match.
Your role: a network engineer on the enterprise team. Level: Intermediate Done when: all 3 goals show as passed (100 points).
Network status
Devices: 2 switchs, 2 PCs, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. SW-A starts with and VLAN 10, VLAN 99 already configured. SW-B starts with and VLAN 10, VLAN 99 already configured. PC-A is a host with 10.0.10.11/24. PC-B is a host with 10.0.10.12/24. Known problems
The network worked and now does not: the scenario above says what users report. The cause is a configuration fault, not a cable. Find it with show commands (interfaces, routes, neighbours, running-config) before you change anything; fix only what is wrong. Goals
SW-B’s trunk uses native VLAN 99 like SW-A (50 pts) No native VLAN mismatch is reported on SW-A (30 pts) The PCs still reach each other (20 pts) The activity, in order
SW-B’s trunk uses native VLAN 99 like SW-A Why: This is the task that checks your configuration directly: the line must really be there, not just look right. Verify with: show running-config No native VLAN mismatch is reported on SW-A Why: Reading a show command is the core skill of troubleshooting: the output is the truth, not your configuration. Verify with: show interfaces trunk The PCs still reach each other Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.0.10.12 Tips
Work in the order given: later goals depend on the earlier ones. After every change, run the verify command before moving on; do not guess. Stuck? Read the verify output first. It names the interface, neighbour or route that is wrong. Common mistake: Fixing only one end Common mistake: Using VLAN 1 as the native VLAN Common mistake: Ignoring the CDP mismatch message Why this matters
Untagged frames on a trunk belong to the native VLAN, and both ends must agree. A mismatch makes traffic leak between VLANs and is reported by CDP and spanning tree.
What you will be able to do
Read native VLAN on both trunk ends
Make them agree
Explain the security risk of a mismatch
The network
Catalyst 2960-24TT, PC — 4 devices in all, already cabled.
Gi0/1 Gi0/1 Fa0/1 Fa0 Fa0/1 Fa0 SW-A SW-B PC-A 10.0.10.11/24 PC-B 10.0.10.12/24 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
SW-A Catalyst 2960-24TT configure it SW-B Catalyst 2960-24TT configure it PC-A PC 10.0.10.11 / 255.255.255.0 PC-B PC 10.0.10.12 / 255.255.255.0
Cabling
SW-A Gi0/1 ↔ SW-B Gi0/1 SW-A Fa0/1 ↔ PC-A Fa0 SW-B Fa0/1 ↔ PC-B Fa0
What is configured at the start
These are already configured when the lab opens — including, where this is a troubleshooting lab, the fault itself:
SW-A vlan 10
vlan 99
exit
interface Fa0/1
switchport mode access
switchport access vlan 10
exit
interface g0/1
switchport mode trunk
switchport trunk native vlan 99
exit SW-B vlan 10
vlan 99
exit
interface Fa0/1
switchport mode access
switchport access vlan 10
exit
interface g0/1
switchport mode trunk
exit Tasks, and how each one is checked
Every task is decided by reading the live network — a neighbour in FULL, a ping that succeeds, a line in the running configuration — not by looking for a keyword you typed. The same command shows you what the lab sees.
# Task Pts How to verify it yourself
1 SW-B’s trunk uses native VLAN 99 like SW-A 50 On SW-B: show running-config contains the line(s) matching interface GigabitEthernet0/1 … switchport trunk native vlan 99. 2 No native VLAN mismatch is reported on SW-A 30 On SW-A: show interfaces trunk output matches Gi0/1\s+\S+\s+\S+\s+\S+\s+99. 3 The PCs still reach each other 20 On PC-A: ping 10.0.10.12 must succeed.
Stuck? Ask the coach
Tell the coach what you see or what you tried. It gives you a hint, not the answer, the way a senior engineer would. Included with a free trial or any plan.
Ask the coach
Common mistakes
Fixing only one end
Using VLAN 1 as the native VLAN
Ignoring the CDP mismatch message
Reference solution
Try it yourself first — the lab checks your work either way. This is one configuration that makes every task pass, device by device:
SW-B interface g0/1
switchport trunk native vlan 99
exit Where this is examined
CCNA (trunking), CCNP ENCOR, Network+.
This lab practises these topics in the zNetLaB certification courses:
Related labs
Where to start
The lessons all guided labs plans
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