OSPF Interview Questions
While preparation can be stressful, we aim to assist your preparation with OSPF interviews by offering pertinent and valuable questions and guidelines on responding professionally and ethically to these OSPF interview questions and answers for experienced and Freshers.
We hope this resource proves helpful in that endeavour.
We wish you the best of luck for all future interviews!
1. What is OSPF?
OSPF (Open Shortest Path First) is a link-state routing protocol that works by forming adjacencies and neighborships between routers, forming a network map.
It uses link state advertisements (LSAs) to educate others about available networks and interconnections.
These LSAs are then grouped to create a link state database. OSPF uses the Dijkstra shortest path first algorithm to determine the optimum path between any point in the network.
2. How does OSPF determine the best path?
OSPF determines the best path using the Dijkstra shortest path first algorithm.
This algorithm is similar to how a car’s navigation system uses the Dijkstra algorithm to determine the fast estroute.
OSPF judges the best path based on the cost of a link, which is a function of the reference bandwidth.
3. What is a link state advertisement (LSA)?
An LSA is a message router exchange that informs each other about available networks and interconnections.
These LSAs are then grouped to form a link state database.
4. What is an adjacency in OSPF?
An OSPF adjacency is a relationship between two routers that do not exchange information but say hello.
OSPF adjacent routers are neighbours but exchange information to build the link state database.
5. What is a neighborship in OSPF?
A neighborship in OSPF is a relationship between two routers on the same network segment that exchange hello messages using multicast.
6. What is a link state request (LSR) in OSPF?
A link state request in OSPF is a request that a router makes to another router for missing information.
If a neighbour gives the missing information, it will say thank you in the form of an LSA.
7. What is OSPF’s default reference bandwidth?
On Cisco routers, the default reference bandwidth for OSPF is 100 megabits per second.
8. What is an OSPF area?
An OSPF area is a region of a network covered by a map. Some networks have one OSPF area, but the Dijkstra algorithm can divide it into multiple regions.
9. What is an area border router (ABR) in OSPF?
An area border router (ABR) sits at the border of at least two areas and sends information back and forth between them.
It is responsible for advertising networks from Area Zero to Area One.
10. When adding areas in OSPF, what is essential to consider?
When adding areas in OSPF, it’s essential to consider the region’s size.
Cisco recommended breaking up an area into multiple locations to avoid burdening router processors during the Dijkstra algorithm.
However, modern routers have faster processors and faster router speeds, making this not a significant issue.
11. What is the OSPF deep dive event, and when?
The OSPF deep dive event is part of Black Friday, a marathon training event taught by Cisco on the day after Thanksgiving.
The session will cover basic theory, OSPF routers, designated routers, network types, LSAs, timers, and OSPF’s metric calculation.
The replay will be posted tomorrow, but some details may be missing.
12. What is the OSPF version three configuration to support IP version six?
OSPF version three configuration to support I P version six, including traditional methods like setting up OSPF version two for IPV four and OSPF V3 for IPV six.
An address family’s configuration approach will allow for one hierarchical structure that does both OSPF and I P version four and I P version six.
13. What is the authentication between routers and all routers in an area?
The presentation will conclude with authentication between routers and all routers in an area.
A Cisco Certified Associate CCAE and network design will provide demonstrations and troubleshooting tips.
14. What are the three sections of the presentation?
The three sections of the presentation are OSPF, ISIS, EIGRP, and RIP.
15. What is the concept of LSA in networking?
LSA stands for Link State Advertisement.
It is a method used in networking to link state advertisements (LSAs) and adjacencies, which are routers sharing information to determine the shortest route to a point.
LSAs are sent throughout an area, creating a map stored in a link state database.
16. What is the difference between a neighbour and an adjacency in networking?
A neighbour is a router that a router exchanges hello messages with, while an adjacency is a relationship between two routers that exchange information.
Adjacencies are closer relationships, such as visiting each other’s homes and helping each other out.
On the other hand, a neighborship is when two neighbours exchange hello messages, knowing about each other but not very pointed with one another.
17. What is the administrative distance in routing information?
Administrative distance is the believability of routing information in routing information.
The lower the administrative distance, the more believable the routing source.
The administrative distance is zero if a network is directly connected to the router.
18. What is a floating static route in networking?
A floating static route is more believable than a route with an ad and administrative distance of one in routing information.
It is less specific than a fixed route and allows a router to switch to a less optimal route if a better one becomes available.
19. How do routers exchange LSAs in networking?
Routers initiate conversations by saying hello and exchanging information with adjacencies using LSA packets.
After exchanges, routers build a database by requesting missing information from neighbours through link state requests.
If someone has the missing information, they provide it within a link state acknowledgement.
20. What is the concept of adjacency in OSPF?
Adjacency is a relationship between OSPF routers and their neighbours.
It requires being a neighbour and exchanging link state updates and database description packets.
To be a neighbour, one router must be in the same area, the entire network, or a group of OSPF routers.
21. What are the different areas of networking?
There are different areas in networking, including regular, stub, and stubby regions. Stub flags and the MTU must match the maximum transmission unit.
22. What is the Exstart slash exchange state in IP OSPF?
The Exstart slash exchange state in IP OSPF refers to a situation where a router has received information about a link’s cost from other routers but has not yet converged with them on a unique path to that link.
This means the router still considers multiple paths to that link, and its cost value may differ from other routers.
23. Why is understanding the Exstart slash exchange state in IPOSPF critical?
Understanding the Exstart slash exchange state in IPOSPF is essential because it can affect the cost values of links and the efficiency of routing within the network.
If multiple routers are in this state, they may have different cost values for the same link, leading to suboptimal routing and potential network issues.
By understanding this state and working to resolve it, network administrators can ensure that their networks are running efficiently and effectively.
24. What is OSPF’s metric calculation, and how has it changed over time?
OSPF calculates the cost of a link based on the reference bandwidth divided by the interface bandwidth.
The reference bandwidth is typically 100 meg, which is 100 megabits per second.
Over time, new equipment in data centres has addressed issues with OSPF’s metric calculation, particularly about gig links.
25. What is authentication in networking?
Authentication is a process used in networking to verify that two sides match before a relationship occurs.
It requires a match between the two sides and must be on the same subnet.
26. How does OSPF calculate the cost of a 1G link?
OSPF calculates the cost of a 1G link as zero point zero, which is a significant issue with the protocol.
The default value is 100 meg divided by 1000 meg, which is zero point zero.
This means that fast ethernet and gig links have the exact cost, which can lead to suboptimal networks.
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A designated router (DR) in a broadcast network forms adjacencies with other routers, reduces the number of adjacencies needed, and ensures that information is replicated across the area.
This is particularly helpful for multi-axis networks or those with multiple routers sharing the same subnet.
28. What is the process of sending adjacency information to a DR or BDR in IPOSPF?
The process of sending adjacency information to a DR or BDR in IPOSPF involves the routers exchanging information about their interfaces and link costs.
This information is then used to determine the most efficient path to each link, which can improve the network’s overall efficiency.
29. How is the router priority determined in IPOSPF?
The router priority in IP OSPF is determined based on the highest priority value set by the router. By default, the priority is set to one.
Still, network administrators can set the priority to a higher value to ensure that certain routers are given higher priority in the event of a tie.
30. What is an entire mesh topology in IP OSPF?
An entire mesh topology in IPOSPF refers to a network topology where every router is connected to another router.
This can provide a highly efficient and fault-tolerant network but does not scale well for many nodes.
31. How can the number of adjacencies needed in IPOSPF be determined?
The number of adjacencies needed in IP OSPF can be determined using the following formula: (n-1) * (n-2) / 2, where n is the number of nodes in the network.
This formula considers that each node needs to establish an adjacency with every other node in the network.
32. What is the Exstart slash exchange state in IP OSPF?
The Exstart slash exchange state in IP OSPF refers to a situation where a router has received information about a link’s cost from other routers but has not yet converged with them on a unique path to that link.
This means the router still considers multiple paths to that link, and its cost value may differ from other routers.
33. How can the router priority in OSPF be set?
Entering the router interface configuration mode allows you to set its priority in OSPF.
A higher priority, such as ten or five for DR, is better. If a router is busy, it can be set to zero.
If everything is tied, routers in OSPF are known by their router ID.
34. How can the router ID be set in router configuration mode?
Setting a router ID like an R1 gives an intuitive notion of R1, while setting R2 gives a router ID 2.2.2.2. These routers are just names, not advertising them.
35. What happens if no loopback interfaces are present?
The router’s highest IP address is assigned if no loopback interfaces exist.
36. What is the difference between a backbone area and an area border router (ABR)?
An area border router lets one area know what networks are available in another location, but not the Dijkstra algorithm.
On the other hand, the backbone area must be present in every other area.
37. Why is it important to consider each area’s specific needs and limitations when designing a network?
Dividing a network into different areas can help optimise routing and reduce the burden on the router processor.
However, it is essential to consider each area’s specific needs and limitations when designing a network to ensure its effectiveness and efficiency.
38. Why is understanding the Exstart slash exchange state in IP OSPF essential?
It is essential to understand the Exstart slash exchange state in IP OSPF because it can affect the cost values of links and the efficiency of routing within the network.
If multiple routers are in this state, they may have different cost values for the same link,
39. What is a link state advertisement (LSA) in OSPF?
An LSA is information that can be grouped into router LSA and type two LSA. A router generates a type one LSA for every connected interface.
In contrast, a type two LSA is generated for a link that matches two criteria: a transit area going from one OSPF-speaking router to another and a link on which a designated router would be elected.
40. What is the difference between a router LSA and a type two LSA in OSPF?
A router LSA is generated by a router for every network it attaches, while a type two LSA is flooded within an area.
41. What is a summary LSA?
A summary LSA, known as a type three LSA, lists available networks in a particular area. It breaks a network into smaller areas and lets routerssee about them.
42. What is the difference between summary LSA and network summarization?
Summary LSA is not about network summarisation but lists available networks.
It generates a type three LSA for every network in a particular area, allowing routers to know about them.
43. What is a stub area?
A stub area summarises routes coming in from another autonomous system.
In a stub area, a network receives all five types of LSAs from EIGRP, and the only way to advertise is through a default gateway.
44. What is the problem with sending type seven LSAs from RIP into area one?
When R3 connects to RIP, the network becomes less stubby. R3 recognises these are from another autonomous system and sends them as type seven LSAs.
R2 then types these LSAs into area zero, indicating they come from another external system. This makes the area less stubby.
45. What is a default LSA?
A default LSA is a type three LSA that specifies that if there is no specific route in the database for a network, it should be sent to R2.
It is more efficient than sending multiple types of five LSA advertisements and makes the database more efficient.
46. What is the purpose of sending summary LSAs into area one?
The purpose of sending summary LSAs into area one is to provide routers with information about the available networks in a particular area.
However, it may not be efficient for routing specific networks.
47. What is the basic configuration of OSPF version two?
To configure OSPF version two, start by configuring router R1 and assigning a locally significant process ID (I D), which is not an autonomous system number.
Then, advertise the networks R1 knows about using a wild card mask, which is the inverse of a subnet mask.
48. What is a wild card mask in OSPF configurations?
A wild card mask is not an autonomous system number but a way to remember the process ID.
In OSPF configurations, it advertises the networks a router knows about.
49. Why should a network interface be set as passive in OSPF configurations?
A network interface should be passive in OSPF configurations to prevent malicious users from forming adjacencies and corrupting the database.
It is also more reasonable than setting the reference bandwidth to 100 megabits per second.
50. What is the importance of understanding Layer 2 Protocols (LSAs) for certification purposes?
Understanding LSAs is essential for network management certification. They advertise networks and exchange information between routers.
A good understanding of these protocols ensures smooth network operation and efficient routing.
51. What is the concept of network types in OSPF?
Network types in OSPF refer to how networks and links are advertised using a single command, which includes all possible IP addresses within an area.
This is done by setting the reference bandwidth to 100,000 and learning routes on R3.
The concept of network types is essential in understanding the role of OSPF in advertising networks and in understanding the compatibility of different network types.
52. What is the role of OSPF’s link state database?
OSPF’s link state database shows the type one LSA for every network and link within an area.
Type one LSAs advertise information about the network’s topology.
Two type one LSAs are created for each link, a transit link, and one type three LSA is designed for every network from another area.
The link state database advertises the network’s topology and ensures that all network routers have the same information about the topology.
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OSPF’s routing information base is used to view the networks known to OSPF.
It shows the routes that OSPF knows about, and if there are multiple routes, it chooses the best route with a greater than sign.
The routing information base is also used to determine the best path for data to take through the network.
54. What is the cost of an interface in OSPF?
Adjusting the reference bandwidth, which determines the priority of a link in the network, increases the cost of an interface in OSPF by 100.
55. What is the compatibility of different network types in OSPF?
OSPF compatibility with different network types depends on the type of network being used.
OSPF can be used in broadcast-to-broadcast, non-broadcast-to-non-broadcast, and broadcast-to-non-broadcast network types.
Timers can be adjusted manually as long as they match and both ends support or elect DRS. If both ends don’t select DRS, they can be compatible.
56. What are the different neighbour states in OSPF?
OSPF has three neighbouring states: down, attempt, and exchange. In the down state, neighbours are not heard from.
In the attempt state, the router sends Hello messages to a configured neighbour but hasn’t received anything from that neighbour.
In the exchange state, adjacencies are established, and database descriptor packets describe the database structure.
57. What are the basics of OSPF?
OSPF essentials are down net and nits. In a depressed state, neighbours remain silent.
Net neighbours greet each other without their routers. In the exchange stage, adjacencies are formed, and database descriptor packets define the structure. Link-state routing protocol OSPF advertises network topology.
It stores network topology information in a link state database and ensures all routers have the same information.
OSPF works in broadcast-to-broadcast, non-broadcast-to-non-broadcast, and broadcast-to-non-broadcast networks.
Manually adjusting timing is allowed if both ends support or elect DRS. DRS-free ends may be compatible. Down, neighbouring states try,
58. What is OSPF, and what does it involve?
OSPF is a network management system that allows bidirectional communication between neighbouring nodes.
It involves forming neighbours and adjacencies and creating DRs and BDRs.
59. What is route filtering in OSPF?
Route filtering is a feature that can be added to OSPF to ensure that only necessary routes are included in the link state database.
It works by configuring a distribution list on a router to remove specific routes before they are added to the router’s IP routing table.
60. What is a distribution list in OSPF?
A distribute-list is a list of rules that can be used to remove specific routes from the link state database.
It is typically used with a prefix list to filter out specific network addresses.
61. What is a prefix list in OSPF?
A prefix list lists rules that filter out specific network addresses from the link state database.
It is typically used with a distribute list to remove particular routes from the router’s IP routing table.
62. What is the difference between inbound and outbound filtering in OSPF?
Inbound filtering removes specific routes from the link state database before they are added to the router’s IP routing table, and outbound filtering removesparticular routes from the router’s IP routing table after they have been added.
63. What is the importance of cautiousness when using summarised addresses in OSPF?
Be cautious when using summarised addresses or address summaries in routing tables because they can lead to potential issues.
For example, if the third octet is a two, the advertisement will still work but may be more specific than the original entry.
This can lead to problems if other networks or subnets in the network are not configured to use the same address summaries.
It is essential to consider the potential impact of using summarised addresses or address summaries before implementing them in a routing table.
64. What is the process of route summarisation in OSPF?
Routesummarization in OSPF involves using the summary-address command to summarise routes in Autonomous System Border Routers (ABR) and Autonomous System Boundary Routers (ABR).
This reduces the number of routes that need to be advertised in an area, making routing more efficient.
65. What is the summary-address command used for?
The summary-address command summarises routes in OSPF.
It is used on the ASBR area range and the ABR and for OSPF version three to reduce the number of routes that need to be advertised in an area.
66. What is the concept of virtual links in OSPF version three?
Virtual links in OSPF version three form a virtual connection between two routers in different areas, allowing one of the areas to have an interface in area zero.
This is useful in networks with multiple locations, allowing for a more efficient routing configuration.
67. What is the need for a transit area in OSPF?
In OSPF, a transit area is a unique area used to connect multiple regions.
It is crucial to have one because it allows for more efficient routing between areas.
68. What is the configuration for a virtual link?
Configuring a virtual link involves specifying the IP address on the far side of the virtual tunnel and then configuring OSPF version three to route IP version six traffic.
This creates two different OSPF instances running for version two and version three.
69. What is the difference between the traditional and efficient configurations for virtual links in OSPF?
The traditional configuration for virtual links in OSPF involves overlaying an OSPF version three configuration on top of an IPv4 configuration.
The efficient configuration consists of using a different topology with IPv6 addressing on the live interface, which is more efficient.
70. What is the purpose of using the Cisco Express forwarding (IPv6) protocol to enable IPv6 unicast routing?
The purpose of using the Cisco Express forwarding (IPv6) protocol to enable IPv6 unicast routing is to configure the router to support IPv6 routing and to allow it to advertise its IP address to other routers in the network.
71. How does the configure OSPF version three?
The configures OSPF version three by setting a router ID in interface configuration mode and creating two different OSPF processes: one for OSPF version two, which writes IPv4, and one for OSPF version three, correspondingto IPv6.
72. What is the purpose of using the address family approach in OSPF routing?
The address family approach in OSPF routing allows for more specific configurations and removes the traditional IPv6 router from the network, enabling IPv6 routing. The router syntax is different, with OSPF V3 for version three.
73. How does the verify the configuration of OSPF version three?
The configuration of OSPF version three is verified by using commands such as the show OSPF version three neighbour command and the show OSPF version three database command.
74. What is the purpose of using the OI flag in IPv6 routes?
Using the OI flag in IPv6 routes indicates that the route is inter-area, which is the traditional way of doing it, as it allows for more efficient forwarding and routing decisions.
75. What is the improved name of the inter-area prefix in the show OSPF version three database command?
The improved name of the inter-area prefix in the show OSPF version three database command is “inter-area prefix.”
76. What is the address family approach, and how does it help configure IPv4 and IPv6 networks?
The address family approach allows for more specific and straight forward configurations of IPv4 and IPv6 networks.
This new approach will enable users to configure their network and improve their routing performance quickly.
77. What is OSPF version three, and how does it improve security?
OSPF version three provides enhanced security by allowing users to set up authentication between two routers or for an entire area, such as area zero.
The speaker demonstrates how to set up OSPF authentication between R2 and R3 to prevent rogue actors from connecting their routers and corrupting the OSPF database.
78. How can OSPF be configured to prevent rogue actors from connecting their routers and corrupting the OSPF database?
OSPF can be configured to prevent rogue actors from connecting their routers and corrupting the OSPF database by setting up authentication between two routers or for an entire area, such as area zero.
79. Can routes be filtered selectively out of an OSPF database for just one router in an area?
No, it violates OSPF’s basic concept, which requires all routers inside an area to agree on what that area looks like.
We cannot filter selective routes out of an OSPF database for just one router in an area.
However, we can configure a distribution list to block a specific route before injecting it into the IP routing table.
This way, OSPF knows about the route, but the IP routing table doesn’t learn it because we have blocked it.
80. What are the three primary approaches to OSPF route filtering?
Two of them are setting up a filter list on an ABR to block a type 3LSA and configuring a distribute list on a specific router to secure a route from being learned just for that router’s IP routing table.
Interview Questions for OSPF blog add that OSPF (Open Shortest Path First) is a popular network link-state routing protocol.
OSPF calculates its route link by link using various metrics, including cost, bandwidth usage and load on links, to calculate the shortest paths between routers.
Overall, this OSPF protocol interview questions presents an in-depth exploration of OSPF, its interview questions and the OSPF Interview Questions and Answers pdf. It is a valuable resource for network engineering interviews or anyone seeking a greater understanding of OSPF.
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