OSPF routing uses the concept of areas. What are the characteristics of OSPF areas? (Choose
Three.)
A.
Each OSPF area requires a loopback interface to be configured.
B.
Areas may be assigned any number from 0 to 65535.
C.
Area 0 is called the backbone area.
D.
Hierarchical OSPF networks do not require multiple areas.
E.
Multiple OSPF areas must connect to area 0.
F.
Single area OSPF networks must be configured in area 1.
Area 0 was chosen to be the backbone, the number 0 or 0.0.0.0 is just a number. Because OSPF is link state the LSDB must be identical within an area. This is to ensure that the SPF calculation is consistent and to prevent routing loops.
1
0
An OSPF network can be divided into sub-domains called areas. An area is a logical collection of OSPF networks, routers, and links that have the same area identification.. A router within an area must maintain a topological database for the area to which it belongs. The router doesn’t have detailed information about network topology outside of its area, thereby reducing the size of its database.
Areas limit the scope of route information distribution. It is not possible to do route update filtering within an area. The link-state database (LSDB) of routers within the same area must be synchronized and be exactly the same; however, route summarization and filtering is possible between different areas. The main benefit of creating areas is a reduction in the number of routes to propagate—by the filtering and the summarization of routes.
Each OSPF network that is divided into different areas must follow these rules:
A backbone area—which combines a set of independent areas into a single domain—must exist.
Each non-backbone area must be directly connected to the backbone area (though this connection might be a simple logical connection through a virtual link, ).
The backbone area must not be partitioned—divided into smaller pieces—under any failure conditions, such as link or router down events.
0
0
Why is area 0 the backbone area in OSPF –
Why must all other areas connect to it –
In OSPF domains the area topology is restricted so that there must be a backbone area (area 0) and all other areas must have either physical or virtual connections to the backbone. The reason for this star-like topology is that OSPF inter-area routing uses the distance-vector approach and a strict area hierarchy permits avoidance of the “counting to infinity” problem. OSPF prevents inter-area routing loops by implementing a split-horizon mechanism, allowing ABRs to inject into the backbone only Summary-LSAs derived from the intra-area routes, and limiting ABRs’ SPF calculation to consider only Summary-LSAs in the backbone area’s link-state database.
0
0
2 to the power of 16 = 65536
0
0