configure terminal
!
Use the command below to add a Read-Only community string:
!
snmp-server community public RO
!where "public" is the Read-only community string.
!To add a Read-Write Community string, use the command below:

!snmp-server community private RW
!where "private" is the Read-write community string.
!Exit the configuration mode and save the settings:
exit
!
write memory
!
Building configuration...
[OK]
Router#