A network administrator has enabled OSPF across an NBMA network and has issued the
command ip ospf network nonbroadcast. Given those facts, which two statements are true?
(Choose two.)
A.
Interfaces will automatically detect and build adjacencies with neighbor routers.
B.
DR and BDR elections will not occur.
C.
DR and BDR elections will occur.
D.
The neighbor command is required to build adjacencies.
E.
All routers must be configured in a fully meshed topology with all other routers.
Explanation:
Even if there is only one router, broadcast multiaccess networks elect a DR and a BDR to
serve as focal points for routing information. In contrast, point-to-point OSPF networks do
not elect a DR because they can never include more than two nodes.Another type of OSPF network, Nonbroadcast Multiaccess (NBMA), can include more than
two nodes. Therefore, NBMA will try to elect a DR and a BDR. Common NBMA
implementations include Frame Relay, X.25, and SMDS. NBMA networks follow rules at
Layer 2 that prevent the delivery of broadcasts and multicasts.
A non-broadcast environment requires that all OSPF neighbors be manually configured. This
is the default setting for physical interfaces with Frame Relay encapsulation, as well as for
their point-to-multipoint subinterfaces. By manually configuring each neighbor, OSPF knows
exactly which neighbors need to participate and which neighbor is identified as the DR. Also,
communication between neighbors is done via unicast instead of multicast. This
configuration also requires a full mesh and has the same weakness as the broadcast
environment.
For non-broadcast networks the default Hello interval is 30 seconds and the Dead interval is
four times the Hello interval, 120 seconds. Non-broadcast multi-access networks do elect a
DR and BDR, due to their multi-access nature. In order to set which router you want as the
DR, you must set the priority in the neighbor statement to elect the neighbor as the DR. In
order to manually configure who your neighbors are, the following command must be
entered in router configuration mode for the selected OSPF process:
neighbor ip_address
ip_address = the ip address of the neighbor.
If you would like to set the priority of this router to become the DR, you will need to append
the priority of the neighbor:
neighbor ip_address priority value