DTP: The One Negotiation Combination That Silently Does Nothing
Both switch ports are dynamic auto, so neither proposes a trunk and one show command prints nothing at all. Diagnose it and fix it yourself, live, right below.
Symptom
Both switchports are up, but the link never becomes a trunk. Diagnose why VLAN 1 works while VLANs 10, 20, and 99 stop at the boundary, then fix the one passive DTP combination.
Your mission
Prove the cause, then fix it
Format
Guided CLI investigation
Investigate before you configure.
Use the CLI evidence to isolate the fault, make the smallest safe correction, then verify the network state changed.
Observe the symptom and link state.
Diagnose by comparing the protocol evidence.
Verify the expected device state and confirm the original symptom is resolved.
Console access: SW1 · SW2
This workspace stacks for portrait phones. Rotate to landscape for the full split-screen console.
Need a hint?
Reveal the root cause when you're ready.
Try the investigation first, then use this as your escape hatch, not a dead end.
Lab debrief
Turn the session into a repeatable troubleshooting pattern.
Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.
The problem
SW1 and SW2 are connected on Gi0/1. VLANs 10, 20, and 99 exist on both switches, but both interfaces are set to switchport mode dynamic auto. The link is up in access VLAN 1, so VLAN 1 traffic works while the other VLANs cannot cross.
What you'll practice
- Recognise when an up switchport is operating as access instead of trunk
- Use the blank show interfaces trunk result as evidence that no port is trunking
- Compare administrative mode, operational mode, encapsulation, and VLAN evidence on both switches
- Make one side actively form a trunk and verify VLANs 10, 20, and 99 are carried
1. Why auto plus auto is silent
Dynamic auto accepts a trunk proposal, but it does not send one. When both ends wait, IOS leaves the connected link in static access mode without a dramatic error, which makes this fault easy to mistake for a VLAN problem.
2. The blank trunk output is evidence
show interfaces trunk prints no sections when the switch has no trunking interface. That empty result is the first useful fact, not a missing screen or an incomplete command.
3. A trunk must exist before its parameters matter
Allowed VLANs and dot1q encapsulation describe a trunk, but neither setting asks DTP to form one. First make one end trunk or dynamic desirable, then verify that VLANs 10, 20, and 99 are active and forwarding.
The topology
SW1 and SW2 are directly connected on Gi0/1. Both switches contain VLANs 10 SALES, 20 ENGINEERING, and 99 MGMT. Before the fix, both Gi0/1 ports are dynamic auto and operate as static access in VLAN 1. After one side becomes an active trunk, the peer forms a trunk and carries the required VLANs.
Commands to run yourself
The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.
show interfaces trunkshow interfaces GigabitEthernet0/1 switchportshow vlan briefTopology diagram
Frequently asked
Why does show interfaces trunk print nothing?
Because neither Gi0/1 is operating as a trunk. The adapter models the IOS result as a genuinely empty response, while show interfaces GigabitEthernet0/1 switchport reveals dynamic auto on both ends and static access as the operational mode.
Why does VLAN 1 work when the link is broken?
Both ports are connected in access VLAN 1, and show vlan brief places Gi0/1 in the VLAN 1 Ports column. VLANs 10, 20, and 99 are present locally but cannot cross an access link as tagged traffic.
Should I use dynamic desirable or an explicit trunk?
Both work in this lab. Dynamic desirable sends a proposal that dynamic auto accepts. An explicit switchport mode trunk is the clearer production choice, and switchport nonegotiate can be added when you want a statically defined trunk without DTP.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.