native vlan...

why... because before 802.1q started inserting tags into frames there were original 802.3 ports still hanging out that didn't understand tagging...

to make this work with non 802.1q the ieee created the concept of native vlan... the native vlan supports protocol traffic like cdp, vtp, pagp and dtp... the only of those that are relevant today is cdp (i love cdp)  when there is a native vlan mismatch, cdp v2 complains vociferously... i can stand that for about 7 or 2 seconds before i start losing my mind...  stp also places the port in an inconsistent state...  if you change the native vlan on one side, make sure you match it on the other...

dls1(config-if)#sw trunk native vlan 200                                       
dls1(config-if)#end  

                

sw trunk native vlan x
sw trunk native vlan x
sw trunk native vlan x
sw trunk native vlan x....

on both sides of the trunk...