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OSPF Interview Questions

OSPF (Open Shortest Path First) is a widely-used link-state routing protocol. Nowadays, a career in networking can be quite lucrative, and its opportunities are increasing rapidly. If you plan to apply for an OSPF post, then there are a set of OSPF interview questions you must be prepared for. This article acquaints you with top OSPF interview questions covering different areas. Take a look!

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Do not worry! You can totally rely on us if you want to practice OSPF Interview Questions. We have researched, picked up, and listed some OSPF interview questions that are most frequently asked.

It has become increasingly important to protect our data in this fast-changing technological era. Indeed our life has become easier due to the digital revolution. But the world is becoming more complex due to this technological advancement. The network is a crucial element for using technology. And we can access that with the help of a router. But the process is not that simple. It involves Internet protocol. OSPF is one such routing Internet protocol that takes care of the algorithm from source to destination. It is a task for experts, and thus, the demand for OSPF professionals is constantly growing.

OSPF

We will move to OSPF Interview Questions- 2022 (Updated) for the following:

Top OSPF Interview Questions: 

  1. Explain the functioning of OSPF
  2. Name the five packets used in OSPF
  3. What is the OSPF router ID?
  4. What is OSPF adjacency?
  5. Describe the different router types
  6. What do you think IP-OSPF-Transmit-Delay is used for?
  7. Do you need any special commands to run OSPF on BRI/PRI links?
  8. Describe the role of topology and routing table in OSPF
  9. Name the different network types in OSPF
  10. Differentiate between E1 and E2

OSPF Interview Questions for Freshers:

If you are new to the world of OSPF, here are some fundamental questions to prepare for your interview. 

1. Explain the functioning of OSPF.

OSPF utilizes the Dijkstra Shortest Pathway First algorithm to decide the shortest and the most suitable path. It shows the status of straight related links using LSA- Link State Advertisements. It also notifies in case of any updates or changes in any of the links. Link State Advertisements are refreshed every 30 minutes. 

2. Which LSA is used for intra-communication in OSPF?

Type-3 Link State Advertisement is used for communication amongst the regions. Type 4 and Type 5 Link State Advertisements are for communicating within other external protocols.  

3. Name the different types of LSA- Link State Advertisement used in OSPF.

Different types of LSA used in OSPF are given below:

  • External LSA
  • Router LSA
  • Network LSA
  • Network Summary LSA
  • ASBR Summary LSA
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4. Name the five packets used in OSPF.

There are five types of OSPF packets. These are:

  • HELLO
  • DBD
  • LSR
  • LSU
  • LSack

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5. What are the different area types in OSPF?

Different types of OSPF are:

  • Type 1: Represents a router
  • Type 2: Represents the pseudo node for a multi-access link
  • Type 3- A network link summary that follows the internal route.
  • Type 4- Represents an ASBR
  • A route that is external to the OSPF domain
  • Used in stub areas in place of a type 5 LSA

OSPF LSA Types

6. What are timers in OSPF?

There are two types of intervals in OSPF. These are:

  • Hello Interval: This interval defines the duration of how frequently one OSPF router will send the hello packet to the other OSPF router
  • Dead Interval: This interval defines the duration of how much the extended router must wait for hello packets before the neighbors are declared dead.

Timers in OSPF

7. Which multicast address does the OSPF use?

OSPF uses two multicast address- 224.0.0.5 and 224.0.0.6

8. What is the OSPF router ID?

OSPF router ID works as an identifier. It is used to recognize the router. It is a 32-bit number.

9. How can you change the reference bandwidth in OSPF?

The 11.2 Cisco IOs Software Release allows the change of Bandwidth. You can change it with the help of the OSPF auto-cost-reference bandwidth command under “router OSPF.” The default bandwidth is 100 Mbps. And the OSPF link cost is a 16-bit number. Thus, the maximum value for this change is 65,535.

10. Are OSPF routing protocol exchanges authenticated?

Yes, the OSPF routing protocol exchanges are authenticated. OSPF can authenticate all the packets to be exchanged between the neighbor routers. This authentication can occur through easy passwords. For configuring the authentication, you can use the command IP OSPF authentication key.

This will assign an eight-octet password to the interface attached to each area. However, if you want to configure the authentication on some interfaces, you can go for alternate methods or different interfaces belonging to the same area.

11. What is OSPF adjacency?

An OSPF adjacency is a theoretical link for a neighbor over which we can send the Link State Advertisements (LSAs).

12. How can you change the neighboring ships into adjacency?

Following are the steps to change neighboring ships into adjacency:

  • Firstly, we need to send a state request to create a link. This link will inform packets.
  • Then both the routers will exchange database description packets. This will ensure database synchronization.
  • Once the database synchronization is established, the two routers can be measured as adjacent routers.

13. What is the full form of LSA, LSU, and LSR in OSPF?

In OSPF, LSA means Link State Advertisement. While LSU is Link State Update, LSR stands for Link State Request.

14. What multicast IP does DR/BDR router and non-DR use?

In DR/BDR, it uses 224.0.0.5 and 224.0.0.6.

15. Describe the different router types.

Different router types can be described as follows:

  • Autonomous System Boundary Routers: An OSPF Router that advertises exterior routes into the OSPF domain.
  • Internal Routers: An OSPF Router whose all interfaces belong to the same area.
  • Area Border Routers: An OSPF Router that has interfaces in more than one area.
  • Backbone Routers: An OSPF Router that is inside the Router in Area 0.

OSPF Interview Questions for Experienced:

If you are up skilling yourself in the OSPF field and looking for advance level questions to prepare for, you can take help from the below list.  

1. Can you use the distribute-list in/out command with OSPF to filter the routes?

The distribute-list commands are supported in OSPF but only in some conditions. It is also different from a distance vector routing protocol like Routing Information Protocol and Enhanced Interior Gateway Routing Protocol (EIGRP). And it is noteworthy that OSPF routes cannot be filtered while entering the OSPF database. The distribute-list in command only filters from entering the route. It doesn’t prevent the propagation of link-state packets. Thus, the route memory is not conserved in this process. Also, this propagation might affect other routers as well.

Also, it would help if you were careful as the distribution list in command needs to be implemented carefully. If not, there are chances that loops will occur. This command only works on the routes of Autonomous System boundary Routers.

2. What is Link State Retransmit Interval? Which command is used to set it up?

OSPF is required to send an acknowledgment of every new link state advertisement. This is done by sending LS packets. These packets keep on transiting until they are acknowledged. Retransmissions are defined by this Link State Retransmit Interval. The command for this IP OSPF retransmits interval. The value is 5 seconds by default. 

3. What do you think IP-OSPF-Transmit-Delay is used for?

IP-OSPF-Transmit-Delay is used to add a specified time to the age field of an update. It needs to be added before the transmission over a link. If not, then the time in which LSA propagates won’t be considered. The value is 1 second by default. This is more significant on low-speed links for certain reasons. 

4. Can an OSPF default be originated into the system based on external information?

Not really. OSPF can only generate a default when it is configured using the command default-information originate. It also works if there is already a default network in the box from a different process. Remember that the default route is 0.0.0.0. If an OSPF-enabled router does not have a default router and you want to generate it, use the command “default-information originate.”

5. Do you need any special commands to run OSPF on BRI/PRI links?

Apart from using the normal commands for OSPF configuration, we require some special commands for running it in the Primary Rate Interface and the Basic Rate Interface. When you use the normal OSPF configuration commands, you should use the dialer map command. Also, broadcast keywords are essential here to indicate that the broadcasts need to be forwarded. 

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6. Do you need any special commands to run OSPF on asynchronous links?

We require special commands to run OSPF on links that do not occur at a certain predetermined time. For this, we can use the async default routing command on the asynchronous interfaces. This enables the router to continue passing updates to other routers with the same interfaces. 

7. What is the significance of area 0 in OSPF?

Area 0 is known as the backbone area in OSPF. In simple language, all other areas send their transfer through this area only. It cannot function without this. The entire routing area is also distributed through this.

8. What are the four types of areas in OSPF?

Four types of areas in OSPF are given below:

  • Backbone area: The backbone area (Area 0) is the core of an OSPF network. Every single area connects with it, and all traffic among the areas must travel through it. The backbone area distributes all the routing among areas. While all other OSPF areas must connect to the backbone area, this connection doesn’t need to be direct and can be made through a link. 
  • Normal OSPF area: In a normal OSPF area, there are no restrictions; the area can carry all types of routes.
  • Stub OSPF area: This area does not receive routes from other autonomous systems. We perform routing from the stub area via the default route to our backbone area.
  • NSSA area: NSSA means The Not So Stubby Area. External routes can be imported by it with some exceptions.

9. Describe the role of topology and routing table in OSPF.

OSPF maintains the below three tables:

  • Topology table: A topology contains all the routes in a particular network area.
  • Routing table: A routing table contains the best routes for each network. 
  • Neighbor Table: The neighbor table contains a list of all neighboring routers. 

10. List the benefits of OSPF summarization?

The benefits of OSP summarization are:

  • It reduces the update messages.
  • It takes the OSPF to larger scales.
  • It limits the quantity of the information stored in the routing tables.
  • It reduces the load on the router processor. 
  • It reduces the bandwidth practice.

11. Name the different network types in OSPF.

The different network types in OSPF are listed below:

  • Broadcast network type- Data packets are sent from one router to another.
  • Non-broadcast network type- The network here is not capable of broadcasting. It only supports access to devices.
  • Point-to-point network type- Data packets are sent between two routers.

Different network types in OSPF

12. Describe the function of the neighbor router.

The functions of the neighbor router are given below:

  • It contains the router ID of each neighbor router.
  • It knows about the current state of each neighbor router.
  • It knows the IP address of the remote interface of each neighbor.

13. What is a Virtual link in OSPF?

According to the OSPF rule, inter-area communication happens when at least one area is 0 or the backbone area. A virtual link has to be used to form a logical association in case it’s not possible. And a virtual link is not feasible via the stub area.

14. What are DR and BR in OSPF?

In OSPF, DR means Designated Router, and BDR means Backup Designated Router. Both of them are used for exchanging information between different routers. DR is the contact point for the network segment by default. BDR is selected for the network to avoid problems with DR whenever these occur. 

15. Differentiate between E1 and E2.

E1 is an External Type route that defines the cost that is required to the destination from ASBR (Autonomous System Boundary Router). Here, the internal cost and the external metric are considered.
E2 is also the External Type route, but it doesn’t include the internal costs. In E2, it calculates only the external metrics for reaching a particular destination.

Difference Between E1 and E2

Frequently Asked OSPF Interview Questions: 

1. What is OSPF?

The full form of OSPF is Open Shortest Path First. It is a routing Internet protocol. It is the open link and works by utilizing Dijkstra’s SPF algorithm to find straight paths from your source to your destination. It is an intra-domain protocol. This means that it is used within a particular area of a network. It is designed only for a single autonomous system.

2. What is the OSPF protocol used for?

The work of a router is to learn routes. OSPF does the same. It learns about each router within the entire network. The link-state routing algorithm present in it gives out the information of every domain. OSPF learns the information by sending Link State Advertisements.

3. How does OSPF Work?

The steps are given below:

  • Step 1: The first step in the procedure is to become OSPF neighbors. Two routers running on OSPF that too on the same link will create a neighbor relationship.
  • Step 2: In the second step, OSPF exchanges LSDB information on the database. 
  • Step 3: Once the LSDB information is exchanged, OSPF chooses the best route with the help of the SPF calculation.

4. Name the critical attributes of OSPF.

Following are the critical attributes of OSPF:

  • Equal Cost Routes management: CEF Load corresponding
  • Protocol Type: Link State
  • Transport IP (Port 89) submit to network walks website
  • Metric: Cost (Bandwidth)
  • Standard: RFC2328 (OSPFv2), RF C2740 (OSPFv3/IPv6)

5. What are the characteristics of OSPF?

The characteristics of OSPF are given below:

  • OSPF falls under the category of a classless routing procedure that supports VLSM and CIDR.
  • The organizational distance of OSPF routes is 110.
  • OSPF does not have any hop-count limit. The support is limitless.
  • OSPF takes cost as its metric. This is computed based on the link bandwidth.
  • OSPF also supports an independent system.

6. Why are DR and BDR important in OSPF?

BDR and DR are important because they solve the following two problems in OSPF:

  • Excessive LSA flooding
  • High No. of Adjacencies

7. Is it possible to have OSPF over a GRE tunnel?

Yes. It is possible to have OSPF over a GRE tunnel.

8. What do you understand by OSPF summarization?

OSPF summarization, also known as Route summarization, helps in reducing OSPF traffic and computation. This results in preventing the memory from getting wasted.

9. Which parameters should match for two routers to become neighbors?

To make two routers neighbors, the following parameters should match:

  • Area ID
  • Subnet
  • Authentication
  • Hello and Dead Interval

10. What are the seven stages of OSPF?

OSPF goes through the below seven stages:

  • Down 
  • Attempt
  • Two ways
  • Exstart
  • Exchange
  • Loading
  • Full

Conclusion:

These were some of the OSPF Interview Questions that could be asked in your next interview. Remember that it is an essential part of every dimension of life, especially work. And it opens the doors of opportunities in many ways. You cannot miss a chance to grab them, and this article will make you even well-versed with the knowledge. Keep practicing, and good luck!

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Last updated: 03 December 2022
About Author
SaiKumar Kalla

Kalla Saikumar is a technology expert and is currently working as a content associate at MindMajix. Write articles on multiple platforms such as ServiceNow, Business Analysis, Performance Testing, Mulesoft, Oracle Exadata, Azure, and other courses. And you can join him on LinkedIn.