Which two OSPF network types support the concept of a designated router? (Choose two.)
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A. B. C. D. E.AB.
The OSPF (Open Shortest Path First) protocol defines five network types: broadcast, NBMA (Non-Broadcast Multiple Access), point-to-point, point-to-multipoint, and point-to-multipoint nonbroadcast.
Out of these, the OSPF network types that support the concept of a designated router (DR) are broadcast and NBMA.
The concept of DR was introduced in OSPF to reduce the amount of OSPF traffic on a network. When multiple OSPF routers are connected to a common network segment, they must communicate with each other to form an adjacency and exchange routing information. However, if all routers send their updates to every other router on the segment, the amount of traffic can become unmanageable as the number of routers increases.
To mitigate this issue, OSPF designates one router as the DR for the segment. The DR is responsible for exchanging routing information with all other routers on the segment. The DR then sends the updates to the other routers in a multicast packet, reducing the amount of traffic on the network.
In the broadcast network type, all routers are directly connected to a common network segment, such as an Ethernet LAN. OSPF elects a DR and a backup DR (BDR) for the segment, and all other routers form an adjacency with them. The DR and BDR are responsible for exchanging routing information with all other routers on the segment.
In the NBMA network type, the routers are connected to a non-broadcast network, such as Frame Relay or ATM. OSPF elects a DR and BDR for the network, and all other routers form an adjacency with them. However, because non-broadcast networks do not support multicast, OSPF packets must be unicast to each neighbor. This can lead to an increase in traffic and processing overhead. To mitigate this, OSPF also defines the concept of a neighbor relationship, which allows routers to form an adjacency without exchanging updates with each other.
Point-to-multipoint (P2MP) and point-to-multipoint nonbroadcast (P2MP NBMA) network types do not have a DR, as there is no common network segment that connects all routers. Instead, each router forms a separate adjacency with all other routers in the network.
Finally, a loopback interface is not a network type in OSPF but is used as a virtual interface for a router. It can be used as a source address for OSPF routing updates, among other things, but it does not support the concept of a DR.
Therefore, the correct answers to the question are A. broadcast and B. NBMA.