encapsulation dot1q command
B1# show running-config
! Only pertinent lines shown
interface gigabitethernet 0/0
! No IP address up here! No encapsulation up here!
!
interface gigabitethernet 0/0.10
encapsulation dot1q 10
ip address 10.1.10.1 255.255.255.0
!
interface gigabitethernet 0/0.20
encapsulation dot1q 20
ip address 10.1.20.1 255.255.255.0