1. What is a function of OSPF hello packets?
- to discover neighbors and build adjacencies between them
- to ensure database synchronization between routers
- to send specifically requested link-state records
- to request specific link-state records from neighbor routers
Explanation: The OSPF hello packet serves three primary functions: discover OSPF neighbors and establish adjacencies, advertise parameters that OSPF neighbors must agree on, and elect the DR and BDR.
2. Which OPSF packet contains the different types of link-state advertisements?
Explanation: Link-state update (LSU) packets contain different types of link-state advertisements (LSAs). The LSUs are used to reply to link-state requests (LSRs) and to announce new information.
3. Which three statements describe features of the OSPF topology table? (Choose three.)
- After convergence, the table only contains the lowest cost route entries for all known networks.
- The topology table contains feasible successor routes.
- The table can be viewed via the show ip ospf database command.
- Its contents are the result of running the SPF algorithm.
- It is a link-state database that represents the network topology.
- When converged, all routers in an area have identical topology tables.
Explanation: The topology table on an OSPF router is a link-state database (LSDB) that lists information about all other routers in the network, and represents the network topology. All routers within an area have identical link-state databases, and the table can be viewed using the show ip ospf database command. The EIGRP topology table contains feasible successor routes. This concept is not used by OSPF. The SPF algorithm uses the LSDB to produce the unique routing table for each router which contains the lowest cost route entries for known networks.
4. What does an OSPF area contain?
- routers that share the same process ID
- routers that share the same router ID
- routers that have the same link-state information in their LSDBs
- routers whose SPF trees are identical
Explanation: An OSPF area contains one set of link-state information, although each router within the area will process that information individually to form its own SPF tree. OSPF process IDs are locally significant and are created by the administrator. Router IDs uniquely identify each router.
5. A router is participating in an OSPFv2 domain. What will always happen if the dead interval expires before the router receives a hello packet from an adjacent DROTHER OSPF router?
- A new dead interval timer of 4 times the hello interval will start.
- SPF will run and determine which neighbor router is “down”.
- OSPF will run a new DR/BDR election.
- OSPF will remove that neighbor from the router link-state database.
Explanation: On Cisco routers the default dead interval is 4 times the hello interval, and this timer has expired in this case. SPF does not determine the state of neighbor routers; it determines which routes become routing table entries. A DR/DBR election will not always automatically run; this depends on the type of network and on whether or not the router no longer up was a DR or BDR.
6. What is the order of packet types used by an OSPF router to establish convergence?
- LSAck, Hello, DBD, LSU, LSR
- LSU, LSAck, Hello, DBD, LSR
- Hello, DBD, LSR, LSU, LSAck
- Hello, LSAck, LSU, LSR, DBD
An OSPF router progresses in this order to convergence, using the following packets:
- Hello packet, used for OSPF election and establishing neighbor adjacencies
- DBD packet, used to synchronize databases with neighbors
- LSR packet, used to request more information in synchronizing databases
- LSU packet, used to send link-state updates to neighbors
- LSAck packet, used to acknowledge receipt of an LSU
7. What is a feature of the OSPF routing protocol?
- OSPF authentication is configured in the same way on IPv4 and IPv6 networks.
- Routers can be grouped into autonomous systems to support a hierarchical system.
- It scales well in both small and large networks.
- The SPF algorithm chooses the best path based on 30-second updates.
Explanation: OSPF uses the SPF algorithm to choose the best path. Routing changes trigger routing updates (no 30-second updates). In IPv4, OSPF uses MD5 authentication between two neighboring OSPF routers. In IPv6, OSPFv3 does not include any authentication capabilities of its own. Instead it relies entirely on IPsec to secure communications between neighbors. Routers can be grouped into areas to support a hierarchical system.
8. What is used to create the OSPF neighbor table?
- link-state database
- forwarding database
- routing table
- adjacency database
Explanation: The adjacency database is used to create the OSPF neighbor table. The link-state database is used to create the topology table, and the forwarding database is used to create the routing table.
9. What is identical on all OSPF routers within a single area?
- neighbor table
- routing table
- link-state database
- static routes
Explanation: When the LSP flooding process completes, all OSPF routers will learn the same link-state information in the routing area. This information is used to build a complete link-state database, which will be the same on all OSPF routers within that specific area.
10. What function is performed by the OSPF designated router?
- summarizing routes between areas
- maintaining the link-state database
- redistribution of external routes into OSPF
- dissemination of LSAs
Explanation: OSPF designated routers are elected on multiaccess networks to disseminate LSAs to other OSPF routers. By having a single router disseminate LSAs, the exchanging of LSAs is more efficient.
11. What are two reasons for creating an OSPF network with multiple areas? (Choose two.)
- to simplify configuration
- to ensure that an area is used to connect the network to the Internet
- to reduce use of memory and processor resources
- to reduce SPF calculations
- to provide areas in the network for routers that are not running OSPF
Explanation: If a router is not running OSPF, it is not configurable with an OSPF area. OSPF areas have no direct relationship with the Internet. Routers that run OSPF can connect to the Internet, but multiple OSPF areas are not required for this purpose. OSPF areas help to decrease the demand for router memory and processing power by limiting OSPF protocol traffic, keeping link-state databases small, and requiring fewer SPF recalculations. Multiarea OSPF requires additional steps to configure and therefore does not simplify the configuration process.
12. At which OSPF state are neighbor routers converged and able to exchange routing updates?
Explanation: OSPF neighbors that reach the Full state are converged and can exchange routing information.
13. The OSPF hello timer has been set to 15 seconds on a router in a point-to-point network. By default, what is the dead interval on this router?
- 30 seconds
- 60 seconds
- 45 seconds
- 15 seconds
Explanation: By default, the dead interval is calculated as 4 times the hello interval.
14. What happens immediately after two OSPF routers have exchanged hello packets and have formed a neighbor adjacency?
- They exchange abbreviated lists of their LSDBs.
- They negotiate the election process if they are on a multiaccess network.
- They exchange DBD packets in order to advertise parameters such as hello and dead intervals.
- They request more information about their databases.
Explanation: During the exchange of hello packets, OSPF routers negotiate the election process and set the OSPF parameters. DBD packets are exchanged after that step has been completed. DBD packets contain abbreviated lists of link-state information. After that information has been exchanged, OSPF routers exchange Type 3 LSR packets to request further information.
15. Which statement is correct about multiarea OSPF?
- OSPF can consolidate a fragmented OSPF area into one large area.
- Arranging routers into areas partitions a large autonomous system in order to lighten the load on routers.
- All routers are in one area called the backbone area (area 0).
- OSPF multiarea increases the frequency of SPF calculation.
Explanation: A company with one large autonomous system or AS can be divided into smaller areas. When this occurs and the OSPF routing protocol is implemented, the design is called multi-area OSPF. Multi-area OSPF decreases the frequency of the SPF calculation, thus lightening the load on the router. In a single area OSPF design, all the routers are located in area 0 or the backbone area.