Switching query

Hey i am using 4 vlans in a switch namely vlan 10 ,vlan 20 ,vlan 30,vlan 40
and vlan 50
I am assigning vlan 10 to switch port fa 0/1
vlan 20 to fa 0/2
vlan 30 to fa 0/3
vlan 40 to fa 0/4
vlan 50 to fa 0/5

when i am configuring anything for a particular vlan can we use interface
vlan command or else inteface fa command

For example i want to configure ospf authentication for vlan 20 means we
have to go on interface vlan 20 or else int fa 0/2

int fa 0/2
ip ospf authentication message-digest
ip ospf mesaage-digest-key 1 md5 cisco

or else

int vlan 20
ip ospf authentication message-digest
ip ospf mesaage-digest-key 1 md5 cisco

Confusion between interface vlan and int fa commands

pls clear my head???

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