vlan 1

port 21 untagged

exit

vlan 1001
 port 21 tagged
 port 18-19 untagged
 exit

port 18-19

pvid 1001

exit

port 21

pvid 1

exit

设置port 21口为trunk口，且允许vlan1001通过。18、19口为access口

interface gigabitethernet0/21
 switchport mode trunk
 switchport trunk allowed vlan add 1001
 switchport trunk pvid vlan 1
 exit