Top 30 OSPF Interview Questions and Answers (2024)

Introduction

OSPF is one of the most important topics in Networking and IT domain interviews. Every employer wants a candidate who has deep knowledge of OSPF protocol, and that is why we came up with the most asked OSPF interview questions.

These OSPF interview questions and answers are very carefully curated by some of the leading minds in this domain to help freshers and experienced candidates crack the OSPF interviews. PyNet Labs has launched another batch of OSPF BGP Training for network engineers to learn OSPF in depth.

So, no need to get nervous before any interview anymore as we have the perfect questions that will most likely be on your interviewers’ list. You can also get OSPF interview questions and answers pdf at the last of this blog.

About OSPF

OSPF (Open First Short Path First) is a link-state routing protocol used to determine the optimum path for forwarding network traffic for which it uses the Shortest Path First (SPF) algorithm. It is widely used in enterprise networks and service provider networks due to its scalability, fast convergence capabilities (adapts quickly to changes), and excellent support for network designs.

OSPF is designed as an Interior Gateway Protocol (IGP) to move packets within a large autonomous system or routing domain. With OSPF, networks can increase speed, reliability, and optimal performance. Moreover, it is capable of automatically adapting to changes in network topology and recalculating the best paths if needed which makes it a dynamic routing protocol. Let’s start with OSPF Interview Questions for Freshers.

Basic OSPF Interview Questions and Answers

Here is a list of the most asked OSPF interview questions and answers:

Q1. What is the OSPF routing protocol?

OSPF stands for Open Shortest Path First. It is a link-state routing protocol that uses the Shortest Path First (SPF) algorithm to find the best path between the source and the destination router.

It was developed by Internet Engineering Task Force (IETF).  It was developed as an Interior Gateway Protocol (IGP). A protocol has a primary aim to move the packets within a large autonomous system or routing domain.

It is a network layer protocol.

Q2. What is the area in OSPF?

The term “area” refers to a collection of OSPF routers, networks, and links with the same identification for the area. The router in an area has to keep an operational database of the area it is part of.

Q3. What is the Backbone area?

The area known as the backbone (Area 0) is the center of an OSPF network. The other areas are linked to it, and all communication between the areas has to pass through it. The entire route between regions is carried out through the backbone region.

Q4. What is an Autonomous System Boundary Router ASBR?

An Autonomous System Boundary Router (ASBR) is a type of router that is capable of running multiple protocols. It also acts as a gateway for the routers that are placed outside the OSPF domain and to the routers too that operate on different protocols.

An Autonomous System Boundary Router is capable of importing and translating various protocol routes into OSPF via a process called redistribution.

Q5. What is a Designated Router (DR)?

A Designated Router (DR) is used to reduce the number of adjacencies created. A DR has the responsibility of distributing the LSAs to all the other routers. In a broadcast network, a DR is chosen, and all the other routers share their DBD to this DR.

In a broadcast network, routers make a request to the DR for an update, to which DR answers with an update.

Q6. How OSPF DR and BDR are elected?

The election between OSPF DR and BDR takes place in the broadcast network or a multiaccess network. The criteria for the selection are as follows:

• DR is declared on the basis of the router priority. The router with the highest priority becomes the DR.

• However, if both routers have the same priority, then the router with the highest router ID is selected.

Firstly, the loopback address (highest) is taken into consideration. However, if, in any case, the loopback is not configured, then the active IP address on the router interface is taken into consideration, and the highest active IP address takes it.

Q7. What are the OSPF packet types?

OSPF uses packets to send and receive requests, ensure delivery of Link-state advertisements and establish & maintain adjacencies. Packets are also used to describe the link-state databases. The OSPF packet types are as follows:

• Hello – The Hello packet type is used to discover neighbors, build adjacencies and maintain them.

• DBD (Database Description)– is used to determine whether the LSDB between two routers is identical. The DBD is like an overview of LSDB.

• LSR (Link-state request) – It is used to request link-state records of a specific type from an OSPF neighbour.

• LSU (Link-state update)– This packet sends specific links-state records to those who requested them. This packet is similar to an envelope that has multiple LSAs inside.

• LSAck (Link-state acknowledgements)– This packet is used to acknowledge others as OSPF is a very reliable protocol.

Q8. What are the OSPF network types?

The default OSPF network type is based on the media used for the connection, and it can be changed independently of the media used. Cisco provides five OSPF network types, as listed in the table below.

Q9. Explain OSPF Virtual Link?

The term “virtual link” does not refer to a physical connection. In fact, it is an intelligent link that uses the cheapest route connecting both the ABR of the area that is not connected to the backbone region and the backbone ABR of the transit zone.

A virtual adjacency along the link is created and routing data will be exchanged.

Q10. Explain the Stub Area and different types of Stub area?

Sometimes, we need to control the advertisement of external routes into an area. This area is called the stub area, and Stub areas are not capable of importing routes external to OSPF.

LSAs Type 4 & Type 5 get filtered from the stub areas. ABR injects a default route into that area in place of external routes.

There are 3 restrictions that apply to the OSPF stub area: –

1. Virtual links are not allowed in the stub area.

2. The stub area is not allowed to be a backbone area.

3. Autonomous System Boundary Routers are not allowed.

Different types of Stub areas are:

• Totally Stubby Area

Like stub areas, the totally stubby area does not receive type4 and type5 LSA from their ABRs. However, they also do not receive type 3 LSAs, and it allows the advertisement of the internal router in that area.

• Not-So-Stubby Areas

The motivation behind NSSA is to allow the OSPF stub area to carry external routes. The external router is imported into OSPF NSSA as Type 7 LSA by ASBR. Now, ABR converts the Type 7 LSA back into Type 5 LSA as Type 7 LSA is not allowed to go into area 0.

• Totally NSSA

In Totally NSSA, the Type3 also gets filtered with Type4 & Type5.

These are the best OSPF Interview Questions and Answers for Freshers. Let’s move on and see the advanced OSPF Interview Questions for Experienced Candidates.

Advanced OSPF Interview Questions and Answers

Here is a list of most asked advanced OSPF Interview Questions and Answers.

Q11. Can you filter the routes using the distribute-list in/out of the command in OSPF?

OSPF supports distributing list instructions, but only under certain conditions. It differs from distance vector routing protocols such as the Routing Information Protocol and the Enhanced Interior Gateway Routing Protocol. It is also worth noting that OSPF routes cannot be filtered when they enter the OSPF database. The distribute list in the command merely prevents the information from entering the route. It does not prevent link-state packets from propagating.

As a result, the route memory is not preserved in this process. This spread may also damage other routers. It would also assist if you were cautious, as the distribution list in the command must be implemented correctly. Loops are possible if this is not done. This command only applies to the routes of Autonomous System boundary Routers.

Q12. What exactly is the Link State Retransmit Interval? Which command is used to configure it?

In OSPF, an acknowledgment is required of every new link state advertisement. This is carried out by sending LS packets. These packets continue to transit until they are acknowledged, and this Link State Retransmit Interval defines retransmissions. The command IP OSPF is used to retransmit the interval and its by default value is 5 seconds.

Q13. To run OSPF on asynchronous links do you need any special command?

To run OSPF on asynchronous links we require special commands when they do not occur at a certain predetermined time. In this scenario, we can use the async default routing command on the asynchronous interfaces. This allows the router to keep sending updates to other routers with the same interfaces.

Q14. What is the importance of OSPF Area 0?

In OSPF, Area 0 is referred to as the backbone area. In simple words, we can say, all other areas send their transfers through this Area 0. It is not possible to function without this as the entire routing area is distributed through the Area 0.

Q15. What is the role of topology, routing, and neighbour tables in OSPF?

The role of all three are mentioned below:

• Topology table: A topology table comprises all the routes in a specific network area.
• Routing table: A routing table comprises all the best routes for each network.
• Neighbour Table: A neighbour table comprises a list of all neighboring routers.

Q16. What are the different area types in OSPF?

Different types of OSPF are:

• Type 1: It represents a router 
• Type 2: It represents the pseudo node for a multi-access link 
• Type 3: A network link summary that follows the internal route. 
• Type 4: It denotes an ASBR 
• A route that is not within the OSPF domain 
• In stub regions, it is used in place of a type 5 LSA.

Q17. How can the neighboring ships into adjacency be changed?

The steps to change neighbouring ships into adjacency are as follows:

Step 1: To create a link, we must make a state request and this link will relay information to packets.

Step 2: Then, both routers will send database description packets to each other, and this will ensure that the databases are synchronized.

Step 3: Once the database is synchronized, the two routers can be measured as adjacent routers.

Q18. What are the advantages of OSPF summarization?

The advantages of the OSPF summarization are given below:

• OSPF minimizes update messages.
• OSPF takes OSPF to a higher level.
• OSPF limits the amount of information stored in the routing table.
• OSPF minimizes the load on the router processor.
• OSPF minimizes bandwidth usage.

Q19. Is it possible in OSPF to allocate a number to one party and leave the other party without a number?

No, in OSPF, it is not possible to assign a number to one party and leave the other party without a number. If you attempt to do this, the OSPF database will become inconsistent, and routes will not be entered into the steering table.

Q20. What multicast addresses does OSPF allow?

OSPF allows two multicast addresses, which are 224.0.0.5 and 224.0.0.6

Q21. How can the reference bandwidth be changed in OSPF?

Cisco IOS Software Release 11.2 is used to change the bandwidth. You can modify this via the OSPF auto-cost-reference bandwidth command under “Router OSPF”. Bandwidth is 100 Mbps by default and OSPF link expenditure is a 16-bit number. Thus, 65,535 is the high price for this change.

Q22. Why is IP-OSPF- transmit-delay used?

It is used to add a specified time to the age field of the update, which is important before sharing any link. If not, LSA propagation time will not be allowed. The default value is 1 second and this is more important on low-speed links for some factors.

Q23. How many types of OSPF timers are there?

There are two types of OSPF timers:

• Dead Interval Timer: It allows you to specify how long the extended router holds a Hello packet before declaring the neighbour dead.
• Hello Interval Timer: It allows you to specify how typically an OSPF router shares Hello packets with other OSPF routers.

Q24. Why does OSPF refer to a loop-free protocol?

OSPF refers to a loop-free protocol due to the existence of a link-state database. In OSPF, the database provides a full view of the network and prevents the routing loops common in distance vector protocols. OSPF avoids loops by implementing an inter-area loop-free topology, which is why it is known as a loop-free protocol.

Q25. What are the main attributes of OSPF?

Following are some of the key attributes of OSPF:

• Equal Cost Route Management: CEF Load Compatible
• Protocol Type of OSPF: Link State
• Transport: Give IP (port89) to the Network Walk website
• Metric of OSPF: Cost (Bandwidth)
• Standard: RFC2328 (OSPFv2), RFC2740 (OSPFv3/IPv6)

These are the most asked OSPF Interview Questions and Answers for Experienced. Now, it is time to see some scenario-based OSPF Interview Questions and Answers.

Scenario-Based OSPF Interview Questions and Answers

Here are some most-asked scenario-based OSPF Interview Questions with their answers:

Q26. Assume you have a network with multiple routers in a single area. One of these routers is advertising incorrect routes. How would you troubleshoot this issue?

To troubleshoot this issue, you first need to check the OSPF neighbor adjacencies with the “show ip ospf neighbor”. After that, verify OSPF configurations and check if the router IDs are unique across the network.

Use “show ip ospf database” to compare the OSPF LSAs and ensure they are consistent across all the routers. Now, you should check for incorrect router redistribution in OSPF configuration and confirm if OSPF areas are correctly defined. You can also use commands like “show ip route” and “debug ip ospf events” to troubleshoot issues further.

Q27. Two routers in the same OSPF area are not able to form a neighbor adjacency. How to troubleshoot this issue?

First, you need to verify the connectivity between layer 2 and layer 3 of the two routers. After that, check whether the IP Addresses are correctly assigned and it is reachable using the command “ping”.

Now, use the command “show ip ospf interface” to verify that the Hello and Dead Timers are the same on both sides. Also, ensure that both the routers have compatible network types. Lastly, if OSPF authentication is configured on both routers, and if yes, it is consistent on both routers.

Q28. Suppose that the OSPF routers in your network are generating LSAs of type 1 and 3, but suddenly, some type 5 LSAs start appearing. What could be the reason for that, and how can this issue be addressed?

Type 5 LSAs are external routes injected by ASBRs and are used to advertise routes learned from the redistribution of other routing protocols, such as BGP. If you see unexcepted type 5 LSAs in your network, you should check for the routers that are acting as ASBRs. Use the command “show ip ospf border-routers” to do so.

To resolve this issue, you should use route filtering or control redistribution using route maps.

Q29. You have to configure OSPF over a Frame Relay Network. What consideration should you undertake when configuring OSPF on non-broadcast multi-access networks?

OSPF has the ability to run on various network types, including broadcast, non-broadcast, point-to-point, or point-to-multicast. OSPF treats Frame Relay or other NBMA networks as non-broadcast networks. So, it requires manual neighbor configuration using the “neighbor” command.

Now, you can choose to configure the OSPF network type as point-to-point or point-to-multicast as per your requirement. It will simplify the neighbor discovering process. You need to use “ip ospf network” to configure the network type on the interface. Now, make sure to define OSPF neighbors manually in NBMA Configurations to a suitable network type as per your topology.

Q30. You have to advertise a default route from an OSPF router to the rest of the OSPF Domain. How will you achieve it?

To advertise a default route throughout the OSPF Domain, you should configure the router with a static default route (ip route 0.0.0.0 0.0.0.0 [next-hop]) and use the “default-information originate” in the OSPF Configuration. It will tell the router to advertise the default route as a Type 5 LSA into other OSPF areas.

However, suppose you want to advertise the default route only when the router has an active default route in its routing table. In that case, you need to use “default-information originate always”.

These are some of the most asked scenario-based Interview Questions on OSPF.

OSPF Interview Questions and Answers PDF

If you are looking for a PDF with more OSPF interview questions and answers, you should go to:

CCNA Interview questions and answers

Conclusion

In this blog, we have explained the most important OSPF Interview questions and their answers for freshers as well as experienced professionals for you to go through it and make your concept much stronger.

In conclusion, OSPF is an essential topic for freshers and experienced networking professionals preparing for their interviews and its thorough understanding is very important. Along with it you must know what are the question that an interviewer may ask. We hope you liked this article; please give your valuable feedback in the comment box below. Don’t forget to subscribe to our free newsletter for more awesome content.

All the best for your interview.