question on: switchport trunk allowed vlan

[danny9797]

Hi everyone,

I wouldn't mind getting some clarification on this commant.

By default, a trunk carries ALL Vlans.

My basic question is, if I use the "switchport trunk allowed vlan 4" command, will the trunk DROP traffic from every Vlan except 3?

Eg,

interface GigabitEthernet5/34

description cisco

switchport trunk allowed vlan 109

switchport mode trunk

If I configure the above port in this manner, am I only allowing Vlan109 traffic through?

Thanks

[cadet alain]

Hi,

yes you only enable vlan 109 out the trunk port.

Regards.

Alain

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[danny9797]

Hi Alain,

I appreciate the response because I read differently on other documents and websites.  I read the following comman sets would be required in order to just allow Vlan 109 out of the trunked interface:

interface GigabitEthernet5/34

description cisco

switchport trunk allowed vlan remove 1-108, 110-4094

switchport trunk allowed vlan 109

switchport mode trunk

I assumed the allowed vlan xx command would automatically discard traffic traversing that given interface which does not reside in that specified vlan.

Dan

[Steve Chapman]

you are correct once you enter the command it will remove all vlans.  Just remember if you need to add a 2 or 3 make sure you use the add commnad

interface GigabitEthernet5/34

switchport trunk allowed vlan ADD 110

This would add vlan 110 to the existing vlan 109.

Steve

[cadet alain]

Hi,

you don't need to remove the vlan  first.

Regards.

Alain

Don't forget to rate helpful posts.

[danny9797]

Thank you for the clarification.