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Native VLAN question

I asked this in another forum, but was hoping for some other explanations...

switchport mode trunk

switchport native vlan 80

switchport trunk allowed vlan 50, 80

Can someone provide a line by line explanation of whats being done?

If I understand correctly, the first line lets ALL vlans through this port. The second line lets all untagged traffic that comes from VLAN 80 through. Line three perplexes me, because if we are trunking the port (letting all VLANs through) why explicitly let these two VLANs through when they are already allowed.

Thank you.

Hall of Fame Super Blue

Re: Native VLAN question


"switchport mode trunk" means configure the link as a trunk link ie. a link that can carry traffic for multiple vlans. By default it will allow all vlans.

"switchport native vlan 80" means the vlan on the trunnk link that will not be tagged will be vlan 80. So all other vlan traffic is tagged but not this vlan.

"switchport trunk allowed vlan 50, 80" means only allow vlan 50 and vlan 80 traffic across this link. There a number of reasons you may want to do this. Perhaps at the other end of the link you know that the switch only has ports in vlan 50 and vlan 80 so there is no need to forward traffic for any other vlan. By not allowing those vlans across the trunk you not only stop broadcast traffic from going across the trunk (which can be achieved with the "vtp pruning" command) but you also stop STP for any other vlans than 50 & 80 across the link.