@glen.grant wrote: You need to specifically set the encapsulation first , "switchport encapsulation dot1q" then issue your trunk command... specially in cisco 3500 series..   Switch (config-if) # switchport trunk encapsulation ( dot1q | isl ) If you...

specially in cisco 3500 series..   Switch (config-if) # switchport trunk encapsulation ( dot1q | isl ) If you are using cisco use command #switchport trunk encapsulation isl #switchport mode trunk   OR ( multi-vendors like hp, juniper & also for cisc...