You attach six routers (R1, R2, R3, R4, R5, and R6) to a single Ethernet LAN and configure the routers with addresses in the same subnet. You do not modify the default priority of any router. R1 is chosen as the designated router and R2 is chosen as the backup designated router.

How many full adjacencies does R3 have?

A.
0

B.
1

C.
2

D.
5

Explanation:
Configuring Point-to-Point Adjacency over LAN Interfaces

You can configure OSPF point-to-point adjacency over Ethernet and aggregated Ethernet interfaces. OSPF treats point-to-point interfaces and broadcast interfaces differently. The differences comprise the establishment of neighbor adjacencies, flooding link-state information, representation of the network topology, the SPF calculation, and protocol packets destination addresses.

You can configure OSPF so that it treats LAN interfaces with only two routers connected to them as point-to-point interfaces. The point-to-point extension modifies the shortest-path-first (SPF) calculation so that the correct next hop is calculated. The next-hop address is null for point-to-point interfaces by default and is rejected.

To configure an interface as a point-to-point interface in OSPF, include the point-to-point statement:

[edit protocols ospf area area-id interface interface-name]
p2p;

For a list of hierarchy levels at which you can configure this statement, see the statement summary section for this statement.

Two routers are supported for each subnet. The LAN interface must be numbered.