Trunk Not Coming Up (DTP Negotiation Failure)
If both switch ports are left at dynamic auto, neither side initiates and the link stays an access port — up, but carrying only one VLAN. A trunk forms only if at least one side actively asks for it. The fix is to stop negotiating and set trunk mode statically.
What you see
SW1# show interfaces gigabitEthernet0/1 switchport Administrative Mode: dynamic auto Operational Mode: static access Trunking Native Mode VLAN: 1 (default)
Why it happens
Dynamic Trunking Protocol (DTP) decides whether a link becomes an access port or a trunk. dynamic desirable actively proposes a trunk; dynamic auto will only accept a proposal. Two ports both set to auto therefore agree to do nothing — the link comes up as an access port in VLAN 1.
The giveaway is the difference between Administrative Mode (what you configured) and Operational Mode (what actually happened). Admin dynamic auto with operational static access is this exact fault.
How to confirm it
show interfaces GigabitEthernet0/1 switchport | include Mode show interfaces trunk show vlan brief
If the port does not appear in show interfaces trunk at all, it is not trunking regardless of what you intended.
The fix
! Configure both ends explicitly and disable DTP interface GigabitEthernet0/1 switchport trunk encapsulation dot1q switchport mode trunk switchport nonegotiate switchport trunk allowed vlan 10,20,30
switchport nonegotiate stops DTP frames entirely. On a link where you know both ends, static trunk plus nonegotiate is faster, deterministic, and removes a known attack surface.
How to stop it happening again
Never rely on DTP in production. Configure access ports as switchport mode access and trunks as switchport mode trunk, both with nonegotiate.
Questions people ask
Why do I need switchport trunk encapsulation dot1q?
Only on older switches that also supported ISL. On modern switches 802.1Q is the only option and the command is rejected as unnecessary — that rejection is not an error.
Can a trunk form if one side is access and the other is trunk?
No, and it is worse than not working: you get a mismatch where one side tags and the other does not, causing dropped VLANs and native VLAN warnings.
Related reading
Vlans Trunking ExplainedHow To Configure A Trunk PortAccess Vs Trunk PortLab Trunk Configuration
Reading the fix is not the same as doing it
Every fault on this page can be broken and repaired on real Cisco routers and switches in our Ahmedabad lab — which is how it stops being theory. Come and see the rack before you pay anything.