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Top 35+ Most Asked OSPF Interview Questions and Answers

1) What do you understand by OSPF? / What is the OSPF Routing protocol?

OSPF is an acronym that stands for Open Shortest Path First. It is an open average link-state routing protocol that utilizes Dijkstra's SPF algorithm and is used to find the straight and the shortest path. It follows that by populating the routing table with the resulting best path.

2) What is the algorithm used by OSPF?

OSPF uses the SPF (Shortest Path First) algorithm, also known as the Dijkstra algorithm. It is used for calculating the best path and preparing the OSPF database.

3) Why is it required to divide the autonomous system into various areas in OSPF?

In the OSPF algorithm, dividing the autonomous system into various areas keeps the route updates to a minimum. It reduces the number of routing updates in the network, conserves resources, and keeps problems from propagating throughout the network.

4) What are the key attributes of OSPF?

Following is a list of some key attributes of OSPF:

  • Equal Cost Routes management: CEF Load corresponding
  • Protocol Type of OSPF: Link State
  • Transport: IP (Port89) submit to network walks website
  • Metric of OSPF: Cost (Bandwidth)
  • Standard: RFC2328 (OSPFv2), RFC2740 (OSPFv3/IPv6)

5) What are the different functioning and working of OSPF?

Following is the list of different functioning and working of OSPF:

  • OSPF uses the Dijkstra Shortest Path First algorithm to determine the shortest path.
  • OSPF follows a hierarchical network design using Areas.
  • OSPF forms neighbor relationships with adjacent routers in the same area.
  • OSPF is a classless protocol and supports VLSMs.
  • Instead of advertising the distance to connected networks, OSPF advertises the status of directly connected links using Link-State Advertisements (LSAs).
  • OSPF sends updates (LSAs) when there is a change to one of its links and will only send the change during the updation. LSAs are additionally refreshed every 30 minutes.

6) What is the benefit of dividing the entire network into areas?

Following are the advantages of dividing the entire network into areas:

  • It decreases the routing overhead.
  • It can speed up the convergence.
  • It can also confine the network instability to single areas of the network.

7) What are the key characteristics of OSPF?

Following is a list of the key characteristics of OSPF:

  • OSPF is a classless routing protocol that supports VLSM and CIDR.
  • OSPF allows for the creation of areas and an autonomous system.
  • OSPF supports both IPV4 and IPV6.
  • OSPF uses cost as its metric, which is computed based on the bandwidth of the link.
  • OSPF has no hop-count limit. It supports unlimited Hop count.
  • OSPF routes have an administrative distance of 110.

8) What is OSPF Router ID, and how is it elected?

The OSPF Router ID is a 32-bit number similar to a normal IP address. It is elected as the highest IP Address of Loopback interfaces or the Highest IP of the active physical interfaces if loopback is not present. We can set it manually, but when the OSPF process is reset completely, or the device is reloaded, it will take effect. To avoid database issues, we should run only one router-id per device.

9) What is the role of topology and routing table in OSPF?

The OSPF process is used to build and maintain three separate tables:

  • Neighbor table: This table contains a list of all neighboring routers.
  • Topology table: This table contains a list of all possible routes to all known networks within an area.
  • Routing table: This table contains the best route for each known network.

10) How does OSPF work? / Explain the working of OSPF.

OSPF protocol is used by routers to send routing information and associated changes in the table. Instead of sending the entire table, OSPF only sends the part that has been altered. This configuration information is stored in a Link State Database, and using this data determines the best and fastest route to send data packets to a destination.

11) What are the key benefits of using OSPF Summarization?

The key benefits of using OSPF Summarization are:

  • It minimizes the update messages and scales the OSPF network to large scales.
  • It is also used to limit the quantity of information stored in routing tables.
  • It takes off the weight placed on the Router processor and reduces the bandwidth practice.

12) What is the full form of LSA, LSU, and LSR in OSPF?

In OSPF, the full form of LSA is Link State Advertisement. It is used in the Link State database for sharing the basic routing topology information to the other local routers. OSPF fills the Link State database with the LSA information. These LSAs are exchanged between routers until all of them have the same database of topology data.

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

When two neighbors try to exchange routes, they have to send each other a list of all LSAs in their respective topology databases. Then each Router checks its topology database and sends an LSR message requesting all LSAs that were not found in its topology table. Other Router responds with the LSU that contains all LSAs requested by the neighbor.

13) What are the different types of LSA in OSPF?

Following are the different types of LSA in OSPF:

  • LSA Type 1: Router LSA
  • LSA Type 2: Network LSA
  • LSA Type 3: Summary LSA
  • LSA Type 4: Summary ASBR LSA
  • LSA Type 5: Autonomous system external LSA
  • LSA Type 6: Multicast OSPF LSA
  • LSA Type 7: Not-so-stubby area LSA
  • LSA Type 8: External attribute LSA for BGP
  • LSA Type 9: Link Scope Opaque (OSPFv2) / Intra Area Prefix LSA (OSPFv3)
  • LSA Type 10: Area Scope Opaque LSA
  • LSA Type 11: AS (Autonomous System) Scope Opaque LSA

14) What are the different networks types in OSPF?

Following are the different network types available in OSPF:

  • Point-to-point network types: In this network type, the data packets are sent between exactly two routers.
  • Broadcast network types: The data packets are sent from one Router to multiple routers in this network type.
  • Non-Broadcast network types: In this network type, the network here supports access to many devices but does not allow broadcast capability.

15) How can you change the neighbor ships into adjacency?

We need to follow the following steps to change the neighbor ships into adjacency:

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

16) What is the main usage of the OSPF protocol?

The OSPF protocol is mainly used for exchanging routing information between the neighboring routers. The LSA (Link State Advertisement) helps the routers ensure that all of the routers have the same topology information in their databases and helps determine the nearest hop address by using the SPF algorithm.

17) What are the different OSPF Router types?

Following is a list of four OSPF Router types:

  • Internal Routers (IR): Internal Routers are OSPF Router whose all interfaces belong to the same area.
  • Backbone Routers (BR): Backbone Routers are the OSPF Router that acts as Internal Router in Area 0.
  • Area Border Routers (ABR): Area Border Routers are the type of OSPF Routers that has interfaces in more than one area.
  • Autonomous System Boundary Routers (ASBR): Autonomous System Boundary Routers are the type of OSPF Routers that advertise external routes into the OSPF Domain.

18) What are the issues that DR and BDR solve in OSPF?

DR and BDR solve the following two problems in OSPF:

  • Excessive LSA flooding
  • High No. of Adjacenci

19) What do you understand by Link State Retransmit Interval?

OSPF sends recognition of each newly conventional LSA. LSAs are retransmitted until they are approved.

The Link State Retransmit Interval defines the time among retransmissions. We can use the command IP OSPF retransmit-interval to place the retransmit-interval.

20) What are the most commonly known OSPF Neighbor States?

Following is a list of the most commonly known OSPF Neighbor States:

Down State: In the Down State process, no Hello packets have been revealed on the interface. The Hello packets either are deleted or fail to meet the dead timer interval. In this state, the neighbors usually found are manually configured.

Init State: The Init State is progressive and involves acknowledging a "Hello message" by a router from an OSPF neighbor. The Router receives the message in this state but has not properly established two-way communication with the neighbor.

2way state: In this state, two-way communication is established 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 provides a point where OSPF neighbors exchange DBD packets. In this state, the adjacencies between DR & BDR in the network are also built. This state also facilitates to transfer of routing information using Link State Requests in this state.

Loading state: This state follows the exchange state and is responsible for forwarding LSRs (Link State Requests) by routers to neighbors. In return, neighbors send in LSUs (Link State Updates) containing information about requested networks.

21) Is it possible to have one numbered side and leave the other side unnumbered in OSPF?

No. It is not possible to have one numbered side and leave the other side unnumbered in OSPF. If you try to do this, it will create an inconsistency in the OSPF database, which will prevent the routes from being installed in the steering table.

22) What is the difference between E1 and E2 in OSPF?

Difference between E1 and E2 in OSPF:

  • E1: In OSPF, E1 is an External Type route that defines the cost required from ASBR (Autonomous System Boundary Router) to the destination. In this type, the internal cost and the external metric are considered.
  • E2: E2 is also the External Type route, but the internal costs are not considered in this type. In E2, only the external metrics are calculated to reach a particular destination. If you have two E2 routes for reaching a destination, the path having the lowest metric from the ASBR is considered for sending data.

23) What do you understand by OSPF adjacency?

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

24) What are the OSPF Hello and Dead Interval?

OSPF implements two timers and hello packets to analyze whether a neighboring router is alive or not.

  • Hello Interval: The Hello interval is used to describe the time interval in which an OSPF router transmits a Hello packet. The default interval value is 10 seconds for point-to-point links and a shared network in this interval.
  • Dead Interval: The Dead interval is used to define a time interval for which a router will wait before declaring a neighboring router dead. Both Hello and Dead interval values can be different depending on the type of network.

25) What are the five OSPF packet types?

Following is the list of five main OSPF packet types:

  • DBD
  • LSU
  • LSR
  • LSack

26) What do you understand by the OSPF Router ID?

The OSPF Router ID is an identifier used to recognize the Router, and it is a 32-bit number.

27) Is it possible to have OSPF over a GRE tunnel?

Yes, it is possible to have OSPF run over a GRE tunnel.

28) What are the different tables maintained by OSPF?

The router participating in the OSPF routing protocol has to maintain three OSPF tables. Following is the list of these tables:

  • Neighbor table: This table is used to store the information about OSPF neighbors.
  • Topology table: This table is used to store the topology structure of a network.
  • Routing table: This table is used to store the best routes to all known network

29) What are the different OSPF timers?

There are mainly two types of OSPF timers:

  • Dead Interval Timer: The Dead Interval Timer is used to define how the extended router waits for hello packets before it declares the neighbor dead.
  • Hello Interval Timer: The Hello Interval Timer is used to define how frequently the OSPF router sends the hello packet to another OSPF router.

30) What are DR and BDR in OSPF?

In OSPF, DR stands for Designated Router, and BDR stands for Backup Designated Router. Both are used for exchanging information between different routers. It depends upon the network; one router is selected as DR and another as the BDR.

By default, in each broadcast network, DR is the contact point for the network segment. All routers become adjacent with the DR while handling all the LSAs for the network. The information about the DR is sent by the router using the multicast address

If any problem occurs with DR, then to avoid problems in case of DR failure, BDR is selected for the network. It becomes active when the DR fails and listens to the multicast address

31) What is the importance of area 0 in OSPF?

In OSPF, area 0 is the backbone area, and this is important because all other areas must have to send their inter-area transfer through the backbone area.

32) What is the concept of Virtual-link in OSPF? Can we form a virtual link through the stub area?

According to the OSPF rule, inter-area communication happens when at least one area should be area 0 or backbone area. If this is not possible, then we have to use virtual-link to form a logical association. A virtual link is not possible through the stub area.

33) What are the multicast IPs used by OSPF? What multicast IP does DR/BDR router and non-DR use?

OSPF Routers use and multicast IP addresses. In DR/BDR, it uses and DR/BDR uses while replying to OSPF routers in the Broadcast Multi-access network.

34) Is it possible to use OSPF without a backbone area?

Yes, we can use OSPF without backbone area, but only intra-area communication is possible. For inter-area communication, we must use the backbone area, and inter-area communication is not achievable without a backbone area.

35) Why is OSPF called a loop-free protocol?

OSPF is called a loop-free protocol because of the presence of the link-state database. In OSPF, the database offers a complete view of the network and prevents routing loops common in distance vector protocols. OSPF avoids loops by applying the inter-area loop-free topology. That's why it is called a loop-free protocol. s

36) What is the default value of Hello Interval and Dead Interval?

The default value for Hello Interval for OSPF is 10 seconds, and for the Dead, Interval is four times the Hello Interval. By default, it is 40 seconds.

37) What is the difference between EIGRP vs OSPF?

Following is the list of key differences between EIGRP and OSPF:

EIGRP stands for Enhanced Interior Gateway Protocol. It is a hybrid type protocol. OSPF is an acronym that stands for Open Shortest Path First. It is a link-state protocol.
EIGRP requires a very low CPU power and memory. OSPF always requires high processing power and memory.
EIGRP is based on the Cisco Proprietary standards. The OSPF protocol is based on the IETF Open Standard.
In EIGRP, the routing metrics consist of load, reliability, bandwidth, and delay. In OSPF, the routing metrics consist of the interface bandwidth.
EIGRP facilitates summarization and filtering, and these are possible anywhere in the network. OSPF is possible only on ABR or ASBR.

38) What are the different activities that a neighbor router can perform in OSPF?

In OSPF, a neighbor table can include the following information:

  • The Router ID of each neighboring router.
  • The current "state" of each neighboring router.
  • The interface is directly connected to each neighbor.
  • The IP address of the remote interface of each neighbor.

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