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Webcast-Catalyst9k
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allowed vlan on trunk: filtering in or out only ?

Hi, if I configure the allowed vlans on a trunk, does the switch operate the filter on inbound traffic or on outboud traffic only ?

Does the traffic of not-allowed vlan reach the switch anyway ?

Does the switch partecipate on spanning-tree of not allowed vlan ?

thank you in advance

rs

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Re: allowed vlan on trunk: filtering in or out only ?

r.spiandorello wrote:

Hi, if I configure the allowed vlans on a trunk, does the switch operate the filter on inbound traffic or on outboud traffic only ?

Does the traffic of not-allowed vlan reach the switch anyway ?

Does the switch partecipate on spanning-tree of not allowed vlan ?

thank you in advance

rs

RS

It does both ie. it stops a vlan being transmitted if not on the allowed list and it will drop traffic received for a vlan not on the allowed list.

A switch will run STP if -

1) if has the vlan in it's database

AND  either -

i) it has an active port in that vlan up and runing

or

ii) it has a trunk allowing that vlan

so the switchport trunk allowed vlan command is often used to limit the span of STP but obviously you need to make sure there are no active access ports on the switch for the vlan.

To be honest if STP is the issue i would in most cases simply run VTP transparent, that way you simply don't have the vlan in the vlan database on the switch.

Jon

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