What is meant by Trunks Belong to All Vlans and what command if not "show interfaces trunk" would I use to verify this?

[GillverK]

Was taking a few practice exams and noted something peculiar which I couldn't quite grasp.

So on a test the answer to one question was "To which Vlan does a trunk belong?" for which the answer was "Every Vlan the switch knows about." Beyond that being a badly asked question in my opinion, it goes on to explain the ""By default," a trunk belongs to every Vlan the switch knows about"... then "default" should be in the original question!

So my question is, he answer is pretty keen on the fact that it belongs to every Vlan the switch "knows about" and "by default" so I was wondering exactly what this meant especially regarding the following. 

So is the question suggesting that we're accounting for "legacy" systems and explicitly disregarding 8021Q's Native VLAN setting? I mean, to me setting the Native VLAN would constitute what VLAN a trunk belongs to, which is why the original question imo should have included the word default. But beyond that, I decided to experiment in Packet Tracer by creating VLANs 2-5 and not setting a native vlan on a trunk. As expected the native vlan was listed as 1 but there would always be a change inthe numbers of Vlans list in "Vlans in spanning tree forwarding state and not pruned" after I did a shut/no shut on the interface. When I switch the Native Vlan to 2, and created VLANs 5 and 6 after, "Vlans in spanning tree forwarding state and not pruned" would only increase after I did a shut/no shut but nothing really alerted me to anything, regarding either scenario, that indicated that the Trunk belonged to all the vlans the switch knew about even if it was in the default vlan (vlan 1).

So is there another command I could use to make me see that when you create new vlans, then create a trunk and leave it in the Default VLAN, that can allow you to see that it "belongs" to all the vlans, or is this just a reference to something involved in legacy settings and that 8021q really tags a frame which affects its delivery, but the trunk port still belongs to all vlans, or is it that the question is just poory asked, or something else. 

Thanks for answering, guys and gals.

[Giuseppe Larosa · Accepted solution]

Hello GilverK,

the behaviour of real switches is different they don't need any shut/no shut cycle to update list of Vlans allowed in forwarding state and not pruned.

A trunk by defaults allows all defined Vlans on the switch (unless using the switchport trunk allowed vlan command to limit the number of Vlans that can pass over it)

This is the default behaviour on Cisco switches.

We cannot say that a trunk port belongs only to its native Vlan, as the native Vlan in 802.1Q is just the Vlan for untagged traffic.

the correct answer should be that a trunk is a member of all the vlans permitted over it.

Hope to help

Giuseppe

[GillverK]

Thanks a lot @Giuseppe Larosa . One last question, though. So you're saying that by "belongs to" the question is really asking what VLANs are allowed through by Default, or are you saying something else? My main hang up is with the question's use of "belongs to" honestly.

[Giuseppe Larosa]

Hello GilverK,

>> One last question, though. So you're saying that by "belongs to" the question is really asking what VLANs are allowed through by Default, or are you saying something else? My main hang up is with the question's use of "belongs to" honestly.

A trunk port does not belong to a single Vlan it is member of all Vlans permitted over it.

By default all Vlans defined on the switch are allowed on a trunk so the answer follows this logic.

Hope to help

Giuseppe

[GillverK]

Ok. No problem.

Thank you.