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Troubleshoot — the trunk is up but one VLAN never arrives
Enterprise Intermediate 6 devices 3 checked tasks 100 points Plan lab
Two switches, a trunk between them showing connected, VLAN 10 on both sides — and the PCs in VLAN 10 cannot reach each other across it. VLAN 20 works fine. A trunk can be up and still not carry a VLAN: find out why, on which side, and fix the list. 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.
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The scenario
Two switches, a trunk between them showing connected, VLAN 10 on both sides — and the PCs in VLAN 10 cannot reach each other across it. VLAN 20 works fine. A trunk can be up and still not carry a VLAN: find out why, on which side, and fix the list.
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, 4 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 20 already configured. SW-B starts with and VLAN 10, VLAN 20 already configured. PC-10-A is a host with 10.0.10.11/24. PC-10-B is a host with 10.0.10.12/24. PC-20-A is a host with 10.0.20.11/24. PC-20-B is a host with 10.0.20.12/24. 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
The trunk on SW-A carries VLAN 10 and VLAN 20 (40 pts) The two VLAN 10 PCs reach each other across the trunk (40 pts) VLAN 20 keeps working (20 pts) The activity, in order
The trunk on SW-A carries VLAN 10 and VLAN 20 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 two VLAN 10 PCs reach each other across the trunk 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 VLAN 20 keeps working Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.0.20.12 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: “switchport trunk allowed vlan 10” replacing the whole list and dropping every other VLAN Common mistake: Fixing only one end of the trunk Common mistake: Forgetting the VLAN must also exist on both switches Why this matters
A trunk can be up and still not carry a VLAN: the allowed list is explicit. The symptom is “this VLAN works on one switch and nowhere else” while other VLANs cross fine.
What you will be able to do
Read show interfaces trunk (allowed and active VLANs)
Fix the allowed list with add rather than replacing it
Verify both VLANs across the trunk
The network
Catalyst 2960-24TT, PC — 6 devices in all, already cabled.
Gi0/1 Gi0/1 Fa0/1 Fa0 Fa0/2 Fa0 Fa0/1 Fa0 Fa0/2 Fa0 SW-A SW-B PC-10-A 10.0.10.11/24 PC-10-B 10.0.10.12/24 PC-20-A 10.0.20.11/24 PC-20-B 10.0.20.12/24 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-10-A PC 10.0.10.11 / 255.255.255.0 PC-10-B PC 10.0.10.12 / 255.255.255.0 PC-20-A PC 10.0.20.11 / 255.255.255.0 PC-20-B PC 10.0.20.12 / 255.255.255.0
Cabling
SW-A Gi0/1 ↔ SW-B Gi0/1 SW-A Fa0/1 ↔ PC-10-A Fa0 SW-A Fa0/2 ↔ PC-20-A Fa0 SW-B Fa0/1 ↔ PC-10-B Fa0 SW-B Fa0/2 ↔ PC-20-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 20
exit
interface Fa0/1
switchport mode access
switchport access vlan 10
exit
interface Fa0/2
switchport mode access
switchport access vlan 20
exit
interface g0/1
switchport mode trunk
switchport trunk allowed vlan 20
exit SW-B vlan 10
vlan 20
exit
interface Fa0/1
switchport mode access
switchport access vlan 10
exit
interface Fa0/2
switchport mode access
switchport access vlan 20
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 The trunk on SW-A carries VLAN 10 and VLAN 20 40 On SW-A: show interfaces trunk output matches Gi0/1\s+[\d,-]*\b10\b|Gi0/1\s+1-4094. 2 The two VLAN 10 PCs reach each other across the trunk 40 On PC-10-A: ping 10.0.10.12 must succeed. 3 VLAN 20 keeps working 20 On PC-20-A: ping 10.0.20.12 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
“switchport trunk allowed vlan 10” replacing the whole list and dropping every other VLAN
Fixing only one end of the trunk
Forgetting the VLAN must also exist on both switches
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-A interface g0/1
switchport trunk allowed vlan add 10
exit Where this is examined
CCNA (trunking), CCNP ENCOR (Layer 2), Network+ (switching troubleshooting).
This lab practises these topics in the zNetLaB certification courses:
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Where to start
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