OSPF – Dumps For All

OSPF Interview Questions

1: What is an OSPF neighbor?

*** From the perspective of an OSPF router, a neighbor is another OSPF router
that is attached to one of the first router’s directly connected links.

2: What is an OSPF adjacency?

***An OSPF adjacency is a conceptual link to a neighbor over which LSAs can
be sent.

3: What is an LSA? How does an LSA differ from an OSPF Update packet?

***A router originates a link state advertisement to describe one or more
destinations. An OSPF Update packet transports LSAs from one neighbor to
another. Although LSAs are flooded throughout an area or OSPF domain,
Update packets never leave a data link.

4: What is a link state database? What is link state database
synchronization?

***The link state database is where a router stores all the OSPF LSAs it knows
of, including its own. Database synchronization is the process of ensuring that all
routers within an area have identical link state databases.

5: What is the default HelloInterval?

***The default OSPF HelloInterval is 10 seconds.

6: What is the default RouterDeadInterval?

***The default RouterDeadInterval is four times the HelloInterval.

7: What is a Router ID? How is a Router ID determined?

***A Router ID is an address by which an OSPF router identifies itself. It is either
the numerically highest IP address of all the router’s loopback interfaces, or if no
loopback interfaces are configured, it is the numerically highest IP address of all
the router’s LAN interfaces.

8: What is an area?

***An area is an OSPF sub-domain, within which all routers have an identical link
state database.

9: What is the significance of area 0?

***Area 0 is the backbone area. All other areas must send their inter-area traffic
through the backbone.

10: What is MaxAge?

***MaxAge, 1 hour, is the age at which an LSA is considered to be obsolete.

11: What are the five OSPF packet types? What is the purpose of each type?

*** The five OSPF packet types, and their purposes, are:

Hellos, which are used to discover neighbors, and to establish and maintain
adjacencies

Updates, which are used to send LSAs between neighbors

Database Description packets, which a router uses to describe its link state
database to a neighbor during database synchronization

Link State Requests, which a router uses to request one or more LSAs from a
neighbor’s link state database

Link State Acknowledgments, used to ensure reliable delivery of LSAs

12:What are LSA types 1 to 5 and LSA type 7? What is the purpose of each
type?

*** The most common LSA types and their purposes are:

Type 1 (Router LSAs) are originated by every router and describe the originating
router, the router’s directly connected links and their states, and the router\xd5 s
neighbors.

Type 2 (Network LSAs) are originated by Designated Routers on multiaccess
links and describe the link and all attached neighbors.

Type 3 (Network Summary LSAs) are originated by Area Border Routers and
describe inter-area destinations.

Type 4 LSAs (ASBR Summary LSAs) are originated by Area Border Routers to
describe Autonomous System Boundary Routers outside the area.

Type 5 (AS External LSAs) are originated by Autonomous System Boundary
Routers to describe destinations external to the OSPF domain.

Type 7 (NSSA External LSAs) are originated by Autonomous System Boundary
Routers within not-so-stubby areas.

13: What are the four OSPF router types?

***The four OSPF router types are:

# Internal Routers, whose OSPF interfaces all belong to the same area

# Backbone Routers, which are Internal Routers in Area 0

# Area Border Routers, which have OSPF interfaces in more than one area

# Autonomous System Boundary Routers, which advertise external routes into
the OSPF domain
14: What are the four OSPF path types?

***The four OSPF path types are:

Intra-area paths

Inter-area paths

Type 1 external paths

Type 2 external paths
15: What are the five OSPF network types?

*** The five OSPF network types are:

i)Point-to-point networks

ii) Broadcast networks

iii) Non-broadcast multi-access (NBMA) networks

iv) Point-to-multipoint networks

v) Virtual links
16: What is a Designated Router?

***A Designated Router is a router that represents a multiaccess network, and
the routers connected to the network, to the rest of the OSFP domain.
17: How does a Cisco router calculate the outgoing cost of an interface?

***Cisco IOS calculates the outgoing cost of an interface as 108/BW, where BW
is the configured bandwidth of the interface.
18: What is a partitioned area?

***An area is partitioned if one or more of its routers cannot send a packet to the
area’s other routers without sending the packet out of the area.
19: What is a virtual link?

*** A virtual link is a tunnel that extends an OSPF backbone connection through
a non-backbone area.
20: What is the difference between a stub area, a totally stubby area, and a
not-so-stubby area?

***A stub area is an area into which no type 5 LSAs are flooded. A totally stubby
area is an area into which no type 3, 4, or 5 LSAs are flooded, with the exception
of type 3 LSAs to advertise a default route. Not-so-stubby areas are areas
through which external destinations are advertised into the OSPF domain, but
into which no type 5 LSAs are sent by the ABR.
21: What is the difference between OSPF network entries and OSPF router
entries?

*** OSPF network entries are entries in the route table, describing IP
destinations. OSPF router entries are entries in a separate route table that record
only routes to ABRs and ASBRs.
22: Why is type 2 authentication preferable over type 1 authentication?

***Type 2 authentication uses MD5 encryption, whereas type 1 authentication
uses clear-text passwords.
23: Which three fields in the LSA header distinguish different LSAs? Which
three fields in the LSA header distinguish different instances of the same
LSA?

***The three fields in the LSA header that distinguish different LSAs are the
Type, Advertising Router, and the Link State ID fields. The three fields in the LSA
header that distinguish different instances of the same LSA are the Sequence
Number, Age, and Checksum fields

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What is the difference between OSPF and RIP?

RIP and OSPF are both routing protocols likely you have used without even knowing it. A routing protocol describes the way your data (or signals) get from your PC on a network to another PC or device on the network.

RIP = Hop count Algorithm

The only metric used to calculate the cost of a path (path is from point A to destination B) is the hop count from router A to router B, even if you have a path with more hops and more bandwidth available.

OSPF = Link State Algorithm

This is a more intelligent algorithm, tha build a topology of network and build the cost regarding some characteristics of path like bandwidth, load, reliability, etc.A real life example would be like when work is done and its time to drive home. You have multiple routes usually to choose from to get home. The shortest way home would obviously be a straight line from work to home. Unfortunately no magic road exists from where ever you are directly to where ever you need to go. So if you were using RIP to get home you would drive in a straight line (or take the roads that would more or less offer you a straight route home). Thing is, these roads may be BUSY, very busy in fact, you can sit in traffic for a half hour moving less than 1km sometimes. So technically the shortest way (straight line) may not be the most efficient way. Now if you were using OSPF, you would take into account traffic and other variables to decide how to get home. The quickest route home may not be the direct path, it may be a back road that has less traffic on it, through a sub division, and then to your house. However because you were using OSPF you got home quicker than if you were to use RIP as RIP didn’t really look at what the traffic was like. Now in the above paragraph turn the word “work” into “client” and the word “home” into “server” and replace the word “you” with “the packet” – magically we turn the above paragraph into tecnobabble

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CCNA – OSPF Questions

Question 1:

Which of the following statements below best describe the process identifier that is used to run OSPF on a router? (Choose two)

A – It is an optional parameter required only if multiple OSPF processes are running on the router
B – It is locally significant
C – It is needed to identify a unique instance of an OSPF database
D – All routers in the same OSPF area must have the same process ID if they are to exchange routing information

Answer: B C

Question 3:

Which items are correct about the routing protocol OSPF? (Choose three)

A – Support VLSM
B – Increase routing overhead on the network
C – Confine network instability to one area of the network
D – Allow extensive control of routing updates

Answer: A C D

Explanation:

Routing overhead is the amount of information needed to describe the changes in a dynamic network topology. All routers in an OSPF area have identical copies of the topology database and the topology database of one area is hidden from the rest of the areas to reduce routing overhead because fewer routing updates are sent and smaller routing trees are computed and maintained (allow extensive control of routing updates and confine network instability to one area of the network).

Question 4:

Which three features are of OSPF routing protocol? (Choose three)

A – Converge quickly
B – OSPF is a classful routing protocol
C – Identify the best route by use of cost
D – Before exchanging routing information, OSPF routers find out neighbors

Answer: A C D

Question 5:

OSPF routing uses the concept of areas. What are the characteristics of OSPF areas? (Chose 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

Answer: B C E

Explanation:

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.

But TT commented on 01-11-2010:

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.