Interview Questions.

OSPF Interview Questions and Answers - Jul 16, 2022


OSPF Interview Questions and Answers

Q1.  What OSPF stands for?

Ans: OSPF is a standardized Link-State routing protocol, designed to scale successfully to supportlarger networks.

Q2. What is the set of rules utilized by OSPF?

Ans: OSPF makes use of SPF (Shortest Path First) algorithm for calculating the great direction and getting ready OSPF database.

Q3. What are the traits of OSPF ?


OSPF supports best IP routing.

OSPF routes have an administrative distance i.E. One hundred ten.

OSPF uses value as its metric, that's computed based totally at the bandwidth of the link. OSPF has no hop-count restrict.

Q4. Briefly provide an explanation for functioning and operating of various OSPF ? 

Ans: Following are running and functioning of OSPF

 OSPF employs a hierarchical community layout the use of Areas.

 OSPF forms neighbor relationships with adjacent routers inside the equal Area.

 Instead of advertising the space to related networks, OSPF advertises the popularity of directly related  hyperlinks the usage of Link-State Advertisements (LSAs).

 OSPF sends updates (LSAs) while there is a trade to one among its hyperlinks, and will most effective ship the alternate all through the updation. LSAs are moreover refreshed each half-hour.

 OSPF uses the Dijkstra Shortest Path First algorithm to decide the shortest path.

 OSPF is a classless protocol, and for that reason helps VLSMs.

Q5. What is OSPF Router ID and how is it elected?

Ans: Router identity is 32 bit quantity similar to IP address. It is elected as maximum IP Address of Loopback interfaces or Highest IP of the active physical interfaces if loopback isn't gift. It also can be set manually however it will take impact when OSPF system is reset absolutely or device is reloaded. There should be most effective one router-identity in line with device strolling ospf to avoid database problems.

Q6.  What are specific neighbour states in OSPF ?

Ans: Neighbor adjacencies will development via several states, inclusive of:

Q7.  What is function of topology and routing table in OSPF ? 










The OSPF technique builds and keeps 3 separate tables:

neighbor table – carries a list of all neighboring routers.

 Topology desk – incorporates a list of all possible routes to all recognized networks inside an area.

 Routing desk – consists of the first-rate course for each known community.

Q8. What specific activities neighbour router carry out in OSPF? 

Ans: A neighbor table consists of the subsequent facts:

 The Router ID of each neighbouring router

 The modern-day “country” of every neighbouring router

 The interface immediately related to each neighbour

 The IP cope with of the faraway interface of every neighbour

Q9.  Name few types of LSA ?


Router LSA

Network LSA

Network Summary LSA

ASBR Summary LSA

External LSA

Q10. What will appear if we make precedence of a OSPF router as “zero”?

Ans: OSPF Router with Priority 0 will now not participate in DR/BDR election.

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Q11. What are the multicast IPs utilized by OSPF? What multicast IP does DR/BDR router and non-DR use?

Ans: OSPF Routers use 224.Zero.0.5 multicast IP deal with. However, in DR/BDR it uses 224.Zero.0.5 and 224.Zero.Zero.6. DR/BDR will use even as replying to OSPF routers in Broadcast Multi get entry to community.

Q12. On which algorithms OSPF depends? What is the principle at the back of it?

Ans: OSPF uses the Dijkstra Shortest Path First algorithm to determine the shortest route and Link-State Advertisements (LSAs).

An instance might be Frame Relay.

Q13. Name tables which OSPF keep ?

Ans: The OSPF process builds and continues 3 separate tables:

A neighbor table – incorporates a listing of all neighboring routers.

A topology desk – carries a listing of all viable routes to all recognized networks within an area.

A routing desk – consists of the best direction for each known community.

Q14. What are the packet types does OSPF use? Explain Hello, Dead Timers, LSA, LSU, LSR packets.

Ans: Hello is a handshake packet tells approximately the alive popularity of OSPF interface.

Dead is the time it's going to wait and then it'll break the neighborship and inform routing desk for removing the direction or related route.

LSA- Link kingdom commercial.  It contains the records of the community  or replace.

LSU – It incorporates multiple updates at one cross.

LSR – It’s a request packet whilst router going for walks OSPF wishes higher and updated data.

Q15. What could be Hello/Dead timers in network having hyperlink capability of >T1 and <=T1?

Ans: For link above T1 i.E. 1.544Mbps Hello = 10 seconds, Dead is forty seconds

For link = or underneath Hello = 30 seconds, Dead = one hundred twenty Seconds.

Dead interval is actual four times than Hello and its fee can’t be set manually.

Q16. Does DR/BDR election take place in OSPF Point to Point Network

Ans: No election occurs in point to point network

Q17. What is “Process identity”? Does it have nearby or global importance? Does it want to be same on OSPF neighbor routers?

Ans: Process identification is used similarly to Autonomous system but it has local importance most effective. It can be one of a kind on neighbors running OSPF.

Q18. Can area 1 and region 2 change or put it on the market networks if they may be not related to region 0 (Backbone area)? What we want to do make it occur?

Ans: Every place should be related to area zero else there received’t be trade of inter location routes. If it isn't possible then use the idea of digital hyperlink

Q19. What is the idea of Virtual-hyperlink in OSPF? Can digital hyperlink be formed via stub area?

Ans: Per OSPF Rule, Inter place communication will show up while atleast one vicinity have to be area zero or backbone region. If this isn't always viable, then we will use virtual-link to shape a logical affiliation. Virtual link isn't feasible through Stub region.

Q20. What LSA is used for inter place conversation? What LSA kind is used for inter protocol conversation in OSPF?

Ans: Type three LSA Is used for inter area verbal exchange. For conversation with different protocols or outside routes , type four and five could be used.

Q21. What is the DR/BDR in OSPF? How those are elected and in what situation?

Ans: DR/BDR election happens in Shared Multi get right of entry to network to avoid full mesh state of affairs. In shared environment, each OSPF router will attempt to form neighborship with different router, so DR/BDR is available in to movement and act as unmarried point of contact. Neighborship will handiest shape with DR and Backup DR. DR router might be the very best priority router and BDR may be 2nd highest priority. If there's a tie, then Highest router-identification will be used to make the choice. Router ID is elected as the best numerical Loopback IP or the best Physical Active IP address or the Interface IP which comes up first.

Q22. Explain LSA and its all Types along with LSA 1, 2, 3,4,five,7. When will we use LSA type 6


LSA Type 1 :- Every OSPF router inside the region speaks this LSA.

LSA Type 2 :- This LSA might be utilized in Shared multi-get entry to community in DR/BDR election.

LSA Type 3 :- This LSA facilitates in exchange of inter location summary records. It may be used by ABR( Area Border Router)

LSA Type four :- This LSA is supposed for external verbal exchange i.E. With different protocols. This LSA receives the records approximately role of ASBR.

LSA Type five :- This may be generated by means of ASBR and helps in alternate of outside records of routes.

LSA Type 7:- It is utilized in special place i.E. NSSA Not So Stubby Area. It receives converted in to Type 5 with the aid of ABR.

Q23.  What are the special OSPF network types and provide an instance for every ?

Ans: Different OSPF network sorts with their examples are given under:

Broadcast Multi-Access – shows a topology wherein broadcast occurs.

Examples consist of Ethernet, Token Ring, and ATM.

Point-to-Point – indicates a topology wherein two routers are immediately connected.

An instance might be a factor-to-point T1.

Point-to-Multipoint – suggests a topology in which one interface can hook up with a couple of destinations. Each connection between a supply and destination is dealt with as a point-to-factor hyperlink.

An example could be Point-to-Multipoint Frame Relay.

Non-broadcast Multi-get admission to Network (NBMA) – shows a topology where one interface can hook up with more than one locations; however, proclaims cannot be despatched across a NBMA network.

Q24. What is the idea of “Area” in OSPF? What is Backbone area, stub area, Totally Stubby Area, Not So Stuby area (NSSA)?

Ans: OSPF is hierarchical in nature and therefor it uses areas to manipulate and manage its LSA or database site visitors. The concept of area brings stability within the surroundings and suppresses the useless convergence updates within the vicinity until meant for different regions. Backbone vicinity is “location 0” – this spine area is should be bodily or logically connected to other areas to communicate. No  areas can percentage information if one of the areas isn't backbone area. Stub area blocks the LSA four and five i.E. External information which is despatched via ABR ( Area Border Router)- It does so to reduce the external visitors. However, connectivity can happen the use of Default path injection completed via ABR as an alternate manner.  Any router wants to send packets outside network from stub region could have default direction in its routing desk.

Totally Stubby Area – It blocks external LSA and precis LSA i.E. LSA 3, four and five. Default path will be injected by ABR to routers of region.

Not So Stubby location

Q25. What troubles can purpose OSPF neighborship in “down” nation?

Ans: Steps in forming neighborship in OSPF is sort of similar to EIGRP’s. Two ospf gadgets have to be related lower back to back with cables connected on correct interfaces. IP and mask should be accurate. Then run the ospf technique if now not run already and put it up for sale the interface segments on each routers. Hello, Dead timers and MTU size should be equal at each ends. There need to not be any get admission to-listing or obvious firewall blocking the OSPF Packets or IP addresses. Neighborship have to establish now. Area wide variety and authentication parameters have to match. If problems are still there, then check the router logs and try to research the problem.