if vlan 15 doesn't exists in db
SV3# sh run
... // Pas de vlan 15
SV3# vlan database
SV3# vlan 15
SV3# vtp transparent
SV3# apply
Vlan added !!!!!
Vlan :
SV3>enable
SV3#config t
SV3(config)#interface fastEthernet 0/1
SV3(config-if)#switchport mode access
SV3(config-if)#switchport access vlan 15
SV3(config-if)#ex
Trunk :
SV3>enable
SV3#config t
SV3(config)#interface fastEthernet 0/11
SV3(config-if)#switchport mode trunk
SV3(config-if)#switchport trunk encapsulation dot1q
SV3(config-if)#switchport trunk native vlan 15
SV3(config-if)#end