Top 25 OSPF Interview Questions And Answers in 2023

OSPF Interview Questions And Answers

OSPF, which stands for open short path first, is a standard link-state routing protocol. It helps distribute IP routing information in single autonomous systems by locating the right paths for packets passing through them. You must anticipate OSPF-related questions if you are scheduled to interview for a computer networking job. We will cover some of the most common OSPF interview questions together and their possible responses to help you get ready for your upcoming interview. Let’s get started.

1. Define OSPF Routing Protocol

OSPF stands for Open Shortest Path First. This open link-state routing protocol uses the Dijkstra SPF algorithm to figure out the best route( straightest and shortest) for packets in a set of connected networks, which is then used to complete the routing database. OSPF routing protocol also recognizes and notifies of any changes or updates in straight related links’ statuses via LSA-Link State Advertisements.

2. Walk Us Through How You Would Change Neighboring Ships Into Adjacency

I would leverage the main steps needed to change neighbouring ships to adjacency. I’d first create a link to inform the packets by sending a state request, empowering the routers to exchange database description packets for successful database synchronization. The two routers would then achieve adjacency once database synchronization is complete.

3. Mention The Different Types Of OSPF Routers

There are three main types of OSPF routers. The most common is the autonomous system boundary router, which is pretty unique as it indicates exterior routes into the main OSPF domain. The second type is the area border router, known for having interfaces in multiple locations, while the third is the internal router, whose interfaces are found in the same area. Lastly, we have backbone routers; located in Area 0, an exceptional location in OSPF routers.

4. Mention The Purpose Of OSPF Areas

Areas are unique features in OSPF that guarantee its proper functioning. They divide large networks into far, much smaller, and easier manageable components. Every area has an area ID, acting as a unique identifier. Additionally, all the routers in a given area have the same topology databases, built by exchanging Link-State Advertisements or LSAs. On the other hand, the routers in different areas strive to learn more about the topology of other areas by exchanging summary LSAs. Some of the benefits of OSPF areas include improved scalability, faster convergence, and better network traffic.

 5. Mention The Different Types Of Areas In OSPF

There are four main OSPF areas. We have the normal OSPF area, which has no restrictions and can therefore carry all route types, the NSSA area, which allows the export of external routes but with certain exceptions and the stub OSPF area, which does not accept routes from autonomous systems but allows routine performance through the default route to the backbone areas. Lastly, we have the backbone area (Area O), which functions as the core. Every traffic travels through it given that it is connected to all the other areas. It is charged with routing distribution which can be successfully done via links, not necessarily direct connections.

6. Define OSPF Cost And Link-State Advertisement

The primary function of OSFP is to determine the shortest and straightest path for packets in a set of connected systems. It uses the OSPF cost to calculate the shortest path to a given destination. Generally expressed as an integer value, the OSPF cost is grounded on the link bandwidth, which it is usually inversely proportional to. As a result, links with higher bandwidths usually have a lower cost and vice versa. In the event of multiple paths, OSPF always settles for the one with the lowest cost, arrived at by adding the costs of all the links along a given path.

Link-State Advertisements, popularly known as LSA, refer to the packets that allow OSPF routers to communicate. They generally carry details about the routers and the existing links between them. They allow the building and maintenance of topology databases in a given network since every router is required to maintain LSAs databases obtained from other routers. It is also important to note that these link-state advertisements are usually updated whenever there are changes in the network topology.

7. What Are The Roles Of Topology And Routing Tables In  OSPF

OSPF has several unique features, such as topology and routing tables, needed for its optimal functioning. Out of these features arises three primary tables, i.e., topology, routing, and neighbour table. Routing tables have the best routes for every network, while the neighbour table has all the neighbouring networks. The last and main table is the topology table which has all the routes in a given network area.

8. Explain The Three Main Network Types In OSPF

The main network types in OSPF are the broadcast network type, point-to-point network type, and non-broadcast network type. The broadcast network type allows sending of data packets from one router to the other, while the point-to-point network type allows sending of packets between two routers. Lastly, the non-broadcast data type does not do any broadcasting as the name suggests but instead allows access to devices.

9. Differentiate OSPF From BGP

Here are the main differences between OSPF and BGP, which are both routing protocols:

  • BGP is an exterior gateway protocol (EGP), while OSPF is an internal gateway protocol (IGP).
  • OSPF allows the distribution of routing information within single autonomous systems, while BGF exchanges routing information between several autonomous systems.
  • OSPF works with more extensive networks and can support several routes and devices compared to BGP.
  • BGP operates at the application layer of the OSI model, while OSPF operates at the network layer.
  • BGP offers more routing control than OSPF but calls for more configuration owing to its complexity.
  • BGP allows inter-domain routing between several networks, while OSPF is used for internal routing within a given network.

10. Can You Tell The Difference Between OSPF And RIP

Even though OSPF and RIP are both routing protocols, they are different. As a link-state protocol, OSPF allows information exchange on network topology among routers, while RIP uses a hop count to figure out the best path to a destination, given that it is a distance-vector protocol. OSPF also offers faster convergence and highly complex network topologies, making it the best option for larger networks between the two.

11. You Have Already Defined LSA. Can You Define OSPF LSDB?

Each OSPF router in an OSPF network maintains a database known as the link-based database (LSDB). It carries information about the network topology, such as the link-state advertisements from other routers. It also helps the router calculate the shortest path to a destination, explaining why they all have identical LSDBs, updated upon receiving new LSAs.

12. What’s The Use Of OSPF Hello Packets?

OSPF routers need to discover, build and maintain external relationships with their neighbours, which calls for OSPF hello packets. They are occasionally sent out to all enabled interfaces, loaded with information such as the subnet mask, router ID, and OSPF version. Upon receiving a Hello packet, the router weighs the information on its own to determine whether the sender is indeed a neighbour. A relationship is established if the sender is a neighbour and LSAs exchanged for topology database building and maintenance.

13. What Do You Understand By OSPF Load Balancing?

Load balancing is a unique process in OSPF, guaranteeing the distribution of traffic across several paths. Its main aim is to improve network reliability and performance. Some of the ways of balancing load include configuring equal-cost paths, which have similar OSPF cost, or unequal-cost paths ( those with different costs) with path costing. Configuration is done based on the destination or packet. For destination, this process distributes traffic based on the destination IP address, while in per-packet configuration, traffic is distributed across several paths based on a packet-to-packet basis.

14. How Do You Normally Guarantee That OSPF Packets Are Only Received From Trusted Sources? 

The best way to ensure that OSPF packets come from trusted sources is by leveraging OSFP authentication. This feature supports three authentication types, i.e., plain text authentication, null authentication, and cryptographic authentication. A simple password sent in clear text is used in plain text authentication, while cryptographic authentication encrypts OSPF packets using a shared key. Null authentication means no authentication and can, therefore, be used to up security since anyone can send data packets. The first two, plain text and cryptographic authentications can prevent unauthorized access to a given network, making them the best options.

15. Define OSPF SPF Algorithm, And Summarization

Routers use OSPF Shortest Path First Algorithm to determine the shortest path to a given destination in an OSPF network. To arrive at the best (shortest) path, the algorithm calculates the costs of all available options and then settles for one with the lowest cost, determined by summing up the costs of the links along the path. The algorithm runs whenever a change in network topology is detected. The same also happens when the router receives a new link-state advertisement.

Summarization reduces the size of routing tables by summarizing the routes within an OSPF area. It is done by ABRs, mandated to advertise summary LSAs with subnet masks and network addresses of area subnets as well as a metric of the cost needed to reach a given destination within an area. OSPF summarization can improve network performance by reducing the amount of routing information exchanged between OSPF routers.

16. What Do You Understand By OSPF LSA Flooding?

OSPF Link-State Advertisement flooding is a vital process in OSPF networking. It allows OSPF routers to distribute Link-State Advertisements to other network routers. The LSA header is examined upon LSA receipt by the recipient router to determine if the latter is new or has already been received. A new LSA is added to the router’s database and forwarded to all the neighbouring routers, a process that continues till the LSA is received by all the OSPF routers in a given network. LSA flooding, therefore, guarantees the flooding of routers throughout a network, explaining its name.

17. What’s The Importance Of OSPF Virtual Links?

OSPF virtual links are required to connect areas not directly connected by a physical link. They come in handy when it is practically difficult or impossible to create physical links between two areas by allowing the exchange of routing information between routers in different areas. OSFP virtual links also reduce the hops necessary to reach a given destination, thus improving network performance. To create a virtual link, configure a router within the backbone area as a VLE, or virtual link endpoint, and specify the router ID at the other end of the link.

18. Mention Some Of The Advantages Of OSPF

Here are the four main advantages of OSPF:

  • It allows the authentication of protocol exchanges, ensuring the involvement of only trusted routers for better security.
  • It supports several equal-cost paths.
  • It uses the Dijkstra algorithm to recalculate routes to reflect network topology changes, supporting minimal routing protocol traffic generation.
  • It offers area routing, a multi-level hierarchy feature that hides a defined area’s topology information from external environments.

19. Mention The New Features That Came With The Third Version Of OSPF

OSPFv3 was updated to make it compatible with IPv6’s 128-bit address space, and as a result, it has several new features worth exploring. They include:

  • More flooding scope, either area, AS-wide or link-local
  • It processes protocol per link, not per subnet.
  • It supports several OSPF instances per link
  • Elimination of opaque LSAs.
  • Introduction of new LSA and packet format changes
  • Elimination of addressing semantics.

20. Why Would You Advise Someone To Use OSPF Protocol?

I would advise my network to use the OSPF protocol for the following reasons:

  • It is a route-free routing protocol obtained from the SPF algorithm merits.
  • It supports area division, dividing autonomous systems into regions, subsequently lowering the amount of routing information to be transmitted.
  • It allows fast transmission of route changes to the whole autonomous system
  • It minimizes several overhead costs
  • It guarantees more reliable routing thanks to the strict division of routing level
  • It offers different types of authentications for added security
  • It adapts to several network scales.

21. Differentiate Between OSPFv2 And OSPFv3

Here are the main differences between OSPFv2 and OSPFv3:

  • OSPFv2 supports IPv4, while version 3 supports IPv6.
  • The header size of OSPFv2 is 24 bytes compared to OSPFv3’s 16 bytes.
  • OSPFv2 is specified in RFC2328, while the latter is specified in RFC5340
  • OSPv4 uses seven LSAs, while its counterpart uses 9 link-state advertisements, the additional two being intra-area-prefix LSA and link-LSA.
  • OSPFv2 uses networks, while Version 3 uses links.
  • MD5 is used for authentication in OSPFv2, while OSPFv3 uses IPSec.
  • OSPFv2 can automatically configure its RID, while OSPFv3 cannot.
  • The number of instances run per link is unlimited in OSPFv3, while OSPFv2 can only run one instance.

22. Mention the Key Features of OSFP

Here are the main OSFP features:

  • It has an in-built correction and error detection mechanism
  • It is an interior gateway protocol (IGP)
  • It simplifies administration and optimizes network traffic using areas.
  • It operates within a single routing domain, mostly autonomous systems.
  • Despite running over an IP protocol, it encapsulates its data without using a transport protocol such as UDP or TCP.
  • It computes the shortest route to reach a destination using Dijkstra’s algorithm.
  •  Data is directly encapsulated in Ip pockets thanks to PROTOCOL 89.

23. Mention Some Of The Disadvantages Of OSFP Based On Experience

I can point out  the following four main disadvantages of OSPF based on experience:

  • Running the SPF algorithm to calculate the best route requires extra CPU processing.
  • It can only be configured by skilled or experienced network administrators in large networks owing to its complexity.
  • It isn’t easy to troubleshoot
  • Calculating the best route for destinations in OSFP requires lots of information, meaning a higher memory consumption than other routing protocols.

24. Mention The Use Cases Of OSPF

OSPF has unique use cases. It is mainly recommended for large enterprise networks running on routing equipment from several vendors. Companies that require open standard routing protocols can also use this protocol, allowing them to enjoy increased flexibility when replacing or adding routers. Additionally, it’s recommendable to use OSPF for a network with more than 50 routers, given that it is an advanced routing protocol. Lastly, it is the best option in a mixed vendor environment, which may explain its popularity.

25. Mention The Link State Packet Types In OSPF

There are five main link state packet types in OSPF, namely:

  • Link State Advertisement (LSA), which acts as the primary communication means between OSPF routers. It contains the necessary topology details and information and is generally flooded to perform several functions. They are nine in total.
  • Link State Database- This link-state packet contains exchanged link-state information. Routers within a given area must have identical link-state databases.
  • Link State Request (LSR)-  a request packet used for neighbour adjacency functions, leading to LSDB exchange.
  • Link State Update- Popularly known as LSU, the link state update is a response packet that delivers LSDB information via LSR packet upon request.
  • Link State Acknowledgement (LSAcK)- This is a confirmation package signifying the receipt of a requested link state update.

Conclusion

Computer or hardware networking interviews don’t have to be challenging if you prepare well. Ensure you find the best responses to the questions above for the best results. We wish you all the best in your upcoming interview.