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.
Explanation:
Definition of OSPF areas: An OSPF network may be structured, or subdivided, into routing areas
to simplify administration and optimize traffic and resource utilization. Areas are identified by
32-bit numbers, expressed either simply in decimal, or often in octet-based dot-decimal
notation, familiar from IPv4 address notation.
See discussion following Cisco Learning discussion.
https://learningnetwork.cisco.com/message/90832
Especially to note on choice B, D, and E:
Choice B: we all know that The areas can be any number from 0 to 4.2 billion and 1 to 65,535 for the Process ID. As choice B specifies ‘area’ (be aware, it’s not saying ‘process id), there is no reason to say that we cannot assign numbers from 0 to 65535 for area # (it is using ‘may be’, not ‘have to be’ or ‘ought to be’). Hence, we do not worry about assigning ‘0’.
Choice E: as Area 0 is the backbone, we all understand that any areas in a OSPF network have to be connected to it. And actually this is implicitly saying that multiple areas form a hierarchical OSPF network, as Area 0 being a root and others being its leaves.
Choice D: when it specifies ‘Hierarchical’, at least 2 areas should be required to form such topology (of course that includes Area 0)
Although Choice B is not an absolutely accurate statement since it not only can be assigned up to 65535, it is still a correct answer. And again, it specifies ‘area’, not ‘process id’, so ‘0’ can be included. Finally, it would be meaningless to call OSPF a hierarchical network if no more than one area is present.
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I used to think the answers should be C D E and here is my explanation:
OSPF can use an active interface for its router ID, so a loopback interface is not a must -> A is incorrect.
OSPF Area is a 32-bit number so we can use up to 232 – 1 = 4294967296 – 1 (since Area 0 is the first area). Remember that only process ID is a 16-bit number and ranges from 1 to 65535 -> B is incorrect.
F is incorrect too because single area OSPF netwoks must be configured in Area 0, which is called the backbone area.
For answer D, it is a bit hard to guess what they want to say about “hierarchical” but we should understand “Hierarchical OSPF networks” as “OSPF networks”. D is correct bercause we can only have one area (area 0 – the backbone area) for our networks.
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