1. Which characteristic describes both ABRs and ASBRs that are implemented in a multiarea OSPF network?
- They usually have many local networks attached.
- They both run multiple routing protocols simultaneously.
- They are required to perform any summarization or redistribution tasks.
- They are required to reload frequently and quickly in order to update the LSDB.
Explanation: ABRs and ASBRs need to perform any summarization or redistribution among multiple areas, and thus demand more router resources than a regular router in an OSPF area.
2. Which is an OSPF feature that can provide consolidation of type 3 LSAs?
- automatic summarization
- manual route summarization
- DR/BDR elections
- autonomous system boundary routers
Explanation: OSPF route summarization consolidates multiple type 3 LSAs into a single OSPF network to be shared to other routers. DR/BDR elections are used to provide a smaller number of LSAs between neighbors. An autonomous system boundary router (ASBR) is configured to provide external network access to an OSPF topology. OSPF does not support automatic summarization.
3. Which type of area best describes a stub area?
- a small area
- an area with only one router
- an area with no externally learned routes
- an area that is discontiguous from the backbone area
Explanation: Stub areas reduce the size of the routing table by preventing external routes (type 5 LSAs) and ASBR summary routes (type 4 LSAs) from entering the area. The ABR automatically generates a default route for all the routers in the stub area so each router can reach other areas. The ABR has the external and summary routes in its own routing table.
4. Refer to the exhibit. On which router or routers should OSPF summarization be configured to summarize the two Ethernet LANs in Area 2?
- R0-A
- R0-C
- R2-A
- R0-C and R2-A
Explanation: OSPF summarization is configured on Area Border Routers (ABRs) that connect two areas. Because OSPF Area 2 is to be summarized, RO-C is the router where the configuration must occur.
5. Which command can be used to verify the contents of the LSDB in an OSPF area?
- show ip ospf database
- show ip ospf interface
- show ip ospf neighbor
- show ip route ospf
Explanation: The show ip ospf database command is used to verify the contents of the LSDB. The show ip ospf interface command is used to verify the configuration information of OSPF-enabled interfaces. The show ip ospf neighbor command is used to gather information regarding OSPF neighbor routers. The show ip routeospfcommand will display OSPF-related information in the routing table.
6. An OSPF enabled router is processing learned routes to select best paths to reach a destination network. What is the OSPF algorithm evaluating as the metric?
- The amount of packet delivery time and slowest bandwidth.
- The number of hops along the routing path.
- The amount of traffic and probability of failure of links.
- The cumulative bandwidth that is used along the routing path.
Explanation: The OSPF routing protocol uses the cumulative bandwidth as the metric for route computations.
7. What information is needed when configuring an OSPF virtual link between two Layer 3 devices?
- the OSPF process ID of the remote router
- the IP address of the remote router
- the router ID of the remote router
- the router name of the Area 0 DR
Explanation: The command to create an OSPF virtual link is area area-id virtual-linkendpoint-RID. The endpoint-RIDpart of the command refers to the router ID on the remote router.
8. Refer to the exhibit. While configuring a multiarea OSPF topology, a network administrator implements the redistribute eigrp 100 subnets command. Which type of OSPF router is being configured?
- ASBR
- internal
- internal backbone
- ABR
Explanation: An area border router (ABR) is configured to connect to multiple areas of an OSPF topology. An autonomous system boundary router (ASBR) uses redistribution to share information about the OSPF topology with external networks. Internal routers are configured in a single area and internal backbone routers are configured only in area 0.
9. Which type of OSPF area makes use of LSA type 7s?
- backbone
- stub
- not-so-stubby
- totally stubby
Explanation: When an OSPF not-so-stubby area (NSSA) is implemented, external routes are allowed to travel through a stub area that has other external routes prohibited. The way this is accomplished is through the use of type 7 LSAs which are used to carry the external routes across the stub area, but still keep other type 5 external routes out of the area. When the type 7 LSAs reach the ABR for the stub area, the ABR changes these LSAs to type 5s for transmission through the rest of the OSPF areas that allow type 5 LSAs.
10. What is the correct order of statements for calculating the best path in an OSPF network?
- A, B, C
- B, C, A
- B, A, C
- C, A, B
Explanation: Routers first calculate the best path to destinations within their own area, then to other areas within the internetwork, then to destinations external to the network.
11. Which type of OSPF LSA represents an external route and is propagated across the entire OSPF domain?
- type 1
- type 2
- type 3
- type 4
- type 5
Explanation: Type 5 LSAs are used for external routes and are generated by an ASBR. They are forwarded into other areas by ABRs.
12. An OSPF ABR router summarizes all internal area 1 routes into one OSPF interarea summary route. How does this type of route get inserted into the routing table?
- The router injects into the backbone a single type 5 LSA that describes the summary route.
- Cisco IOS automatically creates a summary route on the router to the Null0 interface in order to prevent routing loops.
- This route summarization was configured via the summary-address address mask router configuration mode command.
- An entry with a router designator OIA will be inserted into the routing table.
Explanation: A summary route to the Null0 interface is created automatically by the Cisco IOS to prevent routing loops when manual summarization is configured. Any packet that is sent to the Null0 interface is dropped. The ABR will summarize all the internal routes for one area before injecting them into another area.
“Do I Know This Already?” Quiz Answers:
1. How many OSPF link-state advertisements (LSAs) are used for routing traditional IPv4 packets?
- Two
- Three
- Five
- Six
- Seven
Explanation: OSPF uses six OSPF types for routing IPv4 packets (Types 1, 2, 3, 4, 5, and 7). Additional LSAs exist for IPv6 and MPLS.
2. What is the LSA Age field in the LSDB used 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, at which point they are purged from the LSDB.
3. Which LSA type exists in all OSPF areas?
- Network
- Summary
- Router
- AS external
Explanation: A router LSA (Type 1) is associated with every OSPF-enabled interface.
4. 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 summary LSAs (Type 3).
5. An OSPF stub area blocks which types of LSAs from being injected into the area by the ABR? (Choose two.)
- Type 1 LSA
- Type 3 LSA
- Type 4 LSA
- Type 5 LSA
Explanation: An OSPF area blocks the injection of Type 5 LSAs into an area and inserts a default route instead. Because Type 5 LSAs are blocked, there is not a need for a Type 4 LSA, so those are blocked as well.
6. True or false: An OSPF not-so-stubby area (NSSA) automatically creates a default route when a Type 5 LSA is blocked on the ABR from being injected into the NSSA.
- True
- False
Explanation: A default route is considered optional in the OSPF NSSA configuration.
7. OSPF automatically assigns a link cost to an interface based on what reference bandwidth?
- 100 Mbps
- 1 Gbps
- 10 Gbps
- 40 Gbps
Explanation: IOS XE uses a reference bandwidth of 100 Mbps for dynamic metric assignment to an interface.
8. True or false: If two different routers are redistributing the same network (such as 10.1.1.0/24) as an OSPF external Type 2 route, and they have the same metric, both paths are installed on a downstream router.
- True
- False
Explanation: OSPF inserts an additional check for the forwarding cost (which consists of the path metric to reach the ASBR). If the forwarding cost is the same, then both paths are installed. If the costs are different, then the path with the lowest forwarding metric is installed.
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. Summarizing external OSPF routes on is accomplished by using the _____.
- interface configuration command summary-address network prefix-length
- OSPF process configuration command summary-address network subnet-mask
- OSPF process configuration command area area-id range network subnet-mask
- interface configuration command area area-id summary-address network subnet-mask
Explanation: You configure OSPF route summarization on the ASBR with the OSPF process configuration command summary-address network subnet-mask.
11. When a Type 3 LSA is received on 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 for which 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 are inserted into the LSDB only for the source area. ABRs do not create Type 3 LSAs for the other areas.
12. True or false: Virtual link is another term for an OSPF-enabled GRE tunnel.
- True
- False
Explanation: A virtual link is an OSPF mechanism that connects an ABR and another multi-area router to extend the backbone and enable proper LSA propagation.