OSPF Interview Questions and Answers

Q1.  What OSPF stands for?
OSPF is a standardized Link-State routing protocol, designed to scale efficiently to supportlarger networks.

Q2. What is the algorithm used by OSPF?
OSPF uses SPF (Shortest Path First) algorithm for calculating the best path and preparing OSPF database.

Q3. What are the characteristics of OSPF ?
1. OSPF supports only IP routing.
2. OSPF routes have an administrative distance i.e. 110.
3. OSPF uses cost as its metric, which is computed based on the bandwidth of the link. OSPF has no hop-count limit.

Q4. Briefly explain functioning and working of different OSPF ? 
Following are working and functioning of OSPF

  •  OSPF employs a hierarchical network design using Areas.
  •  OSPF forms neighbor relationships with adjacent routers in the same Area.
  •  Instead of advertising the distance to connected networks, OSPF advertises the status of directly connected links using Link-State Advertisements (LSAs).
  •  OSPF sends updates (LSAs) when there is a change to one of its links, and will only send the change during the updation. LSAs are additionally refreshed every 30 minutes.
  •  OSPF uses the Dijkstra Shortest Path First algorithm to determine the shortest path.
  •  OSPF is a classless protocol, and thus supports VLSMs.

Q5. What is OSPF Router ID and how is it elected?
Router id is 32 bit number similar to IP address. It is elected as highest IP Address of Loopback interfaces or Highest IP of the active physical interfaces if loopback is not present. It can also be set manually but it will take effect when OSPF process is reset completely or device is reloaded. There should be only one router-id per device running ospf to avoid database issues.

Q6.  What are different neighbour states in OSPF ?
Neighbor adjacencies will progress through several states, including:

Q7.  What is role of topology and routing table in OSPF ? 
1. Down
2. Init
3. 2-Way
4. ExStart
5. Exchange
6. Loading
7. Advertisements
8. Full

The OSPF process builds and maintains three separate tables:

  • neighbor table – contains a list of all neighboring routers.
  •  topology table – contains a list of all possible routes to all known networks within an area.
  •  routing table – contains the best route for each known network.

Q8. What different activities neighbour router perform in OSPF? 
A neighbor table includes the following information:

  •  The Router ID of each neighbouring router
  •  The current “state” of each neighbouring router
  •  The interface directly connected to each neighbour
  •  The IP address of the remote interface of each neighbour

Q9.  Name few types of LSA ?
1.  Router LSA
2.  Network LSA
3.  Network Summary LSA
4.  ASBR Summary LSA
5.  External LSA

Q10. What will happen if we make priority of a OSPF router as “zero”?
OSPF Router with Priority zero will not participate in DR/BDR election.

Q11. What are the multicast IPs used by OSPF? What multicast IP does DR/BDR router and non-DR use?
OSPF Routers use 224.0.0.5 multicast IP address. However, in DR/BDR it uses 224.0.0.5 and 224.0.0.6. DR/BDR will use 224.0.0.6 while replying to OSPF routers in Broadcast Multi access network.

Q12. On which algorithms OSPF depends? What is the principle behind it?
OSPF uses the Dijkstra Shortest Path First algorithm to determine the shortest path and Link-State Advertisements (LSAs).
An example would be Frame Relay.

Q13. Name tables which OSPF maintain ?
The OSPF process builds and maintains three separate tables:
1) A neighbor table – contains a list of all neighboring routers.
2) A topology table – contains a list of all possible routes to all known networks within an area.
3) A routing table – contains the best route for each known network.

Q14. What are the packet types does OSPF use? Explain Hello, Dead Timers, LSA, LSU, LSR packets.
Hello is a handshake packet tells about the alive status of OSPF interface.
Dead is the time it will wait after which it will break the neighborship and inform routing table for removing the route or connected route.

LSA- Link state advertisement.  It contains the information of the network  or update.
LSU – It contains multiple updates at one go.
LSR – It’s a request packet when router running OSPF needs better and updated information.

Q15. What will be Hello/Dead timers in network having link capacity of >T1 and <=T1?
For link above T1 i.e. 1.544Mbps Hello = 10 seconds, Dead is 40 seconds
For link = or below Hello = 30 seconds, Dead = 120 Seconds.
Dead interval is exact four times than Hello and its value can’t be set manually.

Q16. Does DR/BDR election happen in OSPF Point to Point Network
No election happens in point to point network

Q17. What is “Process id”? Does it have local or global significance? Does it need to be same on OSPF neighbor routers?
Process id is used similarly to Autonomous system but it has local significance only. It can be different on neighbors running OSPF.

Q18. Can area 1 and area 2 exchange or advertise networks if they are not connected to area 0 (Backbone area)? What we need to do make it happen?
Every area should be connected to area 0 else there won’t be exchange of inter area routes. If it is not possible then use the concept of virtual link

Q19. What is the concept of Virtual-link in OSPF? Can virtual link be formed thru stub area?
Per OSPF Rule, Inter area communication will happen when atleast one area should be area 0 or backbone area. If this is not possible, then we can use virtual-link to form a logical association. Virtual link is not possible thru Stub area.

Q20. What LSA is used for inter area communication? What LSA type is used for inter protocol communication in OSPF?
Type 3 LSA Is used for inter area communication. For communication with other protocols or external routes , type 4 and 5 will be used.

Q21. What is the DR/BDR in OSPF? How these are elected and in what scenario?
DR/BDR election happens in Shared Multi access network to avoid full mesh scenario. In shared environment, every OSPF router will try to form neighborship with other router, so DR/BDR comes in to action and act as single point of contact. Neighborship will only form with DR and Backup DR. DR router will be the highest priority router and BDR will be second highest priority. If there is a tie, then Highest router-id will be used to make the decision. Router ID is elected as the highest numerical Loopback IP or the highest Physical Active IP address or the Interface IP which comes up first.

Q22. Explain LSA and its all Types such as LSA 1, 2, 3,4,5,7. When do we use LSA type 6
LSA Type 1 :- Every OSPF router within the area speaks this LSA.
LSA Type 2 :- This LSA will be used in Shared multi-access network in DR/BDR election.
LSA Type 3 :- This LSA helps in exchange of inter area summary information. It will be used by ABR( Area Border Router)
LSA Type 4 :- This LSA is meant for external communication i.e. with other protocols. This LSA gets the information about position of ASBR.
LSA Type 5 :- This will be generated by ASBR and helps in exchange of external information of routes.
LSA Type 7:- It is used in special area i.e. NSSA Not So Stubby Area. It gets converted in to Type 5 by ABR.

Q23.  What are the different OSPF network types and give an example for each ?
Different OSPF network types with their examples are given below:
1) Broadcast Multi-Access – indicates a topology where broadcast occurs.
Examples include Ethernet, Token Ring, and ATM.
2) Point-to-Point – indicates a topology where two routers are directly connected.
An example would be a point-to-point T1.
3) Point-to-Multipoint – indicates a topology where one interface can connect to multiple destinations. Each connection between a source and destination is treated as a point-to-point link.
An example would be Point-to-Multipoint Frame Relay.
4) Non-broadcast Multi-access Network (NBMA) – indicates a topology where one interface can connect to multiple destinations; however, broadcasts cannot be sent across a NBMA network.

Q24. What is the concept of “Area” in OSPF? What is Backbone area, stub area, Totally Stubby Area, Not So Stuby area (NSSA)?
OSPF is hierarchical in nature and therefor it uses areas to control and manage its LSA or database traffic. The concept of area brings stability in the environment and suppresses the unnecessary convergence updates within the area unless meant for other areas. Backbone area is “area 0” – this backbone area is must be physically or logically connected to other areas to communicate. No two areas can share information if one of the areas is not backbone area. Stub area blocks the LSA 4 and 5 i.e. External information which is sent by ABR ( Area Border Router)- It does so to reduce the external traffic. However, connectivity can happen using Default route injection done by ABR as an alternate way.  Any router wants to send packets external network from stub area will have default route in its routing table.

Totally Stubby Area – It blocks external LSA and summary LSA i.e. LSA 3, 4 and 5. Default route will be injected by ABR to routers of area.

Not So Stubby area

Q25. What problems can cause OSPF neighborship in “down” state?
Steps in forming neighborship in OSPF is almost similar to EIGRP’s. Two ospf devices should be connected back to back with cables connected on correct interfaces. IP and mask should be correct. Then run the ospf process if not run already and advertise the interface segments on both routers. Hello, Dead timers and MTU size should be same at both ends. There should not be any access-list or transparent firewall blocking the OSPF Packets or IP addresses. Neighborship should establish now. Area number and authentication parameters must match. If issues are still there, then check the router logs and try to analyze the problem.

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