This set of Computer Networks online quiz focuses on “OSPF Configuration”.
1. An OSPF router receives an LSA, the router checks its sequence number, and this number matches the sequence number of the LSA that the receiving router already has. What does the receiving router do with the LSA?
a) Ignores the LSA
b) Adds it to the database
c) Sends newer LSU update to source router
d) Floods the LSA to the other routers
View Answer
Explanation: When the OSPF router receives an LSA, the router checks its sequence number. If this number matches the sequence number of the LSA that the receiving router already has, the router ignores the LSA.
2. An OSPF router receives an LSA. The router checks its sequence number and finds that this number is higher than the sequence number it already has. Which two tasks does the router perform with the LSA?
a) Ignores the LSA
b) Adds it to the database
c) Sends newer LSU update to source router
d) Floods the LSA to the other routers
View Answer
Explanation: An OSPF router receives an LSA. If the router checks its sequence number and finds that the number is higher than the sequence number of the LSA that it already has, the router adds it to the database, and then floods the LSA to the other routers.
3. An OSPF router receives an LSA. The router checks its sequence number and finds that this number is lower than the sequence number it already has. What does the router do with the LSA?
a) ignores the LSA
b) adds it to the database
c) sends newer LSU update to source router
d) floods the LSA to the other routers
View Answer
Explanation: An OSPF router receives an LSA. If the router checks its sequence number and finds that this number is lower than the sequence number that it already has, the router sends newer LSU update to source router. The router then adds it to the database and floods it to the other routers.
4. Each LSA has its own age timer. By default, how long does an LSA wait before requiring an update?
a) 30 seconds
b) 1 minute
c) 30 minutes
d) 1 hour
View Answer
Explanation: Each LSA has its own age timer. By default, an LSA waits for 30 minutes before requiring an update. The router then has to send a LSR (Link State Request) to its neighbors to get an update.
5. Distance vector protocols use the concept of split horizon, but link-state routing protocols, such as OSPF, do not.
a) True
b) False
View Answer
Explanation: In Distance vector routing protocols, there is a problem called count-to-infinity which occurs regularly. So, to make sure that it does not occur, the split horizon algorithm is used. There is no requirement for it in OSPF.
6. The outcome of Dijkstra’s calculation is used to populate the __________
a) Topology table
b) Routing table
c) Neighbor table
d) Adjacency table
View Answer
Explanation: The outcome of Djikstra’s calculation is the main source of entries in the routing table as it is the algorithm that is used to find the shortest path in OSPF. The calculations are done after receiving every new LSU.
7. What is the IP protocol number for OSPF packets?
a) 89
b) 86
c) 20
d) 76
View Answer
Explanation: 89 is the IP protocol number for OSPF packets. 86 is the protocol number for DGP, 76 is the protocol number for Backroom-SATNET-Monitoring and 20 is the protocol number for Host Monitoring Protocol.
8. Which packet is NOT an OSPF packet type?
a) LSU
b) LSR
c) DBD
d) Query
View Answer
Explanation: LSU is the Link State Update packet, LSR is the Link State Request packet and DBD is the Database Descriptor packet in OSPF. Query packet is NOT an OSPF packet type.
9. Which multicast address does the OSPF Hello protocol use?
a) 224.0.0.5
b) 224.0.0.6
c) 224.0.0.7
d) 224.0.0.8
View Answer
Explanation: Hello protocol is used to discover neighboring routers automatically. It makes sure that the communication between neighbors is bidirectional. The multicast address that the OSPF Hello protocol uses is 224.0.0.5.
10. The Hello protocol sends periodic updates to ensure that a neighbor relationship is maintained between adjacent routers.
a) True
b) False
View Answer
Explanation: The Hello protocol sends periodic updates to ensure that a neighbor relationship is maintained between adjacent routers. It’s similar to the real world moral construct of saying “Hello” to initialize the communication.
11. DBD packets are involved during which two states?
a) Exstart and exchange
b) Loading and Two-way
c) Init and Full
d) Down and Loading
View Answer
Explanation: DBD stands for Database Descriptor. DBD packets are involved during the two states Exstart and Exchange. In exstart, the master and the slaves are decided and in the exchange state, the DBD is exchanged among the neighbors.
12. At which interval does OSPF refresh LSAs?
a) 10 seconds
b) 30 seconds
c) 30 minutes
d) 1 hour
View Answer
Explanation: Each LSA has its own age timer. By default, an LSA waits for 30 minutes before requiring an update. So to make sure that each router first has an up-to-date LSA, OSPF refreshes LSAs after every 1 hour.
13. Which field is NOT a field within an OSPF packet header?
a) Packet length
b) Router ID
c) Authentication type
d) Maxage time
View Answer
Explanation: The packet length field gives the length of the packet in bits. The Authentication type field gives the type of authentication used. The router ID field gives the ID of the source router of the packet. In an OSPF packet header, there is no field called Maxage time.
14. Which two commands are required for basic OSPF configuration?
a) “[Network mask] area [area-id]” and “Router ospf [process-id]”
b) “[Wildcard-mask] area [area-id]” and “[Network mask] area [area-id]”
c) Only “Router ospf [process-id]”
d) “[Wildcard-mask] area [area-id]” and “Router ospf [process-id]”
View Answer
Explanation: The “Router ospf [process-id]” command enables OSPF routing protocol in the router and the “[Wildcard-mask] area [area-id]” command is used to select the interfaces that we want to include in the OSPF process. That is enough for the basic configuration of OSPF in a router.
15. Which OSPF show command describes a list of OSPF adjacencies?
a) Show ip ospf interface
b) Show ip ospf
c) Show ip route
d) Show ip ospf neighbor
View Answer
Explanation: The “Show ip ospf neighbor” command is the OSPF show command that can describe a list of OSPF adjacencies i.e. the list of adjacent nodes or neighbors. The router will only communicate with its neighbors directly.
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