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:
5) What are the different functioning and working of OSPF?
Following is the list of different functioning and working of OSPF:
6) What is the benefit of dividing the entire network into areas?
Following are the advantages of dividing the entire network into areas:
7) What are the key characteristics of OSPF?
Following is a list of the key characteristics of OSPF:
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:
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:
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.
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:
14) What are the different networks types in OSPF?
Following are the different network types available in OSPF:
15) How can you change the neighbor ships into adjacency?
We need to follow the following steps to change the neighbor ships into adjacency:
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:
18) What are the issues that DR and BDR solve in OSPF?
DR and BDR solve the following two problems in OSPF:
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:
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.
25) What are the five OSPF packet types?
Following is the list of five main OSPF packet types:
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:
29) What are the different OSPF timers?
There are mainly two types of OSPF timers:
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 22.214.171.124.
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 126.96.36.199.
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 188.8.131.52 and 184.108.40.206 multicast IP addresses. In DR/BDR, it uses 220.127.116.11 and 18.104.22.168. DR/BDR uses 22.214.171.124 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:
38) What are the different activities that a neighbor router can perform in OSPF?
In OSPF, a neighbor table can include the following information: