Top OSPF Interview Questions [+Sample Answers]

OSPF Interview Questions

OSPF Interview questions are directed to applicants who desire to function within the OSPF protocol. Are you a candidate who understands the working and operations of routing protocols? Do you want to get involved in the OSPF operations? If Yes, then this article will provide the help you need to get past certain interview questions. OSPF interview questions exist to test your knowledge of Open Shortest Path First routing protocols. These questions revolve around the basic functioning of routing protocols, and your ability to effectively answer them will rank you as a better applicant.

Top OSPF Interview Questions

  • What Is OSPF Routing Protocol?

Generally known as an Interior Gateway Protocol, OSPF is an IP routing protocol responsible for disseminating IP routing information within a single Autonomous System (AS) in an IP network. It also directs routers when it comes to calculating routes with established criteria. You are expected to know that an OSPF is basically operated as an Open Standard Link that utilizes the Dijkastra algorithm. OSPF routing protocol requires an ID for easy identification. Let’s take a look at how to get one and what it means.

  • What do you understand by an OSPF Router Id?

As I stated earlier, an OSPF ID is defined as an address used to identify the router properly. In the presence of a loopback, you can get your OSPF router ID by picking the highest IP address of the router’s loopback interfaces. And in case you don’t have a loopback. You can also use the highest IP address of the router’s physical interfaces as your OSPF router ID. Now that you know what an ID symbolizes and how to get one, let’s go further to explain the algorithm that governs the OSPF operations.

  • What algorithm anchors the OSPF? 

Understanding the basics of the OSPF will help you properly organize operations within the system. An OSPF utilizes the Dijkastra shortest Path First algorithm to determine the Shortest Path and Link State Advertisements( LSAs). It is crucial to understand that there won’t be effective communication between routing protocols without this algorithm. To that end, it is expedient to know the reason behind the existence of OSPF. Why was it created? What are its core features?

  • What are the key attributes and characteristics of OSPF?

An OSPF was created to regulate the relationship between routing protocols and ensure that they function within the established criteria’ circumference. Some of the basic attributes that you must commit to the heart include the following: OSPF operates a Link State type of protocol, it has an administrative distance of 110 and a reference BW of 108, also its standard revolves around  RFC2328 (OSPFv2), RFC2740 (OSPFv3/IPv6). Its attributes are not limited to these but may also involve the CEF load responding used as an equal cost routes management. It would be best if you also acquainted yourself with the following characteristics: 

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  • Provision of support to VLSM, CIDR, IPV4 &IPV6 creates various areas and autonomous systems and provides unlimited Hop count.
  • OSPF as a routing protocol also utilizes computed cost using the bandwidth of the link as its metric. Neighborships and adjacency are terms common to an OSPF routing protocol. Do you know what they symbolize? If yes, then that’s a good one, but if your answer is negative, you don’t have to worry, because I will tell you about it.
  • What does Neighborship and adjacency represent in an OSPF protocol?

You must understand that an OSPF neighborship means that two routers within the OSPF domain exchange Hello packets. So the process of communication in which Hello packets are transferred from one router to another is called neighborship. While OSPF adjacency is not limited to the exchange of hello packets alone, it also includes exchanging link-state databases. A link-state database can be transferred from one router to another when they both share synchronized views. Going further, you should also know that neighborships can be changed into an adjacency. But how?

  • How can I change neighbor ships into adjacency?

The steps you need to follow to change neighborships into adjacency include the following: First, You need to send a state request to create a link that will inform LSU packets. Second, both routers exchange database description packets to ensure database synchronization. Third, Once database synchronization is established, the two routers can be measured as adjacent. Also, you must know that routers go through several neighbor states in the OSPF before they can become an adjacency. What are the common OSPF states you should be familiar with?

  • List the commonly known OSPF Neighbor States?

    • Down State – This is a process when no Hello packets have been revealed on the interface. It is either the hello packets that were deleted or failed to meet the dead timer interval. Neighbors usually found in the downstate were manually configured.
    • Init State – Please note that these states are progressive, and this stage involves acknowledging a  “Hello message” by a router from an OSPF neighbor. The router receives the message but has not properly established two-way communication with the neighbor.
    • 2way state – This is where two-way communication is established. How is that possible? It is made possible when a router sees its ID in the neighbor’s Hello packets’ field category. A two-way state can also be established when a Data-Based Descriptor packet is received from another neighbor’s init state.
    • Exstart and Exchange state –This state brings you to the point where OSPF neighbors exchange DBD packets. It also involves the building of adjacencies between DR & BDR in the network at this state. You can also transfer routing information using Link State Requests in this state.
    • Loading state – This state follows the exchange state and includes the forwarding of LSRs (Link State Requests) by routers to neighbors. In return, neighbors send in LSUs (Link State Updates) containing information about requested networks. We also have different router types operating within the OSPF protocols that you must be conversant with. So what are they?
  • Briefly explain the router types that exist?

There are four router types namely: 

  • Autonomous System Boundary Routers: This type of router operates by establishing a connection with more than one routing protocol. This connection is built to transfer routing information between two or more routers solely.
  • Internal Routers: This is a limited router type that can only establish OSPF neighbor relationships with devices in a particular area.
  • Area Border Routers: This router type has relationships with devices in multiple OSPF areas. This relationship is possible because it has interfaces in several areas.
  • Backbone Routers: This router type governs the OSPF and has interfaces connected to the backbone area. These router types have their individual significance within the OSPF domain. However, occasions where the neighbor and adjacency encounter certain difficulties while trying to stage a relationship or pass routing information. Let’s quickly examine what could be the possible reasons for these problems.
  • What are the reasons for neighbor adjacency problems?

Neighbor adjacency problems could stem from the following:

  • When OSPF neighbors have the same ID: When neighbors have the same ID, proper identification would not be able to take place, and if neighbors can’t identify a router, then no connection can be established.
  • When OSPF is not configured on one of the routers, all routers should do OSPF configuration for proper connection within the OSPF domain.
  • When an OSPF is not enabled on an interface where needed, you should pay attention to OSPF configurations and enabling when trying to establish a relationship between neighbors and adjacencies. Beyond the problems, OSPF specializes in summarizing areas and autonomous system boundaries. Let’s take a look at some of the benefits attached to the OSPF summarization.
  • What are the benefits of using OSPF Summarization?

Using an OSPF summarization will help you:

  • Minimize update messages and scale your OSPF network to large scales,
  • Limit the quantity of information stored in routing tables,
  • Take off the weight placed on the Router processor and  reduces the bandwidth practice. Having understood that, you must know that the autonomous system need to be divided into areas for certain reasons. 
  • Why should the Autonomous System be divided Into Various Areas?

The autonomous system controls the operation of different routers so to create a balance in management and efficiency. It needs to be divided into various areas. Other reasons which revolve around this can include: to ensure that updates between routers are kept at a minimum level, conserve resources, and prevent network problems. You may have probably seen the OSPF LSA, LSU, and LSR, but do you know what they represent in the OSPF domain? Let me give you a brief illustration of them.

  • Briefly explain OSPF LSA, LSU, and LSR?

    • OSPF LSAs (Link-State Advertisements)- The LSAs facilitate the exchange of routing and topology information.  It ensures proper communication between the router’s local routing topology and all other local routers in the same OSPF area. 
    • OSPF LSR(Link State Request) – is the platform where all routers send requests to all LSAs not found in its topology table. The overall functioning of an OSPF routing protocol rest on the effective utilization of LSR.
    • OSPF LSU (Link State Update)- This is a link used by other routers in responding. It also contains all LSAs requested by the neighbor. It would be best if you got used to these terms so that you misplace one for another.

FREQUENTLY ASKED QUESTIONS

  • Can two Routers in an OSPF domain have same Router ID?

OSPF does not permit neighbor relationship between Routers with same RIDs. So two Routers within an OSPF domain cannot have same IDs.

  • Is it possible to configure an OSPF without the backbone area?

Yes it is possible to configure an OSPF without backbone only in an intra-area communication.

For more information on OSPF protocols, please visit https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13699-29.html.Please feel free to drop your comments in the box below. We would love to hear from you!

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Top OSPF Interview Questions [+Sample Answers]

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