Which of the following statements correctly describes the behavior of a passive interface in OSPF IPv4?
A) A passive interface in OSPF will prevent OSPF from forming neighbor relationships on that interface but still allows OSPF updates to be sent out.
B) A passive interface in OSPF will suppress OSPF hello packets on that interface, preventing OSPF neighbor formation, but OSPF updates are still advertised on that interface.
C) A passive interface in OSPF will prevent both OSPF hello packets and OSPF updates from being sent or received on that interface.
D) A passive interface in OSPF will prevent OSPF hello packets from being sent out but still allow the interface to receive OSPF hello packets and form neighbor relationships.
Correct Answer:
B) A passive interface in OSPF will suppress OSPF hello packets on that interface, preventing OSPF neighbor formation, but OSPF updates are still advertised on that interface.
Detailed Explanation:
Option A:
A passive interface in OSPF will prevent OSPF from forming neighbor relationships on that interface but still allows OSPF updates to be sent out.
This option is incorrect because, while it is true that a passive interface prevents the formation of OSPF neighbor relationships, it does not send out OSPF updates on that interface. In OSPF, the primary function of a passive interface is to stop the sending of OSPF hello packets, which are essential for neighbor discovery and the formation of OSPF adjacencies. However, the interface will still advertise the network it connects to in OSPF routing updates. Therefore, this option is partially correct but inaccurate in stating that OSPF updates are sent out from a passive interface.
Option B:
A passive interface in OSPF will suppress OSPF hello packets on that interface, preventing OSPF neighbor formation, but OSPF updates are still advertised on that interface.
This option is correct and accurately describes the behavior of a passive interface in OSPF. When an interface is configured as passive in OSPF, it will no longer send OSPF hello packets. Hello packets are used to establish and maintain neighbor relationships. By suppressing hello packets, OSPF ensures that no new neighbor relationships are formed on that interface. However, the interface will still advertise the connected network in OSPF updates to other OSPF routers. This allows the network to be included in the OSPF topology without forming unnecessary neighbor relationships, which is especially useful for interfaces that connect to non-OSPF devices or where neighbor formation is not desired.
Option C:
A passive interface in OSPF will prevent both OSPF hello packets and OSPF updates from being sent or received on that interface.
This option is incorrect because it inaccurately describes the function of a passive interface. A passive interface does prevent the sending of OSPF hello packets, which stops the formation of OSPF neighbor relationships. However, it does not stop OSPF from advertising the network connected to that interface in its routing updates. OSPF updates, including the network’s information, are still advertised to other routers, allowing them to reach the network through other non-passive interfaces. This is a key aspect of passive interface functionality—while it blocks neighbor formation, it continues to allow the network to be advertised.
Option D:
A passive interface in OSPF will prevent OSPF hello packets from being sent out but still allow the interface to receive OSPF hello packets and form neighbor relationships.
This option is incorrect because it misrepresents the behavior of a passive interface. A passive interface does indeed stop sending OSPF hello packets, but it also stops receiving them, which prevents the formation of any OSPF neighbor relationships on that interface. If hello packets are not sent or received, OSPF neighbors cannot be discovered or maintained, meaning that no adjacency is formed. The primary purpose of a passive interface is to prevent neighbor formation on an interface where it is not needed or desired, without stopping the advertisement of the network connected to that interface.
Conclusion:
In OSPF, configuring an interface as passive is a useful tool to optimize network performance and security. It allows network administrators to prevent unnecessary OSPF neighbor relationships on specific interfaces while ensuring that the network associated with that interface is still advertised in the OSPF topology. This feature is often used on interfaces connected to stub networks, where no OSPF neighbors are present, or on interfaces where OSPF routing information should be shared without establishing a dynamic OSPF relationship. Understanding the correct behavior of passive interfaces is essential for effective OSPF network design and troubleshooting.