Top 10 most asked OSPF Interview Questions and Answers
OSPF is one of the most important topics in Networking and IT domain interviews. Every employer wants a candidate who has deep knowledge of OSPF protocol, and that is why we came up with the most asked OSPF interview questions.
These OSPF interview questions and answers are very carefully curated by some of the leading minds in this domain to help freshers and experienced candidates crack the OSPF interviews.
So, no need to get nervous before any interview anymore as we have the perfect questions that will most likely be on your interviewers' list. You can also get OSPF interview questions and answers pdf at the last of this blog.
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OSPF Interview Questions and Answers
Here is a list of the most asked OSPF interview questions and answers:
Q1 - What is the OSPF routing protocol?
Answer - OSPF stands for Open Shortest Path First. It is a link-state routing protocol that uses the Shortest Path First (SPF) algorithm to find the best path between the source and the destination router.
It was developed by Internet Engineering Task Force (IETF). It was developed as an Interior Gateway Protocol (IGP). A protocol has a primary aim to move the packets within a large autonomous system or routing domain.
It is a network layer protocol.
You can also watch this video to learn more about the OSPF protocol.
Q2 - What is the area in OSPF?
Answer - The term "area" refers to a collection of OSPF routers, networks, and links with the same identification for the area. The router in an area has to keep an operational database of the area it is part of.
Q3 - What is the Backbone area?
Answer - The area known as the backbone (Area 0) is the center of an OSPF network. The other areas are linked to it, and all communication between the areas has to pass through it. The entire route between regions is carried out through the backbone region.
Q4 - What is an Autonomous System Boundary Router ASBR?
Answer - An Autonomous System Boundary Router (ASBR) is a type of router that is capable of running multiple protocols. It also acts as a gateway for the routers that are placed outside the OSPF domain and to the routers too that operate on different protocols.
An Autonomous System Boundary Router is capable of importing and translating various protocol routes into OSPF via a process called redistribution.
Q5 - What is a Designated Router (DR)?
Answer – A Designated Router (DR) is used to reduce the number of adjacencies created. A DR has the responsibility of distributing the LSAs to all the other routers. In a broadcast network, a DR is chosen, and all the other routers share their DBD to this DR.
In a broadcast network, routers make a request to the DR for an update, to which DR answers with an update.
Q6 - How OSPF DR and BDR are elected?
Answer – The election between OSPF DR and BDR takes place in the broadcast network or a multiaccess network. The criteria for the selection are as follows:
- DR is declared on the basis of the router priority. The router with the highest priority becomes the DR.
- However, if both routers have the same priority, then the router with the highest router ID is selected.
Firstly, the loopback address (highest) is taken into consideration. However, if, in any case, the loopback is not configured, then the active IP address on the router interface is taken into consideration, and the highest active IP address takes it.
Q7 - What are the OSPF packet types?
Answer - OSPF uses packets to send and receive requests, ensure delivery of Link-state advertisements and establish & maintain adjacencies. Packets are also used to describe the link-state databases. The OSPF packet types are as follows:
• Hello – The Hello packet type is used to discover neighbors, build adjacencies and maintain them.
• DBD (Database Description)- is used to determine whether the LSDB between two routers is identical. The DBD is like an overview of LSDB.
• LSR (Link-state request) - It is used to request link-state records of a specific type from an OSPF neighbour.
• LSU (Link-state update)- This packet sends specific links-state records to those who requested them. This packet is similar to an envelope that has multiple LSAs inside.
• LSAck (Link-state acknowledgements)- This packet is used to acknowledge others as OSPF is a very reliable protocol.
Q8 - What are the OSPF network types?
Answer - The default OSPF network type is based on the media used for the connection, and it can be changed independently of the media used. Cisco provides five OSPF network types, as listed in the table below.
Q9 - Explain OSPF Virtual Link?
Answer - The term "virtual link" does not refer to a physical connection. In fact, it is an intelligent link that uses the cheapest route connecting both the ABR of the area that is not connected to the backbone region and the backbone ABR of the transit zone.
A virtual adjacency along the link is created and routing data will be exchanged.
Q10 - Explain the Stub Area and different types of Stub area?
Answer – Sometimes, we need to control the advertisement of external routes into an area. This area is called the stub area, and Stub areas are not capable of importing routes external to OSPF.
LSAs of Type 4 & Type 5 get filtered from the stub areas. ABR injects a default route into that area in place of external routes.
There are 3 restrictions that apply to the OSPF stub area: -
- Virtual links are not allowed in the stub area.
- The stub area is not allowed to be a backbone area.
- Autonomous System Boundary Routers are not allowed.
Different types of Stub areas are:
- Totally Stubby Area
Like stub areas, the totally stubby area does not receive type4 and type5 LSA from their ABRs. However, they also do not receive type 3 LSAs, and it allows the advertisement of the internal router in that area.
- Not-So-Stubby Areas
The motivation behind NSSA is to allow the OSPF stub area to carry external routes. The external router is imported into OSPF NSSA as Type 7 LSA by ASBR. Now, ABR converts the Type 7 LSA back into Type 5 LSA as Type 7 LSA is not allowed to go into area 0.
- Totally NSSA
In Totally NSSA, the Type3 also gets filtered with Type4 & Type5.
OSPF Interview Questions and Answers PDF
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Frequently Asked Questions
What is OSPF explained in the interview?
Answer – OSPF stands for Open Shortest Path First. It is an open link-state routing protocol to find the shortest path between the source and the destination router. It was developed as an interior gateway protocol, and it works as a network layer protocol.
What is the main purpose of OSPF?
Answer – The main purpose of the OSPF (Open Shortest Path First) protocol is to collect link state information from all the available routers and form a topology table which is used by the Internet layer as a routing table to route packets according to their IP Addresses.
Where is OSPF used?
Answer – OSPF is mainly used for internal routing as it is a link-state protocol. Using Dijkstra's algorithm, it computes the shortest path between the source and the destination router. It is widely used in large enterprise networks.
What are the main features of OSPF?
Answer – The main features of OSPF are:
- It can run on most routers
- It is a true loop-free routing protocol
- It allows better load balancing
- No limitation on Hop count
- It is an open standard protocol
- It sends multicast frames rather than broadcast frames.