switchport trunk allowed vlan

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Nov 2nd, 2007
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What is the basic mean of that command. We have configure that command to our core switch trunkports but no to edge switch. Our edge switches are transparents. We suppose that it stops broadcasts to spread from core to edge. Should we configure that command to edge too. What is the basic meaning of that command. Availability or secure meaning. Is it reasonable to use it in vtp transparent edge switch. Any idea.

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glen.grant Fri, 11/02/2007 - 01:27
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This command determines what vlans are allowed across a trunk link . If it is not allowed then you cannot have that vlan on a edge switch and expect to get to it. Myself I think it is good practice though it is bit extra work to do it on all links . We do use it down to all access or edge switches .

Jon Marshall Fri, 11/02/2007 - 03:33
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As Glen says if you need to to access a vlan from the core at the edge then you need to allow it on the trunk.

It is best practice to make sure that both ends of the trunk are allowing the same vlans.

What the command does is to stop all traffic for that vlan crossing the trunk, not just broadcasts. It also limits the STP diameter for that vlan. This is as opposed to vtp pruning which does stop unnecessary broadcast traffic being passed down a trunk for a vlan that the the switch has no ports for. But with VTP pruning you are still running STP across all the switches.




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