A router is configured with 'router ospf 1', 'network 192.168.5.0 0.0.0.255 area 0', and 'passive-interface GigabitEthernet0/1', where Gi0/1 is in the 192.168.5.0/24 subnet. What is the effect of the passive-interface command on this interface?
- AOSPF hello packets continue normally on Gi0/1, but the router refuses to install any OSPF-learned routes from that interface
- BOSPF hello packets are suppressed on Gi0/1, preventing neighbor adjacencies there, but the subnet is still advertised in OSPF LSAs
- CThe interface is administratively shut down until passive-interface is removed
- DThe 192.168.5.0/24 subnet is removed from OSPF and no longer appears in any router's routing table
Show answer & explanationAnswer & explanation
Correct answer: B. OSPF hello packets are suppressed on Gi0/1, preventing neighbor adjacencies there, but the subnet is still advertised in OSPF LSAs
The 'passive-interface' command stops OSPF from sending or receiving hello packets on the specified interface, so no neighbor relationships can form there. However, because the interface's network is still included in a 'network' statement, its subnet continues to be advertised as a stub network in OSPF's LSAs to other routers.
Why the other options are wrong
- A. Hello packets are suppressed entirely, not just ignored for route installation.
- C. passive-interface does not shut down the physical or logical interface state.
- D. The subnet is still advertised; only hello traffic (and thus adjacency formation) is suppressed on that interface.
OSPF Passive-Interface
The 'passive-interface' command under an OSPF process prevents hello packets from being sent or received on a specified interface, stopping neighbor adjacencies there while still allowing the interface's connected subnet to be advertised.
- Common use: LAN-facing interfaces with hosts, not other routers
- Command syntax: passive-interface <interface> under router ospf process
- Subnet still appears in LSAs as a stub network, just without adjacency formation on that link
Memory trick: Passive means quiet on hellos, but still tells the world about the subnet.