1. What is OSPF Routing Protocol?
Open shortest path first is an Open Standard Link State routing protocol which works by using Dijkastra algorithm to initially construct the shortest paths and follows that by populating the routing table with resulting best paths.
2. What are the steps required to change Neighborship into adjacency?
1.Two-way communication (using Hello Protocol).
2.Database Synchronization which means exchange of Database Description (DD) packets, Link State Request (LSR) packets, Link State Update (LSU) packets.
After Database synchronization is complete, the two routers are considered adjacent.
3. Explain LSA (Link-State Advertisement), LSU (Link State Update) and LSR (Link State Request)?
The LSAs (Link-State Advertisements) are used by OSPF routers to exchange routing and topology information. When two neighbors decide to exchange routes, they send each other a list of all LSAs in their respective topology database. Each router then checks its topology database and sends Link State Request (LSR) message requesting all LSAs that was not found in its topology table. Other router responds with the Link State Update (LSU) that contains all LSAs requested by the neighbor.
4. Explain OSPF Router ID?
Router Id is used to identify the Router. Highest IP address of the router's loopback interfaces is chosen as the Router ID, If no loopback is present than highest IP address of the router's physical interfaces will be chosen as Router ID. OSPF prevents neighborships between routers with duplicate RIDs. All OSPF RIDs in a domain should be unique. OSPF Router ID should not be changed after the OSPF process is started and the OSPF neighborships are established. If you change the OSPF router ID, we need to either reload the IOS or use "clear ip ospf process" command (restart the OSPF process) for changed RID to take effect.
To manually configure the router ID
R1(config)# router ospf 5
R1(config-router)# router-id 5.5.5.5
5. Can we use OSPF without backbone area?
Yes, but than only intra-area communication is possible. Inter-area communication is not possible without backbone area.
6. What is the difference between an OPPF neighbor and an adjacent neighbor?
LSAs are exchanged only among adjacent routers not among neighbor routers.
7. What are different neighbour states in OSPF ?
OSPF routers need to go through several state before establishing a neighbor relationship -
1. Down - No Hello packets have been received on the interface.
2. Attempt - In Attempt state neighbors must be configured manually. It applies only to nonbroadcast multi-access (NBMA) networks.
3. Init - Router has received a Hello message from the other OSFP router.
4. 2way - the neighbor has received the Hello message and replied with a Hello message of his own. Bidirectional Communication has been established. In Broadcast network DR-BDR election can occur after this point.
5. Exstart - DR & BDR establish adjacencies with each router in the network. Master-slave election will takes place (Master will send its DBD first).
6. Exchange - Routing information is exchanged using DBD (Database Descriptor) packets, Link-State Request (LSR) and Link-State Update packets may also be sent.
7. Loading - LSRs (Link State Requests) are send to neighbors for every network it doesn't know about. The Neighbor replies with the LSUs (Link State Updates) which contain information about requested networks. After all the requested information have been received, other neighbor goes through the same process.
8. Full - All neighbor routers have the synchronized database and adjacencies has been established.
8. Explain different OSPF LSA Types?
1. Router LSA (Type1) - Each router generates a Type 1 LSA that lists its active interfaces, IP addresses, neighbors and the cost. LSA Type 1 is flooded only within an area.
2. Network LSA (Type2) - Type2 LSA is sent out by the designated router (DR) and lists all the routers on the segment it is adjacent to. Type 2 LSA are ?ooded only within an area.
3. Summary LSA (Type3) - Type 3 LSAs are generated by Area Border Routers (ABRs) to advertise networks from one area to the rest of the areas in Autonomous System.
4. Summary ASBR LSA (Type4) - Generated by the ABR. It contain routes to ASBRs.
5. External LSA (Type5) - External LSAs are generated by ASBRs and contain routes to networks that are external to the current Autonomous System.
6. Not-So-Stubby Area LSA (Type7) - Stub areas do not allow Type 5 LSAs. A Not So Stubby Area (NSSA) allows advertisement of Type 5 LSA as Type 7 LSAs. Type LSA is generated by an ASBR inside a Not So Stubby Area (NSSA) to describe routes redistributed into the NSSA.
9. Why master slave needs to be elected between two neighbour interface?
Master sends its DBD (Database Description) First.
10. Explain different OSPF Network types ?
1.Broadcast
2.Non-Broadcast (NBMA)
3.Point-to-Point
4.Point-to-multipoint
5.Point-to-multipoint non-broadcast
Read More Questions:
OSPF Interview Questions Part1
OSPF Interview Questions Part2
OSPF Interview Questions Part3
Open shortest path first is an Open Standard Link State routing protocol which works by using Dijkastra algorithm to initially construct the shortest paths and follows that by populating the routing table with resulting best paths.
2. What are the steps required to change Neighborship into adjacency?
1.Two-way communication (using Hello Protocol).
2.Database Synchronization which means exchange of Database Description (DD) packets, Link State Request (LSR) packets, Link State Update (LSU) packets.
After Database synchronization is complete, the two routers are considered adjacent.
3. Explain LSA (Link-State Advertisement), LSU (Link State Update) and LSR (Link State Request)?
The LSAs (Link-State Advertisements) are used by OSPF routers to exchange routing and topology information. When two neighbors decide to exchange routes, they send each other a list of all LSAs in their respective topology database. Each router then checks its topology database and sends Link State Request (LSR) message requesting all LSAs that was not found in its topology table. Other router responds with the Link State Update (LSU) that contains all LSAs requested by the neighbor.
4. Explain OSPF Router ID?
Router Id is used to identify the Router. Highest IP address of the router's loopback interfaces is chosen as the Router ID, If no loopback is present than highest IP address of the router's physical interfaces will be chosen as Router ID. OSPF prevents neighborships between routers with duplicate RIDs. All OSPF RIDs in a domain should be unique. OSPF Router ID should not be changed after the OSPF process is started and the OSPF neighborships are established. If you change the OSPF router ID, we need to either reload the IOS or use "clear ip ospf process" command (restart the OSPF process) for changed RID to take effect.
To manually configure the router ID
R1(config)# router ospf 5
R1(config-router)# router-id 5.5.5.5
5. Can we use OSPF without backbone area?
Yes, but than only intra-area communication is possible. Inter-area communication is not possible without backbone area.
6. What is the difference between an OPPF neighbor and an adjacent neighbor?
LSAs are exchanged only among adjacent routers not among neighbor routers.
7. What are different neighbour states in OSPF ?
OSPF routers need to go through several state before establishing a neighbor relationship -
1. Down - No Hello packets have been received on the interface.
2. Attempt - In Attempt state neighbors must be configured manually. It applies only to nonbroadcast multi-access (NBMA) networks.
3. Init - Router has received a Hello message from the other OSFP router.
4. 2way - the neighbor has received the Hello message and replied with a Hello message of his own. Bidirectional Communication has been established. In Broadcast network DR-BDR election can occur after this point.
5. Exstart - DR & BDR establish adjacencies with each router in the network. Master-slave election will takes place (Master will send its DBD first).
6. Exchange - Routing information is exchanged using DBD (Database Descriptor) packets, Link-State Request (LSR) and Link-State Update packets may also be sent.
7. Loading - LSRs (Link State Requests) are send to neighbors for every network it doesn't know about. The Neighbor replies with the LSUs (Link State Updates) which contain information about requested networks. After all the requested information have been received, other neighbor goes through the same process.
8. Full - All neighbor routers have the synchronized database and adjacencies has been established.
8. Explain different OSPF LSA Types?
1. Router LSA (Type1) - Each router generates a Type 1 LSA that lists its active interfaces, IP addresses, neighbors and the cost. LSA Type 1 is flooded only within an area.
2. Network LSA (Type2) - Type2 LSA is sent out by the designated router (DR) and lists all the routers on the segment it is adjacent to. Type 2 LSA are ?ooded only within an area.
3. Summary LSA (Type3) - Type 3 LSAs are generated by Area Border Routers (ABRs) to advertise networks from one area to the rest of the areas in Autonomous System.
4. Summary ASBR LSA (Type4) - Generated by the ABR. It contain routes to ASBRs.
5. External LSA (Type5) - External LSAs are generated by ASBRs and contain routes to networks that are external to the current Autonomous System.
6. Not-So-Stubby Area LSA (Type7) - Stub areas do not allow Type 5 LSAs. A Not So Stubby Area (NSSA) allows advertisement of Type 5 LSA as Type 7 LSAs. Type LSA is generated by an ASBR inside a Not So Stubby Area (NSSA) to describe routes redistributed into the NSSA.
9. Why master slave needs to be elected between two neighbour interface?
Master sends its DBD (Database Description) First.
10. Explain different OSPF Network types ?
1.Broadcast
2.Non-Broadcast (NBMA)
3.Point-to-Point
4.Point-to-multipoint
5.Point-to-multipoint non-broadcast
Read More Questions:
OSPF Interview Questions Part1
OSPF Interview Questions Part2
OSPF Interview Questions Part3
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