Lessons · Network switches · a trunk that is not trunking

The trunk that is not trunking

A port configured as a trunk still needs the far end to agree, the cable to be right, and the port not to be shut, before any VLAN crosses it.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

'I set both sides to trunk and nothing works' is a real ticket, and the answer is usually one of three things none of which is the trunk command.

How to think about it

Read the status before rereading the configuration. Disabled means somebody shut it, notconnect means no cable or a dead one, and only after those does the trunk configuration matter.

Worked example

enable

The # prompt.

configure terminal

Into configuration mode.

interface gi0/1

The uplink.

switchport mode trunk

Configured as a trunk.

shutdown

And administratively down, which is the fault being reproduced.

end

Out.

show interfaces status

Gi0/1 disabled, trunk. The configuration is perfect and the port is off.

configure terminal

Back in.

interface gi0/1

The same port.

no shutdown

The fix.

end

Out.

show interfaces status

Now it reads notconnect until a cable is in it, which is a different problem with a different answer.

Your turn

Bring the trunk port back up.

no

The trap

Retyping `switchport mode trunk` because the trunk is down. The configuration was never the problem, and typing it again changes nothing.

Practise a trunk that is not trunking on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.