1. What does OSPF stand for?
OSPF stands for Open Shortest Path First.
2. What is the purpose of OSPF?
OSPF is a routing protocol used to determine the best path for routing IP packets within an autonomous system (AS).
3. What is an Autonomous System (AS) in OSPF?
An AS is a collection of IP networks and routers under the control of a single organization or entity.
4. What are the key advantages of OSPF over RIP?
OSPF supports variable-length subnet masking (VLSM), classless routing, and faster convergence.
5. What is a Link State Routing Protocol?
OSPF is a link state routing protocol, which means it keeps information about all routers and links in the AS.
6. How does OSPF ensure loop-free routing?
OSPF uses the Dijkstra algorithm to calculate the shortest path tree, which ensures loop-free routing.
7. What are the OSPF area types?
OSPF has five area types: Backbone Area (Area 0), Standard Area (Area 1-49), Stub Area, Totally Stubby Area, and Not-So-Stubby Area (NSSA).
8. What is the OSPF Backbone Area (Area 0)?
Area 0 is the central area in OSPF that connects all other OSPF areas. It is also known as the OSPF Backbone Area.
9. What is the purpose of OSPF area segregation?
Segregating OSPF into areas reduces the amount of routing information exchanged and improves scalability and performance.
10. What are OSPF LSR and LSU packets used for?
- LSR (Link State Request) packets are used to request missing link state advertisements, and LSU (Link State Update) packets are used to send link state advertisements.
11. What is the OSPF LSDB (Link State Database)?
- The LSDB is a database that stores information about OSPF routers and their links. It is used to calculate the SPF tree.
12. What is the OSPF SPF (Shortest Path First) algorithm used for?
- The SPF algorithm is used to calculate the shortest path tree and determine the best path for routing.
13. Explain the OSPF Hello Protocol.
- OSPF routers use Hello packets to discover and establish neighbor relationships. These packets also verify that neighbors are reachable.
14. What are the key OSPF neighbor states?
- OSPF neighbors go through several states: Down, Init, 2-Way, Exstart, Exchange, Loading, and Full.
15. What is the OSPF Router ID?
- The Router ID is a unique identifier for each OSPF router within an AS. It can be manually configured or chosen automatically.
16. How is the OSPF Router ID determined if not configured manually?
- The OSPF Router ID is determined by selecting the highest IP address on an active loopback interface. If no loopback interfaces exist, the highest IP on an active physical interface is chosen.
17. What is OSPF DR (Designated Router) and BDR (Backup Designated Router)?
- In multi-access networks, OSPF elects a DR and BDR to reduce the number of adjacencies. The DR forwards LSAs to other routers.
18. What is OSPF Type 1 LSA (Router LSA) used for?
- Type 1 LSAs describe the router's local links and are flooded within the area.
19. What is OSPF Type 2 LSA (Network LSA) used for?
- Type 2 LSAs describe the network topology, including the DR and BDR, in broadcast and NBMA networks.
20. What is OSPF Type 3 LSA (Summary LSA) used for?
- Type 3 LSAs summarize routes from one area to another, allowing inter-area routing.
21. What is OSPF Type 4 LSA (ASBR Summary LSA) used for?
- Type 4 LSAs provide a summary of ASBR (Autonomous System Border Router) information within an area.
22. What is OSPF Type 5 LSA (AS External LSA) used for?
- Type 5 LSAs advertise external routes from outside the OSPF AS into the OSPF domain.
23. What is OSPF Type 7 LSA (NSSA External LSA) used for?
- Type 7 LSAs advertise external routes in NSSAs (Not-So-Stubby Areas).
24. What is OSPF Type 9 LSA (Opaque LSA) used for?
- Type 9 LSAs are used for OSPF extensions and traffic engineering.
25. What is OSPF Virtual Link and when is it used?
- A Virtual Link is used to connect a discontiguous area to the OSPF Backbone Area (Area 0) when no physical path exists.
26. How does OSPF prevent routing loops?
- OSPF prevents loops by using a hierarchical routing structure, SPF calculations, and aging out of old LSAs.
27. What is the OSPF LSRefresh interval?
- The LSRefresh interval is the time period during which LSAs must be refreshed to prevent them from aging out.
28. How does OSPF handle authentication?
- OSPF supports authentication to ensure that routers within the same area trust each other. Common methods include plaintext, MD5, and SHA authentication.
29. Explain OSPF Path Selection Criteria.
- OSPF selects routes based on cost (OSPF Metric), where the lowest cost path is chosen. The cost is typically calculated based on bandwidth.
30. What is the OSPF Metric Cost Formula for IPv4 networks?
- OSPF cost = Reference bandwidth / Interface bandwidth. The default reference bandwidth is 100 Mbps.
31. What is the OSPF Metric Cost Formula for IPv6 networks?
- OSPF cost = 2^16 / Interface bandwidth.
32. How can you manipulate OSPF cost to influence path selection?
- You can manually set the cost on OSPF interfaces or adjust the reference bandwidth to affect cost calculations.
33. How does OSPF handle route summarization?
- OSPF can summarize routes using the area border router (ABR) or the ASBR.
34. What is OSPF Virtual Router Redundancy Protocol (VRRP)?
- VRRP is used with OSPF to provide high availability by allowing multiple routers to share a virtual IP address.
35. How does OSPF handle external routes?
- External routes in OSPF are redistributed into OSPF using the ASBR, and they are advertised as Type 5 LSAs.
36. What is the OSPF default administrative distance for intra-area routes?
- The default administrative distance for OSPF intra-area routes is 110.
37. What is the OSPF default administrative distance for inter-area and external routes?
- The default administrative distance for OSPF inter-area and external routes is 110 for Type 1 and Type 2 LSAs and 110 for Type 5 LSAs.
38. How can you configure OSPF authentication on an interface?
- You can configure OSPF authentication using the ip ospf authentication or ip ospf authentication-key command on an interface.
39. What is OSPF flooding domain?
- A flooding domain is a collection of routers in the same OSPF area that receive the same LSAs.
40. How does OSPF handle routing loops when a link fails?
- OSPF uses SPF calculations to recompute the shortest path tree and converge the network when a link fails.
41. What is OSPF graceful restart?
- OSPF graceful restart allows routers to maintain their OSPF adjacencies when the OSPF process restarts.
42. What is the OSPF max-metric router-lsa command used for?
- The max-metric router-lsa command sets the OSPF Router LSA metric to its maximum value, making the router less preferred as a transit router.
43. What is the OSPF Fast Hello feature used for?
- Fast Hello reduces OSPF neighbor discovery time by sending Hello packets more frequently.
44. How can you configure OSPF authentication in plaintext mode?
- To configure OSPF authentication in plaintext mode, use the ip ospf authentication and ip ospf authentication-key commands.
45. What is the OSPF STUB area and why is it used?
- A Stub Area is an OSPF area that does not receive external routes. It is used to reduce LSDB size and improve scalability.
46. What is OSPF Totally Stubby Area and when is it used?
- A Totally Stubby Area blocks Type 3 and Type 4 LSAs, allowing only Type 1, Type 2, and Type 5 LSAs. It's used for further route reduction.
47. What is OSPF NSSA (Not-So-Stubby Area) and why is it used?
- NSSA is an OSPF area that allows limited external routes (Type 7 LSAs) while maintaining stub area characteristics.
48. What is OSPF Not-So-Stubby Totally Stubby Area and when is it used?
- It blocks all external routes (Type 3, 4, and 5 LSAs) while allowing Type 1, 2, and 7 LSAs. It's used when even stricter route control is needed.
49. What is the OSPF E1 and E2 route classification?
- E1 routes include the cost of the path to the ASBR, while E2 routes do not consider the path's cost.
50. How can you redistribute routes into OSPF?
- Routes can be redistributed into OSPF using the redistribute command, typically on the ASBR.
51. What is OSPF LSA Type 8 used for?
- LSA Type 8 is used for OSPF Opaque LSA and extensions, such as MPLS TE (Traffic Engineering).
52. How does OSPF handle network failures and reconvergence?
- OSPF uses Hello packets, LSAs, SPF calculations, and the Dead Interval to detect and recover from network failures.
53. What is the OSPF Loopback Interface used for?
- The Loopback interface is often used as the OSPF Router ID, ensuring a stable identifier even if physical interfaces go down.
54. What is the OSPF Administrative Distance (AD) for external routes when summarized by ABR?
- The AD for summarized external routes is 5 when advertised by the ABR.
55. Explain the OSPF Route Summarization command area X range X.X.X.X mask X.X.X.X.
- This command on the ABR summarizes routes into Area X based on the specified range and mask.
56. How can you filter OSPF routes using a distribute-list?
- You can use distribute-lists with the distribute-list command to filter OSPF routes based on an access-list.
57. What is OSPF virtual link authentication?
- OSPF virtual link authentication is a security feature that ensures authentication for OSPF virtual links.
58. How can you configure OSPF route summarization for external routes on the ASBR?
- You can use the summary-address command on the ASBR to summarize external routes into OSPF.
59. What is OSPF SPF throttling and how does it work?
- SPF throttling limits how often SPF calculations are performed to avoid excessive recalculations during network instability.
60. What is the OSPF SPF delay timer?
- The SPF delay timer is a timer that postpones SPF calculations after an event to allow other LSAs to arrive.
61. What is the OSPF LSA flood reduction timer?
- This timer reduces LSA flooding in OSPF during network instability to prevent excessive control plane traffic.
62. How can you manually set the OSPF Router ID?
- You can manually set the OSPF Router ID using the router-id command in OSPF configuration.
63. Explain the OSPF NSSA Totally Stubby Area (NSSA-TS) type.
- NSSA-TS allows only Type 1 and Type 2 LSAs, blocking all external routes including Type 7 and Type 5 LSAs.
64. How does OSPF support IPv6 routing?
- OSPFv3 is used for IPv6 routing, providing similar functionality as OSPF for IPv4.
65. What is the OSPF graceful restart helper mode?
- The helper mode allows OSPF routers to help neighboring routers maintain OSPF adjacencies during a restart.
66. What is OSPF Multi-Area Adjacency?
- Multi-Area Adjacency allows an OSPF router to have multiple adjacent areas.
67. How can you configure OSPF multi-area adjacency?
- Configure multi-area adjacency using the area X range command on the ABR.
68. What is OSPF external route tagging, and why is it useful?
- External route tagging allows the OSPF router to distinguish between redistributed external routes and intra-area routes.
69. How can you tag external routes in OSPF using the tag command?
- You can tag external routes in OSPF by using the tag keyword when redistributing routes.
70. What is the OSPF SPF interface priority used for?
- SPF interface priority determines the order in which interfaces are considered during SPF calculations.
71. How can you change the OSPF SPF interface priority?
- You can change the SPF interface priority using the ip ospf priority command on the interface.
72. What is OSPF BFD (Bidirectional Forwarding Detection)?
- OSPF BFD is used to quickly detect link failures and trigger OSPF reconvergence.
73. How can you enable OSPF BFD on an interface?
- Use the ip ospf bfd command on the interface to enable OSPF BFD.
74. What is the OSPF Area Range command used for?
- The area X range command summarizes external routes injected into OSPF, reducing LSDB size.
75. What is the OSPF Max Metric Router LSA feature used for?
- The Max Metric Router LSA feature allows an OSPF router to temporarily advertise itself as unreachable.
76. What is OSPF graceful restart helper mode used for?
- Helper mode allows a router to assist its neighbors in maintaining OSPF adjacencies during a graceful restart.
77. What is OSPF Synchronization and when is it necessary?
- Synchronization is a process of ensuring that OSPF routers have the latest OSPF LSDB information before advertising external routes.
78. How can you configure OSPF Synchronization?
- OSPF Synchronization can be configured with the no synchronization command on the ASBR.
79. What is the OSPF Stub Router Advertisement (SRA) feature used for?
- SRA is used to advertise the stub flag to OSPF routers, optimizing OSPF routing tables.
80. What is OSPF Flooding Scope?
- Flooding scope indicates whether an OSPF LSA is limited to a single area or can propagate throughout the AS.
81. What are OSPF OSPFv2 and OSPFv3 used for?
- OSPFv2 is used for IPv4 routing, while OSPFv3 is used for IPv6 routing.
82. How does OSPF handle Type 7 LSAs in NSSA areas?
- Type 7 LSAs are converted to Type 5 LSAs by the NSSA ABR when they leave the NSSA area.
83. What is the OSPF dead interval, and how does it affect OSPF adjacency?
- The OSPF dead interval is the time an OSPF router waits without receiving Hello packets before declaring a neighbor as dead.
84. What is the OSPF Router-LSA maximum age, and how does it affect the network?
- The maximum age is the time an OSPF Router LSA can exist in the LSDB before being removed. It ensures that outdated information doesn't persist.
85. What is OSPF Fast Reroute (FRR) and how does it work?
- OSPF FRR provides a backup path to quickly reroute traffic in the event of a link or node failure.
86. What is the OSPF Inter-Area MPLS Traffic Engineering (TE) feature used for?
- It allows OSPF routers to exchange MPLS TE information for traffic engineering purposes.
87. How does OSPF handle unequal cost load balancing?
- OSPF can perform unequal cost load balancing by using the maximum-paths command.
88. What is the OSPF Reverse Metric feature used for?
- Reverse Metric allows OSPF routers to consider the cost of the return path when performing load balancing.
89. What is the OSPF virtual link dead interval timer?
- The virtual link dead interval timer is the time a router waits without receiving Hello packets before declaring a virtual neighbor as dead.
90. How does OSPF handle route summarization within an NSSA?
- Summarization within an NSSA is done by the NSSA ABR, reducing the LSDB size.
91. What is OSPF external route redistribution, and when is it necessary?
- Redistribution is used to inject external routes from other routing domains into OSPF when interconnecting different routing protocols.
92. How does OSPF handle route redistribution into OSPFv3 for IPv6 networks?
- OSPFv3 uses the redistribute command for IPv6 route redistribution, similar to OSPFv2 for IPv4.
93. What is the OSPFv3 default administrative distance for intra-area routes?
- The default administrative distance for OSPFv3 intra-area routes is 110.
94. What is the OSPF OSPFv3 Router LSA used for?
- The OSPFv3 Router LSA describes a router's local link state information for OSPFv3.
95. What is OSPF NSSA to Stub conversion and how does it work?
- NSSA to Stub conversion allows an NSSA area to be converted into a Stub area, blocking Type 7 LSAs.
96. What is OSPFv3 AF (Address Family) support used for?
- OSPFv3 AF support allows OSPFv3 to handle multiple address families, including IPv6 and IPv4.
97. What is OSPF graceful restart helper mode and when is it used?
- Helper mode allows a router to assist its OSPF neighbors in maintaining adjacencies during a restart.
98. How does OSPFv3 handle route summarization for IPv6 networks?
- OSPFv3 uses the area range command to perform route summarization for IPv6 networks.
99. What is the OSPF Max-Metric feature used for, and how is it configured?
- OSPF Max-Metric allows a router to advertise itself as the worst path when it should not be used as a transit router. It is configured using the max-metric router-lsa command.
100. What is the OSPF virtual link dead interval timer and how is it configured?
- The virtual link dead interval timer can be configured using the ip ospf dead-interval command and specifies the time a router waits without receiving Hello packets before declaring a virtual neighbor as dead.
These questions cover a wide range of OSPF topics and should provide a comprehensive understanding of the protocol at CCNA,CCNP & CCIE-level interviews. Be prepared to explain each concept thoroughly and discuss practical applications and configurations.
