Hello,
If an untagged frame is received on a trunk port, it will be assigned to the native VLAN configured on that trunk port. Therefore, if you want to drop received untagged frames on a trunk port, exclude the native VLAN from the list of allowed VLANs on that trunk using the command
switchport trunk allowed vlan remove X
Note that by default, the native VLAN is 1 and older switches do not allow removing the VLAN1 from the allowed VLAN list. In this case, you may need to change the native VLAN to an unused VLAN and remove it from the trunk.
Carl, one more thing. I have noticed that you are frequently posting questions on the NetPro and many great and knowledgeable people here are striving to give you the best possible answers, however, you do not seem to rate their answers. I am certainly not the one to go around begging for ratings but from what I have seen here, many people have really tried to help you so I believe they deserve their ratings from you.
Best regards,
Peter
