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CCNA640-802题库解析–OSPF协议

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本文主要结合例题讲解OSPF协议的概念，定义以及配置，检错。

The OSPF Hello protocol performs which of the following tasks? (Choose two.)

A:It provides dynamic neighbor discovery.

B:It detects unreachable neighbors in 90 second intervals.

C:It maintains neighbor relationships.

D:It negotiates correctness parameters between neighboring interfaces.

E:It uses timers to elect the router with the fastest links as the designated router.

F:It broadcasts hello packets throughout the internetwork to discover all routers that are running OSPF.

Correct Answers:A, C

HELLO协议在OSPF网络中的功能：

1.邻居动态发现(neighbor discovery)

2.邻居关系维持(neighbor keepalive)

A routing protocol is required that supports:

1) routing updateauthentication

2) an addressing scheme that conserves IP addresses

3) multiple vendors

4) anetworkwith over 50 routers

Which routing protocol fulfills these requirements?

A:RIPv1

B:RIPv2

C:EIGRP

D: OSPF

Correct Answers:D

首先RIPv1是不支持认证的，因此答案A我们可以排除。是支持多厂商的，因此是一个开放的标准的协议，可以排除EIGPR，因为这是个CISCO私有的协议。网络的大小可以超过50台路由器，那RIPv2就不满足了，因为RIP最大支持16跳。那么答案就是OSPF了。

On point-to-point networks, OSPF hello packets are addressed to which address?

A: 127.0.0.1

B: 172.16.0.1

C: 192.168.0.5

D: 223.0.0.1

E: 224.0.0.5

F: 254.255.255.255

Correct Answers:E

在OSPF中Hello包发向的是224.0.0.5和224.0.0.6这两个地址的。大家在做OSPF实验的时候，用debug命令是可以看到这两个个地址的。

Which of the following describe the process identifier that is used to run OSPF on a router? (Choose two.)

A:It is locally significant.

B:It is globally significant.

C:It is needed to identify a unique instance of an OSPF database.

D:It is an optional parameter required only if multiple OSPF processes are running on the router.

E:All routers in the same OSPF area must have the same process ID if they are to exchange routing information.

Correct Answers:A, C

OSPF的进程号只在本地有效。在一台路由器上需要为每个进程维护各自的OSPF数据库。

Refer to the exhibit. Why are two OSPF designated routers identified on Core_Router?

A: Core_Router is connected to more than one multiaccess network.

B: The router at 208.149.23.130 is a secondary DR in case the primary fails.

C: Two router IDs have the same OSPF priority and are therefore tied for DR election.

D: The DR election is still underway and there are two contenders for the role.

Correct Answers:A

我们看上图显示Core_Router有多个邻居，进行了DR/BDR选举，它连接的是多路访问网络。

A network administrator is troubleshooting the OSPF configuration of routers R1 and R2. The routers cannot establish an adjacency relationship on their common Ethernet link. The graphic shows the output of the show ip ospf interface e0 command for routers R1 and R2. Based on the information in the graphic, what is the cause of this problem?

A: The OSPF area is not configured properly.

B: The priority on R1 should be set higher.

C: The cost on R1 should be set higher.

D: The hello and dead timers are not configured properly.

E: A backup designated router needs to be added to the network.

F: The OSPF process ID numbers must match.

Correct Answers:D

OSPF邻居建立过程中有几个参数是需要严格匹配的：Hello time和dead time，Area id, MTU,特殊区域标识符。然后比较图中的R1和R2的内容可以看到他们的Hello time和dead time是不同的，因此他们的邻居关系是无法建立的。

Refer to the exhibit. Assume that all router interfaces are operational and correctly configured. In addition, assume that OSPF has been correctly configured on router R2. How will the defaultrouteconfigured on R1 affect the operation of R2?

A: Any packet destined for a network that is not directly connected to router R1 will be dropped.

B: Any packet destined for a network that is not directly connected to router R2 will be dropped immediately.

C: Any packet destined for a network that is not directly connected to router R2 will be dropped immediately because of the lack of a gateway on R1.

D: The networks directly connected to router R2 will not be able to communicate with the 172.16.100.0, 172.16.100.128, and 172.16.100.64 subnetworks.

E: Any packet destined for a network that is not referenced in the routing table of router R2 will be directed to R1. R1 will then send that packet back to R2 and a routing loop will occur.

Correct Answers:E

在R1上产生了一个OSPF的缺省路由，出接口指定为S0/0，这条缺省路由以5类LSA的形式通告给了R2，于是R2上也有了一条标记为O*E2 0.0.0.0/0出接口为Serial0/0的路由。所以R2收到任何路由表中没有的目的网段时，就将指定给R1，而R1根据缺省路由的出接口又将数据包发往R2，这样就形成了一个路由的环路。

Refer to the exhibit. A network associate has configured OSPF with the command:

City(config-router)# network 192.168.12.64 0.0.0.63 area 0

After completing the configuration, the associate discovers that not all the interfaces are participating in OSPF. Which three of the interfaces shown in the exhibit will participate in OSPF according to this configuration statement? (Choose three.)