Native Vlan

[andymorph]

Hi

i understand that on an 802.1q link you need both ends to agree on the native vlan, and that any traffic that isn't tagged will drop into the native vlan.  But whats confusing me is if you've got a network set up then surely your tagging all vlan traffic ?  At what point does the trunk see traffic from somthing thats not tagged.  Is this traffic thats not included in the vlans allowed list?

Also i see that native vlan is there for backwards compatability with kit that might not support 802.1q - so it puts traffic into the native vlan - but then surely your just dealing with an access port as you only have one vlan... -  i've been looking around for some examples but cant find anything that unscrambles my head!

[pwwiddicombe]

Nice description here  https://supportforums.cisco.com/discussion/11700441/what-difference-between-default-vlan-and-native-vlan.

SAY you have a trunk between switches and you don't declare a native vlan (and you don't intentionally use VLAN 1).  Then VLAN 1 traffic will pass from switch to switch with no tag, and all other vlans ARE tagged.

Say you intend to have vlan 10,20,30 passed between switches and you set native vlan 10 on the trunk AT BOTH ENDS.  Then vlan 10 isn't tagged, and 20, 30 will be.  If you'd set the VLANS ALLOWED, then vlan 1 would be blocked if not included.

Remember tagging is normally ONLY between switches; gets added to the packet before leaving one switch (if not native) and gets stripped and redirected appropriately on the "other end" of the link.

If inconsistent tagging is used (i.e. vlan 10 native on one side, and vlan 20 native on the other), I think it's platform dependent...  Some switches won't bring up the trunk at all.  Some older releases then might bleed traffic from one vlan that is native on one end, to the vlan that's native at the other, as there isn't a tag to identify the traffic.