Switch Trunking question

[batumibatumi]

Guys,

If I confiured the port like below:

switchport trunk native vlan 10

switchport trunk allowed vlan 11,12

does the vlan 10 allowed passing ? or it still need add vlan 10 to the allowed vlan list like

switchport trunk native vlan 10

switchport trunk allowed vlan 10,11,12

I know that Native VLAN will transmit between the trunk interfaces but in this particular case DO I NEED TO ALLOW native vlan ?

Thank you in advance !

[Vasileios Bouloukos MSc, ITIL, CCIE]

Hi batumi,

I had a similar question in the past.

If we change native vlan to be 10, then any traffic on vlan 10 that was

leaving the switch would be untagged. Any traffic arriving untagged would be assumed to be on vlan 10. 

You should take into account that there is a bug in all CatOS and early versions of IOS.  In those versions, if you disallow the native VLAN on the trunk, then it also blocks Spanning-Tree BPDUs. This can have a severe effect to your network (L2 Loops)

Please check the next link. You can see what happens in practise when native vlan is not allowed.

http://www.youtube.com/watch?v=Ik7cGfDhV54

Hope that helps!

Vasilis

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