Your question is not very clear. Are you talking about native VLAN on a dot1q trunk? If so, use the keyword native under the sub-interface as shown below
interface GigabitEthernet49
no ip address
no ip directed-broadcast
!
interface GigabitEthernet49.1
encapsulation dot1Q 1 native<<<<<<<<<<<------
ip address 10.1.1.1 255.255.255.0
no ip directed-broadcast
!
interface GigabitEthernet49.2
encapsulation dot1Q 2
ip address 20.1.1.1 255.255.255.0
no ip directed-broadcast
