Which two statements about the OSPF Router ID are true? (Choose two.)

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QUESTION NO: 287

OSPF (Open Shortest Path First) is a link-state routing protocol used to share routing information between routers within an autonomous system (AS). OSPF uses a hierarchical network design with multiple areas to reduce the size of routing tables and improve network scalability.

The Router ID is a unique identifier for a router in an OSPF network. It is a 32-bit number that identifies the router and is used to identify the source of OSPF packets. Here are the two statements about the OSPF Router ID that are true:

A. It identifies the source of a Type 1 LSA: The Link State Advertisement (LSA) is a message used by OSPF routers to communicate their routing information. Type 1 LSA, also known as Router LSA, is used to advertise the state of the router's interfaces to other routers in the same area. The Router ID is used to identify the source of a Type 1 LSA.

B. It should be the same on all routers in an OSPF routing instance: It is recommended that the OSPF Router ID should be unique across the entire OSPF network, including all routers and all areas. It is important to ensure that the same Router ID is not used by more than one router in the same OSPF routing instance. However, the Router ID does not need to be the same on all routers in different OSPF routing instances.

C. By default, the lowest IP address on the router becomes the OSPF Router ID: If the Router ID is not manually configured, OSPF will automatically choose the Router ID based on a selection process. By default, OSPF chooses the Router ID based on the highest IP address configured on the loopback interface. If there is no loopback interface, the Router ID will be based on the highest IP address on the active interfaces. If there are multiple active interfaces with the same highest IP address, the Router ID will be based on the interface with the highest priority.

D. The router automatically chooses the IP address of a loopback as the OSPF Router ID: This statement is not entirely true. OSPF protocol does prefer the IP address of a loopback interface as the Router ID. However, it is not an automatic process; the administrator has to manually configure the Router ID to be the IP address of the loopback interface. If the Router ID is not configured manually, OSPF will follow the default selection process explained in option C.

In summary, options A and B are the two statements about the OSPF Router ID that are true. The OSPF Router ID is a unique identifier used to identify the source of OSPF packets and should be the same on all routers in an OSPF routing instance. The Router ID can be manually configured or chosen by OSPF following a selection process.