1. What is the OSPF transport?
- IP/88
- IP/89
- IP/86
- TCP/179
- UDP/520
Explanation: Topic 4.2.0
OSPF is designed with an independent transport mechanism that operates directly on top of the IP layer. It uses IP protocol number 89 and does not rely on transport layer protocols like TCP or UDP.
2. What is the OSPF default reference bandwidth?
- 100 Kbps
- 100 Mbps
- 1000 Mbps
- 10 Mbps
Explanation: Topic 6.6.0
By default, OSPF uses a reference bandwidth of 100 Mbps to calculate interface cost (Cost = Reference Bandwidth / Interface Bandwidth). This means any interface with a bandwidth of 100 Mbps or higher will have a default cost of 1 unless the reference bandwidth is manually adjusted.
3. What special type of LSA is generated by the ASBR in an OSPF NSSA?
- Type 3
- Type 7
- Type 5
- Type 1
Explanation: Topic 5.4.3
In a Not-So-Stubby Area (NSSA), external routes redistributed by an ASBR are advertised using a special Type 7 LSA. These Type 7 LSAs are local to the NSSA and are converted into Type 5 LSAs by the Area Border Router (ABR) before being flooded into the rest of the OSPF domain.
4. An OSPF router receives an LSA and checks the sequence number of the LSA. This number matches the sequence number of an LSA that the receiving router already has. What does the receiving router do with the received LSA?
- It sends the newer LSU to the source router.
- It adds the LSA to the database.
- It ignores the LSA.
- It floods the LSA to the other routers.
Explanation: Topic 4.11.0
OSPF routers use sequence numbers to determine the "newness" of link-state information. If a received LSA has a sequence number that matches the one already stored in the router's Link-State Database (LSDB), the router recognizes it as a duplicate and ignores the LSA.
5. Which algorithm does OSPF use to calculate the SPF tree?
- Bellman-Ford
- Dijkstra
- DUAL
- Diffie-Hellman
Explanation: Topic 4.1.0
OSPF utilizes the Dijkstra algorithm, also frequently referred to as the Shortest Path First (SPF) algorithm, to analyze link-state information and calculate the loop-free best path tree to all destinations.
6. OSPFv2 supports which two types of authentication? (Choose two.)
- AH
- simple password
- AES
- SHA-256
- MD5
Explanation: Topic 5.7.0
OSPFv2 supports three types of authentication: Type 0 (No authentication), Type 1 (Simple password/cleartext), and Type 2 (Cryptographic, which traditionally uses MD5). While newer versions also support SHA, MD5 and simple passwords are the primary methods defined in the protocol.
7. A normal non-backbone area uses what type of router to connect to the backbone area?
- ABR
- ASBR
- BDR
- DR
Explanation: Topic 4.4.0
An Area Border Router (ABR) is specifically defined as a router that has interfaces connected to at least two different OSPF areas, with at least one interface being in the backbone area. It is responsible for advertising routing information between the backbone and non-backbone areas.
8. OSPF LSAs are contained in which OSPF packet type?
- DBD
- LSR
- LSAck
- LSU
Explanation: Topic 4.6.0
Link State Update (LSU) packets are used for the actual flooding of Link-State Advertisements (LSAs). While other packets like DBDs describe headers or LSRs request data, the LSU is the container that carries the full LSA information to neighbors.
9. Which command modifies the cost of a default route injected into a stub area?
- default-information originate cost
- area stub cost
- default area-cost
- area default-cost
Explanation: Topic 5.6.0
The area <area-id> default-cost <cost> command is used on an ABR to manually set the metric for the summary default route that the ABR injects into a stub or totally stubby area. By default, this cost is 1.
10. Which command displays how often the SPF algorithm has executed on a router?
- show ospfv3 neighbor
- show ospfv3 timers
- show ospfv3 database
- show ospfv3
Explanation: 4.14.0
The show ospfv3 verification command displays general information about OSPFv3 processes, including a specific counter for how many times the SPF algorithm has been executed for each area.
11. What are two reasons why route summarization is important? (Choose two.)
- It reduces the size of the routing table.
- It reduces the size of the neighbor table.
- It reduces LSA type 3 flooding.
- It reduces LSA type 1 flooding.
- It reduces LSA type 2 flooding.
Explanation: Topic 5.1.0
Route summarization consolidation reduces the number of entries in the routing table and limits the number of Inter-Area Prefix (Type 3) LSAs flooded across area boundaries. By advertising one summary prefix instead of many specific ones, it also lowers CPU and memory utilization.
12. Refer to the exhibit. An OSPF virtual link is configured. Based upon the show command output, why is the virtual-link down?
Router# show ip ospf virtual-links
Virtual Link OSPF_VL1 to router 209.165.202.101 is down
Run as demand circuit
DoNotAge LSA allowed.
Transit area 0, cost of using 65535
Transmit Delay is 1 sec, State DOWN,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
- The virtual interface VL1 is shut down.
- The timer intervals for virtual links need to be aggressive (2, 8, 8, and 1).
- Virtual links cannot transit area 0.
- The cost of the virtual link needs to be configured as 1.
Explanation: Topic 5.9.0
An OSPF virtual link is designed to provide a logical path to the backbone (Area 0) through a non-backbone area. The output shows the transit area is configured as Area 0, which is invalid because a virtual link cannot transit the backbone area itself; it must transit a normal non-backbone area.
13. Which command will verify if OSPF is advertising the correct networks?
- show ip protocols
- show ip ospf
- show ip ospf interface
- show ip ospf neighbors
Explanation: Topic 6.5.0
The show ip protocols command verifies which networks are being advertised by OSPF by showing the configured network statements or explicitly enabled interfaces. It is a primary tool for troubleshooting missing routes to confirm if a specific network is included in the OSPF process.
14. What does an area border router maintain?
- two databases: one for the backbone and one for all other areas
- a separate database for each area with which it is connected
- a single database for all areas
- a separate routing table for each area
Explanation: Topic 4.4.0
An Area Border Router (ABR) is connected to at least two areas (one being Area 0). Because OSPF requires all routers within a specific area to have identical Link-State Databases (LSDBs), the ABR must maintain a separate and synchronized database for every area it participates in.
15. What is the difference between an LSA 3 and an LSA 4?
- LSA 3 is a summary of the directly connected Area, and LSA 4 is E1.
- LSA 3 is a summary of the directly connected Area, and LSA 4 is a summary of the ASBRs.
- LSA 3 is E1, and LSA 4 is a summary.
- LSA 3 is a summary of the ASBRs, and LSA 4 is a summary of the directly connected Area.
Explanation: Topic 4.9.0
Type 3 LSAs (Inter-Area Prefix) are generated by ABRs to advertise network prefixes from one area into another. Type 4 LSAs (Inter-Area Router) are also generated by ABRs, but their specific purpose is to inform other areas how to reach an ASBR located outside those areas.
16. Which multicast address does the DR use to flood LSUs to other OSPF routers in a multiaccess network?
- 224.0.0.6
- 224.0.0.4
- 224.0.0.5
- 220.0.0.7
Explanation: Topic 4.13.0
In OSPF, while non-DR routers send updates to the Designated Router (DR) and Backup Designated Router (BDR) using the multicast address 224.0.0.6, the DR is responsible for flooding those updates (LSUs) to all other OSPF routers on the segment using the multicast address 224.0.0.5.
17. You need to check if there is a mismatch in the hello and dead intervals on two connected routers. Which command do you use?
- show ipv6 protocols
- show ospfv3 interface
- show ospfv3 interface brief
- show ospfv3 neighbor
Explanation: Topic 4.8.11
The show ospfv3 interface command (or show ip ospf interface for OSPFv2) displays detailed information about OSPF-enabled interfaces, including the configured Hello and Dead timer intervals. Comparing this output on both routers allows you to identify a mismatch.
18. Which OSPF neighbor state indicates that two neighbors have exchanged routes?
- exchange
- loading
- INIT
- full
Explanation: Topic 4.12.0
The FULL state is the definitive indicator that the link-state database (LSDB) synchronization process is complete and the neighbors are fully adjacent. While the act of exchanging information occurs during the EXCHANGE (sending database headers) and LOADING (requesting specific routes) states, the FULL state signifies that:
The routers have identical and synchronized databases.
All Link State Requests (LSRs) have been satisfied.
The routers are now capable of routing traffic, which can only happen once the FULL state is reached.
Therefore, transitioning to the FULL state is the final confirmation that the two neighbors have successfully exchanged all necessary routing information.
19. Which OSPF neighbor state indicates that the adjacency is established, and the router databases are synchronized?
- 2-Way
- Full
- Init
- Loading
Explanation: Topic 4.12.0
The FULL state indicates that the OSPF neighbor adjacency is completely established and the Link-State Databases (LSDBs) between the routers are fully synchronized. Routers must reach this state before they can begin routing transit traffic.
20. What can stub area design improve?
- LSDB size on routers in the backbone
- the ability to reach outside networks
- the number of adjacencies in the stub
- CPU utilization on routers in the stub
Explanation: Topic 5.1.0
Stub area design is intended to reduce the size of the LSDB and routing tables within that specific area by blocking external LSAs and replacing them with a default route. This reduction in data processing directly results in improved CPU and memory utilization for the routers located inside the stub area.
21. Which command will verify the OSPF cost on an interface?
- show ip ospf interface
- show ip ospf
- show ip protocols
- show ip route
Explanation: Topic 6.6.0
The show ip ospf interface command is used to verify the OSPF cost assigned to a specific interface. It displays detailed OSPF parameters, including the cost calculated based on the reference bandwidth or manually configured values.
22. Route summarization reduces the flooding of which two LSA types? (Choose two.)
- network
- NSSA
- external
- router
- summary
Explanation: Topic 5.2.0
Route summarization reduces LSA flooding by consolidating multiple specific routes into a single advertisement. ABRs summarize inter-area routes into Summary (Type 3) LSAs, while ASBRs summarize redistributed routes into External (Type 5) LSAs.
23. Two OSPF neighbors are stuck in the EXCHANGE/EXSTART state. What could be the reason for this?
- An OSPF interface is passive on one router.
- Multicast is broken on the link that connects the two routers.
- There is an MTU issue.
- An access list is blocking OSPF hellos on the interface.
Explanation: Topic 6.4.0
If OSPF neighbors are stuck in the EXSTART/EXCHANGE state, it is most commonly caused by an MTU mismatch on the connecting link. This prevents the routers from successfully exchanging Database Description (DBD) packets, which is required to progress to the Loading and Full states.
24. External route summarization is performed on which OSPF router type?
- ASBR
- ABR
- Backbone router
- Internal router
Explanation: Topic 5.2.2
External route summarization is the process of consolidating routes redistributed from other routing protocols. This function is performed exclusively on an Autonomous System Border Router (ASBR) before it injects AS External (Type 5) LSAs into the OSPF domain.
25. Which option is not a parameter that must be matched for routers to become OSPF neighbors?
- OSPF cost
- stub area flag
- area ID
- subnet ID and subnet mask
- hello and dead interval
Explanation: Topic 6.4.0
While parameters such as Area ID, Hello/Dead timers, Subnet/Mask, and the Stub flag must match for an adjacency to form, OSPF cost is not a requirement for neighbor establishment. The cost is a local parameter used for SPF path selection calculations after the adjacency is already Full.
26. Which Cisco proprietary special area type allows redistribution with an ASBR?
- NSSA
- Totally stub
- Totally NSSA
- Stub
Explanation: Topic 5.4.4
Totally NSSA is a Cisco proprietary enhancement that combines the features of a totally stubby area and an NSSA. It blocks Type-3, Type-4, and Type-5 LSAs from entering the area but still allows an internal ASBR to redistribute external routes into the area as Type-7 LSAs.
27. What is the default metric type when a default route is injected into a normal non-backbone OSPF area?
- E1
- E2
- N2
- N1
Explanation: Topic 5.3.0
When a default route is injected into a normal OSPF area using the default-information originate command, it is advertised as a Type-5 AS External LSA. By default, OSPF assigns these external routes a Metric Type 2 (E2), which maintains a constant external cost regardless of the internal path cost.
28. Which version of OSPF carries IPv4 routes over an IPv6 transport?
- OSPFv3 for address families
- Traditional OSPFv2
- Traditional OSPFv3
- IPv6 OSPF
Explanation: Topic 4.7.0
The OSPFv3 address families framework (the "new style" OSPFv3) uses IPv6 link-local addresses as its transport mechanism. Unlike traditional OSPFv2, this version is capable of carrying both IPv4 and IPv6 routing information over that IPv6-based transport.
