Top 25 OSPF Interview Questions and Answers in 2023

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Below are the top 25 OSPF interview questions and answers for entry-level and experienced candidates. These questions will help you better prepare for interviews, respond confidently, and acquire OSPF positions more rapidly. They include the following:

1. What Does OSPF Protocol Entail?

Open Shortest Path First is a link-state routing protocol that uses its algorithm to determine the optimal path between the source and destination routers. OSPF is created by the Internet Engineering Task Force (IETF) as one of the Interior Gateway Protocols (IGP). In other words, the protocol whose purpose is to move packets within a sizeable autonomous system or routing domain. It is a network layer protocol with protocol number 89 and AD value 110. OSPF employs multicast address for routine communication and for updates to the DR/BDR (BDR).

2. Describe How OSPF Works In Your Own Words.

Routers will use the OSPF protocol to communicate associated updates to the routing table and relay routing data. OSPF will not communicate the entire table; instead, it will only send the section of the database that is modified. A Link state database stores this configuration data. Using this information, it will select the data transmission route that will get the packets to their destination as quickly and efficiently as possible.

3. How Is The OSPF Router ID Chosen, And What Does It Entail?

A typical IP address is in the form of a 32-bit number, and so is the OSPF Router ID. If there is no loopback interface, then the highest IP address of the active physical interfaces is chosen instead. If there is no loopback interface, then the highest IP address of the dynamic physical interfaces is chosen. We can set it manually, but it won’t take effect until the OSPF process completely reloads. To prevent problems with the database, we should only use one router-id for each device.

4. What Are The Complete Forms Of OSPF’s LSA, LSU, And LSR?

OSPF refers to LSA as Link State Advertisement. It is utilized in the Link State database to send the basic topology information required for routing to neighboring routers. OSPF adds the LSA data to the Link State database. The routers exchange these LSAs until each router has access to the same topology database.

  • LSA = Link State Advertisement
  • LSU = Update Link State
  • Link State Request (LSR)

In OSPF, LSA is known as Link State Advertisement (LSA). It is used in the Link State database to convey routing-related topological information to neighboring routers. OSPF populates the Link State database with LSA information. Until all routers have access to the same topology database, they exchange LSAs.

5. What Does LSDB Mean In OSPF?

LSDB stands for link state Database. The database that OSPF creates is called LSDB, and it is based on the information it has discovered in LSAs, also known as link state advertisements. The Local Spatial Data Base (LSDB) is synchronized across all the routers located in the same area. Each router will keep a record of the LSA packets received in its link-states database (LSDB). OSPF uses the shortest path first (SPF) algorithm to determine the optimum routes after the LSDBs have been synchronized between the routes.

6. What Is Hello And Dead Interval In OSPF?

OSPF employs two timers and hello packets to identify if a neighboring router is operational.

Hello, interval determines the frequency with which an OSPF router transmits a Hello packet. Ten seconds is the default interval setting for point-to-point connectivity. This timeframe must be identical for shared networks.

The Dead interval sets the amount of time a router will wait before declaring an adjacent router dead. The values for the Hello and Dead intervals can vary depending on the network type.

7. In OSPF, What Exactly Are DR. And BDR?

A Backup Designated Router (BDR) and Designated Router (DR) are deployed for information exchange between OSPF routers. Depending on the network, one router is designated as the DR, and another is defined as the BDR.

In each broadcast network, the DR serves as the point of contact for the network segment. All routers move closer to the DR while processing all LSAs within the network. The router sends the DR data via the multicast address A BDR is selected for the network to prevent problems if the DR fails. When the DR fails, it starts listening at the multicast address and turns on.

8. What Does LSR Mean In OSPF?

LSR is an acronym for Link State Request. A router may discover that portions of its topological database are out of current after exchanging Database Description Packets with a neighboring router. The Link State Request packet requests the most recent components of the neighbor’s database. The final stage in establishing an adjacency is the transmission of Link State Request packets what others have (DBDs) minus what I have (LADB) = what I need to order (LSR).

9. What Steps Are Necessary To Convert Neighborship To Adjacency?

The following steps are required for a neighboring relationship to become an adjacency:

  • Two-way interaction (using Hello Protocol)
  • Exchange of Database Description (DD) packets, Link.
  • State Request (LSR) packets and Link State Update (LSU) packets constitute Database Synchronization.
  • After Database synchronization is finished, the two Routers are deemed adjacent.

10. How Does OSPF Virtual Link Work?

OSPF necessitates a backbone area (area 0), with each site linking to area 0 through an Area Boundary Router (ABR). The regular region may sometimes lack a simple connection point to the backbone area. In this instance, OSPF employs a virtual link to connect the standard room to the backbone area. An OSPF virtual connection permits two ABRs relating to the same non-backbone site to develop a neighbor relationship through that non-backbone area, despite being separated by many Routers and subnets. This virtual link is a virtual point-to-point connection between the two Routers, with the link located within area 0. Within area 0, the Routers establish a neighbor relationship and flood the link with LSAs

11. What Are The Different Types Of Networks That OSPF Supports?

The following is a list of the various network types that OSPF supports:

  • Types of Broadcasting Networks: In this network, data packets go from one router to numerous routers as they make their way throughout the network.
  • Point-to-Point Network Types: In a network type known as point-to-point, data packets are transferred only between a pair of specific routers.
  • Non-Broadcast Network Types: This network allows access from a wide variety of devices but does not allow broadcasting.

12. What Is The Election Process For DR And BDR?

Depending on the type of network, an OSPF router will select one router as DR and another as BDR. There is no BDR or DR election on a point-to-point network because the routers are directly connected. As DR, the router having the highest OSPF priority on LANs is the one that is selected. It sets Preference by default at 1. If two routers have equal priority, the router with the highest router ID is the one that is selected. The router with the highest router ID or second-highest priority is the BDR.

13. What Are The Benefits Of Adopting OSPF Summarization?

The benefits of OSPF summarization include the following:

  • OSPF Summarization decreases the number of update messages.
  • OSPF Summarization reduces the volume of data saved in routing tables.
  • OSPF Summarization decreases the burden on the Router’s CPU and memory resources.
  • OSPF Summarization decreases bandwidth usage.

14. Which OSPF Neighbor States Are The Most Well-Known?

  • 2way State: In this condition, two-way communication is established when a router identifies the neighbor’s ID in the Hello packets’ field category. A two-way state can be generated when a Data-Based Descriptor packet is received from a neighbor’s init state.
  • Down State: During the Down State procedure, no Hello packets are broadcast on the interface. The Hello packets do not reach the dead timeout period or are deleted. In this scenario, the typical neighbors are manually configured.
  • Extract and Exchange State: This condition provides an area for OSPF neighbors to exchange DBD packets. Similarly, the DR and BDR network adjacencies are established in this stage. This state also facilitates the transmission of routing information by Using Link Condition Requests.
  • Init State: During the progressive Init State, a router recognizes an OSPF neighbor’s “Hello” message. In this case, the Router gets the message. Still, the two-way communication with the neighbor has not been correctly configured.
  • Loading State: This state follows the exchange state and is responsible for sending its neighbors’ LSRs (Link State Requests). Neighbors reply by providing LSUs (Link State Updates) containing information about the requested networks.

15. What Are The Different Types Of OSPF Routers?

The various OSPF router types are as follows:

  • Routers used internally: Internal routers are OSPF routers with identically grouped interfaces.
  • Backbone routers: These are the OSPF routers that function as Internal Routers in Area 0.
  • Autonomous System Boundary Routers: Autonomous system boundary routers are OSPF routers that promote external routes into the OSPF domain.
  • Area Border Routers: Area border routers are OSPF routers with interfaces in multiple regions.

16. When Using OSPF, Why Do We Utilize Area 0?

The Open Shortest Path First (OSPF) protocol makes it possible to further segment into multiple sector networks that belong to a single autonomous system. As a result, the default generation begins at area 0. It is capable of operating either alone or as the basis for OSPF. In most cases, you will see it defined as It is required to be traversed by the many locations that establish connections to Area 0. Direct contact is not necessary to make the connection; instead, a virtual link is used to make the connection.

17. Explain LSA And Its Many Types, Including LSA 1, 2, 3, 4, 5, 6, And 7. When Is LSA Type 6 Used?

  • LSA Type 1: Each OSPF router in the region communicates this LSA.
  • Type 2 LSA: We use LSA in the Shared multi-access network during the DR/BDR election.
  • LSA Type 3: This LSA facilitates the exchange of inter-area summary data. ABR will make use of it ( Area Border Router)
  • LSA Type 4: We use LSA for external communication, i.e., with other protocols. This LSA receives information regarding the position of the ASBR.
  • LSA Type 5: ASBR generates and facilitates the exchange of external route information.
  • LSA Type 7: We utilize it in NSSA Not So Stubby Area areas. ABR transforms it into Type 5.

18. What Exactly Is A Linked State Protocol?

Link state protocols are the shortest routing protocols and the most straightforward path-first algorithms. Link states routing protocols do not see networks in terms of nearby routers and hop counts; instead, they construct a comprehensive view of the entire network that describes all possible routes and their associated costs. Link state protocols utilize the Shortest Path First (SPF) algorithm to determine the optimal path to a destination. The algorithm Shortest Path First (SPF) is also known as the Dijkstra algorithm.

19. Differentiate Between E1 And E2 In OSPF.

E1 External Type Routes specify the ASBR (Autonomous System Boundary Router) cost to the destination. Here, both the internal cost and the external metric are considered. However, internal expenses are not included while employing E2 External Type routes. Only external metrics are computed to reach a particular destination. Suppose two E2 paths are available to reach a target. In that case, the one with the lowest metric from the ASBR is considered for transferring data.

20. What Tables Does OSPF Maintain?

Participating routers in the OSPF routing system maintain three OSPF tables:

  • Neighbor table – Stores OSPF neighbor information. The command to view # sh ip OSPF neighbors is # sh ip OSPF neighbor.
  • Topology table – Stores a network’s topology structure.The command to view # sh ip OSPF topology is # sh ip OSPF topology
  • The routing database stores the optimal paths to all recognized networks. Observe the # sh ip route OSPF command.

21. Is It Possible With OSPF To Have One Numbered Side And Leave The Other Unnumbered?

No. OSPF does not allow one side to be numbered while the other remains unnumbered. If you attempt to do so, the OSPF database will become inconsistent, and you will be unable to insert routes into the steering table.

22. What Is The “Area” Concept In OSPF? What Is The Difference Between The Backbone Area, Totally Stubby Area, And Not So Stubby Area (NSSA)?

OSPF design is hierarchical; therefore, it utilizes regions to govern and manage LSA or database traffic. The idea of area stabilizes the environment and inhibits unwanted convergence updates within the region unless those updates are also for neighboring areas. This backbone area must be physically or intellectually connected to other sections to communicate. If one of the areas is not a backbone, information cannot be shared between the different areas. The stub region obstructs LSAs 4 and 5, i.e., ABR (Area Border Router) transmits external information to reduce external traffic. However, connectivity is achieved via Default route injection performed by ABR. Any router that wishes to send packets from a stub region to an external network will include a default route in its routing table.

23. What Are The Most Critical Aspects Of The OSPF Protocol?

The following is a list of the primary qualities that OSPF possesses:

  • OSPF is a routing protocol that does not use classes and supports VLSM and CIDR.
  • The Open Shortest Path First (OSPF) protocol enables the establishment of areas and an independent system.
  • OSPF is compatible with IPv4 as well as IPv6.
  • OSPF’s cost metric is computed according to the link’s bandwidth and used as the protocol’s primary measure.
  • OSPF does not have a maximum hop-count limit. It allows for an unlimited number of hops.
  • 110 is the administrative distance that we assign to OSPF routes.

24. What Are The Many Functions And Operations Of OSPF?

The various functions and operations of OSPF include:

  •  OSPF determines the shortest path using the Dijkstra Shortest Path First algorithm.
  •  OSPF adheres to a hierarchical network architecture employing Areas.
  • OSPF is a classless protocol that supports VLSMs and builds neighbor connections with nearby routers within the same area.
  •  Instead of advertising the distance to connected networks, OSPF uses Link-State Advertisements for broadcasting the condition of directly connected links (LSAs).
  •  OSPF transmits updates (LSAs) only when there is a change to one of its links and only during the update. Additionally, OSPF renews LSAs every 30 minutes.

25. What Issues Can Result In A “Down” OSPF Neighbor State?

The formation of neighbors in OSPF is nearly identical to EIGRP. Two OSPF devices are connected back-to-back via their respective interfaces. IP and mask must be accurate. Then, if not already running, start the OSPF process and advertise the interface segments on both routers. Hello, The dead times and MTU size should be identical at both ends. No access lists or transparent firewalls should prevent OSPF packets or IP addresses from being transmitted. Now is the time to establish neighbors. The area number and authentication settings must correspond. If problems persist, inspect the router logs and attempt to diagnose the issue.


These are some of the top  OSPF Interview Questions the hiring managers will ask you at your upcoming interview. Remember that it is integral to all aspects of life, including employment. And it opens the door to numerous opportunities. It would help if you seized these opportunities; this article will make you even more knowledgeable. Continue to practice, and may you obtain the position you desire to further your career.