OSPF Passive Interface

1.Configured on an Interface.
2.Command

Router(config)#router ospf 10
Router(config-router)#passive-interface ?
  Ethernet         IEEE 802.3
  FastEthernet     FastEthternet IEEE 802.3
  GigabitEthernet  GigabitEthernet IEEE 802.3
  Loopback         Loopback interface
  Serial           Serial
  Vlan             Vlan interface
  default          Suppress routing updates on all interfaces
Router(config-router)#passive-interface g0/2 192.168.1.2 

3.What it does ?
 Its a feature you enable on a per interface basis which allows a particular
 interface to participate in a routing process but prevents that interface from
 forming neighbor relationships by not sending hello packets and discarding
 received hello packets.
 Stops sending hello messages down a link.Maybe to restrict hello messages
 being sent down to a L2 Switch.No point sending hello's to a L2 Switch as a
 Router will not form a neighbour relationship with it.
                            Also could be a security risk if someone connected to a
L2 Switch starts replying with fake hello messages and hence forms
neighbour relationships .This could be used to  advertise fake routes.

4.Verification :

             Router#show ip ospf int g0/2
                   >>  No Hellos (Passive interface)