Top 15 OSPF Interview Questions

In the world of networking, there are a number of different protocols that can be used for various purposes. One such protocol is Open Shortest Path First (OSPF). This protocol is often used in larger networks to provide routing and dynamic addressing services. If you’re looking for a job as an OSPF expert, it’s important to know what questions might come up during your interview. In this blog post, we’ll cover some common OSPF interview questions and how you should answer them!

1. What is OSPF?

Ans. OSPF is an open-source routing protocol which stands for Open Shortest Path First.

OSPF is a link-state routing protocol. This means that the router calculates the shortest path to a destination by looking at the physical and logical distance of all possible paths to that destination. This is in contrast to distance-vector protocols like RIP, where the router does not know how other routers are connected, so it just sends its own distances and expects the other routers to send theirs back.

OSPF was developed in 1988 by Dr. John Moy, one of the original authors of TCP/IP networking and co-author of “Internetworking with TCP/IP.”

2. What is router ID and how do you change the router ID in the OSPF domain?

Ans. Router ID is a unique number assigned to each router in the OSPF domain. It is used for routing table calculation. The router ID can be changed with the following steps.

The initial step is to log into the router with admin credentials and go to “interfaces” tab and select one of the interfaces from the drop-down menu. Now, type in a new Router ID under “Router ID”. Finally, hit “apply” button at the bottom of the page for changes to get applied.

3. What are the characteristics of OSPF?

Ans. OSPF is a dynamic routing protocol that is designed for use in large internetworks. OSPF has the capability of scaling to large internetworks and is considered to be a classless protocol.

Some of the benefits of OSPF are that it:

-Performs route calculations with speed and efficiency

-Provides scalability

-Supports variable length subnet masking (VLSM)

4. What is the role of Summarization in OSPF?

Ans. Using OSPF with summarization is a more efficient way of routing information. It reduces the load on a router’s CPU and allows it to handle more data.

Summarization, as the name suggests, is the process of summarizing. It reduces the size of routing tables by combining many routes into one entry in order to reduce router CPU overhead and memory usage.

5. What are the packet types in OSPF?

Ans. The type of packets used in the ospf protocol will be discussed below. These packets are responsible for processing updates, queries, and replies.

Packet types:

  • Hello Packet: The hello packet is at the top of the ospf stack and encapsulates information about the router’s current state. It is used to determine if a neighbour can be seen or not.
  • Database description packet: A database description packet is a document that describes what is in a database and how the information should be used. It is part of the design phase of a database project.
  • Link State Request Packet: This packet is used to request a link state update from a neighbour router that has advertised an adjacency with this router but has not yet received one.
  • Link State Update Packet: This packet is used to convey all link state changes that have taken place since the last time this router sent out an update to its neighbours, including link state changes that it has
  • Link-state acknowledgment packet: This packet is used to acknowledge the newly received LSA by its recipient in order to verify its delivery. The acknowledgment packet can be sent immediately or there can be a delay based on the time interval mentioned.

6. What are the types of OSPF timers?

Ans. The timers involved in OSPF are as follows:

  • Election: The time interval at which an OSPF router will participate in the election process to determine the designated router (DR) and backup designated router (BDR). This timer is set to five minutes by default.
  • The hold time: The amount of time that must pass before a DR becomes ineligible for reverting back to BDR. This timer is set to three minutes by default.
  • The minimum hello interval: The amount of time that must pass without seeing another hello packet before considering the neighbour down.

7. What are the types of LSA’s in OSPF?

Ans. There are three types of LSAs in OSPF:

  • Type 1 LSA – A type 1 LSA is generated by the designated router on segment with topology change.
  • Type 2 LSA – A type 2 LSA is generated by an area border router, or ABR, to summarize its attached segment’s routing information and advertise to other areas.
  • Type 3 LSA – A type 3 LSA is generated by an ASBR to advertise routes from other autonomous systems into your autonomous system.

8. What are the different router types?

Ans. There are five router types: Internal Gateway Protocol (IGP) Autonomous System Boundary Router (ASBR), Type I External Route, Type II External Route, and Not So Stubby Area.

9. What issues do DR and BDR solve in OSPF?

Ans. DR and BDR solve the issue of splitting a broadcast domain into multiple broadcast domains. In OSPF, all routers that connect to a common network become neighbors. DR is elected from among these neighbor devices to control traffic flow on the multi-access segment by determining which device will receive packets first. The router with the highest priority becomes the DR. In the event that two routers have equal priority, a device with a lower RID (Router ID) becomes the DR. BDR is elected from among these neighbor devices to control traffic flow on multi-access segments by determining which router will receive packets second. The router with the highest priority becomes the BDR.

10. What are some OSPF packet types?

Ans. There are a number of different OSPF packet types. These include:

  • Hello packets (used to discover and track neighbors)
  • Database Description Packets (contain all the information that routers need in order to build their link-state database)
  • Link State Request Packet (asks for specific Link-State Advertisements from neighboring routers)
  • Link State Update Packets (contain information that is sent out after routers learn about new neighbors or update their link-state database)

11. What is the default administrative distance of OSPF?

Ans. The default administrative distance for OSPF is 110. This means that if an exact match isn’t found, it will be forwarded to any other defined routing protocols before being sent out onto the network.

12. What are the different OSPF states?

Ans. OSPF routers can be in one of five different state types: Down, Init, Two-way, Exstart, and Exchange.

13. What is the difference between a Type I & II OSPF router?

Ans. Type I routers are able to speak with their neighbor directly as well as to other remote networks (Eg. A direct link). However Type II routers can only talk to remote networks via another router (Eg. A router that is connected to another network).

14. What happens if a link goes down between two routers?

Ans. When a neighbor detects an interface going down, it will transition the state of its relationship from Init to Two-way and wait for a response from its other neighbors. If there are multiple interfaces on this OSPF router, a decision will be made as to which one should be used.

15. What is a Wildcard Mask?

Ans. The wildcard mask determines the pertinent bits from an IP address that are relevant for routing purposes. This allows you to tell OSPF what network IDs, subnets and exact routes it needs to look at in order find optimal paths between router interfaces.

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