Top 20 OSPF Interview Questions and Answers

Question-(1) What is OSPF (Open Shortest Path First)?
Ans: Open Shortest Path First (OSPF) is a link state Interior gateway routing protocol (IGP).
OSPF is a vendor-independent routing protocol, that can be used in a multivendor environment.
i.e OSPF can be run on routers of Cisco, HP, Juniper, Netgear, Asus, Apple, etc.

Question-(2) What is the multicast address used by OSPF?
Ans: OSPF uses multicast address 224.0.0.5 to send a hello to form a neighbor relationship.
OSPF sends only triggered update if something changes in the topology.
OSPF has a view of full network topology in the topology table.

Question-(3) what is the hello and dead interval of OSPF?
Ans: Hello time: 10 Seconds
Dead time: 40 Seconds

Question-(4) what is the AD value of OSPF?
Ans: AD value is 110

Question-(5) how metric is calculated in OSPF?
Ans: metric of OSPF is called COST

COST = 100 /Interface Bandwidth (in Mbps)

Question-(6) Where to use the OSPF routing protocol?
Ans: OSPF routing protocol can be used in a multi-vendor environment

Question-(7) What is the difference between neighbor and adjacent in OSPF?
Ans: If routers have exchanged only Hello packets then they are neighbor only
If routers have exchanged Hello packets and LSA then they are Adjacent.

Question-(8) What parameters should match for OSPF neighbor formation?
Ans: The following parameter MUST match
✓ Hello and Dead interval
✓Network Mask
✓Area ID
✓Authentication password.

Question-(9) how to configure OSPF?
Ans: Connect to Router through a console or Remotely and login into the Router and get access to Terminal
Router> Enable
Router#Configure terminal
Router (config)# Router OSPF process_ ID
Router (config-router)#Network network_ bit Wildcard_mask area {Id}
For example, we want to configure OSPF with process_id 1, one interface has IP 10.0.0.0/24 and the area is 0
Router>Enable
Router#Configure terminal
Router(config)#Router ospf 1
Router (config-router)#Network 10.0.0.0.0.0.0.225 area 0

Question-(10) What is router ID in OSPF?
Ans: Router-ID is just 32 bits number and is used to identify each router in the OSPF domain within an AS

Question-(11) What is DR/BDR in OSPF?
Ans: In the OSPF domain each router sends an update to all routers whenever there is a change in topology. The idea behind DR/BDR is that only DR/BDR can send any updates to all routers only.

Question-(12) Which is elected first DR or BDR in OSPF?
Ans: BDR

Question-(13) What is the router-id selection process in OSPF?
Ans: OSPF Router ID selection algorithm works as below:
✓ Any manually configured OSPF Router ID in OSPF Process is selected as the OSPF Router ID.
✓IF there is no OSPF Router ID configured then :
The highest IP address on any Loopback Interfaces of the router is selected as the OSPF Router ID.
✓if there are no Loopback Interfaces configured then:
The highest IP address on its active interfaces is selected for the OSPF Router ID.

Question-(14) what are the LSA types in OSPF?
Ans: There are 6 Types of LSA in OSPF given below :
Type 1 Router LSA
Type 2 Network LSA
Type 3 Summary LSA
Type 4 Summary ASBR LSA
Type 5 External LSA
Type 6 Not-So-Stubby Area LSA

Question-(15) how to configure router-Id in OSPF?
Ans: Router>Enable
Router#Configure terminal
Router(config)# ospf 1
Router(config-router)#Router- I’d 1.1.1.1

Note: After configuring the Router ID the OSPF process must be reset by using the command:
Router(config)#clear in OSPF process

Question-(16) What is OSPF area Concept?
Ans: In OSPF, a single autonomous system (AS) can be divided into smaller groups called areas.
In OSPF, each router sends Link state advertisement (LSA) link state/Link state update (LSU) to each other.
The OSPF area concept is to control frequent LSA/LSU updates in the OSPF domain.
In OSPF, Area reduces the size of the topology database that each router must maintain.

Question -(17) What are the Router types in OSPF?
Ans: internal routers
backbone router
Area Border Router
Autonomous system area border router

Question-(18) Explain different OSPF network types.
Ans: Broadcast
non-Broadcast
Point-to-point
Point-to-multi-point
Point-to-Multi-point non-broadcast

Question-(19) Does OSPF support authentication?
Ans: Yes OSPF supports two types of authentication.
*Plain text authentication
*MD5 authentication

Question-(20) What is Virtual Link in OSPF in routing Protocol?
Ans: OSPF requires the use of a backbone area (a0) with each area connecting to area zero through an ABR.
However, in some cases, the regular area might not have a convenient point of connection to the backbone area.
In these cases, OSPF uses the virtual link to connect that regular area to the backbone area virtually.