Passive Interfaces allow you to include an IP subnet in the Dynamic routing protocols without sending updates on that interface.
If we configure FE2/0 as a passive interface, RA and RB will learn routes to 10.0.2.0 via their IGP adjacencies, but internal information will not be transmitted to RC.
It is best practice to configure Loopback Interfaces as passive interfaces. This is because a loopback interface cannot form an adjacency, since they are not physical interfaces; making the loopback passive means that it will be advertised by the protocol but it will not send or listen for hello packets.
To enable a passive interface, go into the router routing protocol prompt:
R1(config)#router rip
R1(config-router)# version 2
R1(config-router)# no auto
R1(config-router)#no auto-summary
R1(config-router)# passive-interface loop0
R1(config-router)# passive-interface f2/0
If more of our routes are passive we can use a passive interface default and then set the non-passives individually:
R1(config-router)#passive-interface default
R1(config-router)#no passive-interface f0/0
R1(config-router)#no passive-interface f1/0