13. What are two effects of using multiple OSPF areas? (Choose two.)
- prevention of a flood of queries
- reduction in the size of the LSDB
- reduction in the size of the neighbor table
- limits on the propagation of type 1 and 2 LSAs
- decrease in the number of DR and BDR elections
Explanation: AS OSPF areas grow in size, the size of the LSDB grows and the number of type 1 and type 2 LSAs increases. Breaking an OSPF domain into multiple areas reduces the size of the LSDB and the number type 1 and 2 LSAs in each area.
14. Where can interarea route summarization be performed in an OSPF network?
- ASBR
- DR
- ABR
- any router
15. Refer to the exhibit. What is indicated by the O IA in the router output?
- The route was manually configured.
- The route was learned from within the area.
- The route was learned from outside the internetwork.
- The route was learned from another area.
Explanation: The O IA in the routing table indicates that those routes were learned from other areas. The O indicates OSPF, and the IA indicates interarea routes that were received as summary LSAs.
16. Which statement describes the creation of LSAs by ABRs in the discontinuous network situation?
- Type 3 LSAs received from Area 0 are created for nonbackbone areas.
- Type 1 LSAs received from nonbackbone areas are forwarded into the backbone area.
- Type 3 LSAs received from a nonbackbone area are created in other nonbackbone areas.
- Type 1 LSAs received by the ABR are forwarded into other nonbackbone areas.
Explanation: There are three fundamental rules that ABRs use for creating type 3 LSAs:
- Type 1 LSAs received from an area create type 3 LSAs into the backbone area and nonbackbone areas.
- Type 3 LSAs received from Area 0 are created for the nonbackbone area.
- Type 3 LSAs received from a nonbackbone area only insert into the LSDB for the source area. ABRs do not create a type 3 LSA for the other areas (including a segmented Area 0).
17. What is the default maximum number of equal-cost paths supported by OSPF?
- 2
- 4
- 8
- 16
Explanation: By default OSPF supports up to four equal cost paths. The number of equal cost paths can be modified with the maximum-paths command.
18. When does an OSPF router become an ABR?
- when the router has interfaces in different areas
- when the router is configured as an ABR by the network administrator
- when the router has the highest router ID
- when the router has an OSPF priority of 0
Explanation: When a router has interfaces connected to different areas, it becomes an ABR without requiring extra configuration. The router ID and priority values determine DR/BDR status.
19. Which method can be used to prevent type 3 LSAs on the backbone from being regenerated into a nonbackbone area?
- distribute list
- prefix list
- interarea summarization
- intra-area summarization
Explanation: A distribute list configured on an ABR is applied before the type 3 LSA regeneration process and it can prevent the type 3 LSA from being regenerated into a nonbackbone area.
20. Which two networks are part of the summary route 192.168.32.0/22? (Choose two.)
- 192.168.31.0/24
- 192.168.33.0/24
- 192.168.37.0/24
- 192.168.35.0/24
- 192.168.36.0/24
- 192.168.38.0/24
Explanation: The summary route 192.168.32.0/22 would include networks 192.168.32.0/24, 192.168.33.0/24, 192.168.34.0/24, and 192.168.35.0/24.
21. Which three LSA types build the SPF tree for intra-area and interarea routes? (Choose three).
- type 1
- type 2
- type 3
- type 4
- type 5
- type 7
Explanation: Type 1, 2, and 3 LSAs build the SPF tree. Type 1 LSAs are advertised by every OSPF router and advertise router links. Type 2 LSAs are advertised by DRs and identify all routes attached to a network segment. Type 3 LSAs are sent by ABRs and advertise network prefixes from nonoriginating areas.
22. What type of OSPF LSA is originated by ASBR routers to advertise external routes?
- type 1
- type 2
- type 3
- type 5
Explanation: OSPF has many different LSA types.
- type 1 – contains a list of directly connected interfaces
- type 2 – only exists for multiaccess networks and includes DR router ID
- type 3 – used by ABRs to advertise networks from other areas
- type 4 – generated by ABRs to identify an ASBR and provides a route to it
- type 5 – originated by ASBRs to advertise external routes
23. What period of time must elapse before an LSA is purged from the local LSBD if not updated with a new LSA?
- 900 seconds
- 1800 seconds
- 3600 seconds
- 7200 seconds
Explanation: If not updated with a new LSA, LSAs in the local LSDB will age out at 3600 seconds and be purged from the LSDB.
24. What feature can be configured to filter routes as they are crossing an OSPF ABR?
- prefix list
- summarization
- distribute list
- route map
Explanation: A prefix list configured on an ABR can be used to filter routes as advertised into or out of an area.
“Do I Know This Already?” Quiz Answers:
1. True or false: A router with an interface associated with Area 1 and Area 2 will be able to inject routes learned from one area into another area.
- True
- False
Explanation: A router needs to have an interface in Area 0 so that it can be an ABR.
2. True or false: A member router contains a complete copy of the LSDBs for every area in the routing domain.
- True
- False
Explanation: An OSPF router only contains copies of the LSDBs for the areas it participates in.
3. How many OSPF link-state announcement (LSA) types are used for routing traditional IPv4 packets?
- Two
- Three
- Five
- Six
- Seven
Explanation: OSPF uses six OSPF LSA types for routing IPv4 packets (Types 1, 2, 3, 4, 5, and 7). Additional LSAs exist for IPv6 and MPLS.
4. What is the LSA age field in the LSDB used for?
- For version control—to ensure that the most recent LSA is present
- To age out old LSAs by removing an LSA when its age reaches zero
- For troubleshooting—to identify exactly when the LSA was advertised
- To age out old LSAs by removing an LSA when it reaches 3600 seconds
Explanation: LSAs are deemed invalid when they reach 3600 seconds and are purged from the LSDB.
5. Which LSA type exists in all OSPF areas?
- Network
- Summary
- Router
- AS external
Explanation: A router LSA (type 1) is associated with each OSPF-enabled interface.
6. True or false: When an ABR receives a network LSA, the ABR forwards the network LSA to the other connected areas.
- True
- False
Explanation: Network LSAs (type 2) are not advertised outside the originating area. They are used with router LSAs (type 1) to build the summary LSA (type 3).
7. When a type 3 LSA is received in a nonbackbone area, what does the ABR do?
- Discards the type 3 LSA and does not process it
- Installs the type 3 LSA for only the area where it was received
- Advertises the type 3 LSA to the backbone area and displays an error
- Advertises the type 3 LSA to the backbone area
Explanation: Type 3 LSAs received from a nonbackbone area only insert into the LSDB for the source area. ABRs do not create type 3 LSAs for the other areas.
8. True or false: OSPF uses the shortest total path metric to identify the best path for every internal OSPF route (intra-area and interarea).
- True
- False
Explanation: OSPF prefers intra-area routes over interarea routes as the first logic check. In the event that both paths use the same type, the total path metric is used.
9. True or false: Breaking a large OSPF topology into smaller OSPF areas can be considered a form of summarization.
- True
- False
Explanation: While the number of network prefixes might remain the same, the numbers of type 1 and type 2 LSAs are reduced.
10. How is the process of summarizing routes on an OSPF router accomplished?
- By using the interface configuration command summary-address network prefix-length
- By using the OSPF process configuration command summary-address network prefix-length
- By using the OSPF process configuration command area area-id range network subnet-mask
- By using the interface configuration command area area-id summary-address network subnet-mask
Explanation: OSPF summarization occurs at the area level and is configured under the OSPF process.
11. OSPF supports filtering of routes using which of the following techniques? (Choose two.)
- Summarization, using the no-advertise option
- LSA filtering, which prevents type 1 LSAs from being advertised through a member router
- Area filtering, which prevents type 1 LSAs from being generated into a type 3 LSA
- Injection of an OSPF discard route on the router that filtering should apply
Explanation: LSA filtering occurs on the ABR and can occur with summarization (using the no-advertise keyword) or with area filtering (preventing the Type 3 LSAs from entering into the new area).