CCNA640-802题库解析–Rip协议

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本文主要结合例题讲解RIP协议的定义，配置，检查和排错。
Refer to the exhibit. Thenetworkshown in the exhibit is running theRIPv2routing protocol. The network has converged, and the routers in this network are functioning properly. The FastEthernet0/0 interface on R1 goes down. In which two ways will the routers in this network respond to this change? (Choose two.)
A:All routers will reference their topology database to determine if any backuproutesto the 192.168.1.0 network are known.
B:Routers R2 and R3 mark the route as inaccessible and will not accept any further routing updates from R1 until their hold-down timers expire.
C:Because of the split-horizon rule, router R2 will be prevented from sending erroneous information to R1 about connectivity to the 192.168.1.0 network.
D:When router R2 learns from R1 that the link to the 192.168.1.0 network has been lost, R2 will respond by sending a route back to R1 with an infinite metric to the 192.168.1.0 network.
E:R1 will send LSAs to R2 and R3 informing them of this change, and then all routers will send periodic updates at an increased rate until the network again converges.
Correct Answers:C, D
这涉及到RIP关于环路避免的几种机制了。在这里R1的直连的链路发生了变化，立即触发更新（触发更新），发送flash update出去，将这个条目置为possible down，设置最大跳数（路由毒性），R2收到这个flash update后，也回复一个flash update包（毒性逆转），同时将这个条目也置为possible down，设置最大跳数。
根据split-horizon rule，R2不会向R1发送关于网络192.168.1.0的任何信息，并且当R2收到来自R1关于网段192.168.1.0 down的信息后，R2会反馈一条“192.168.1.0” 被计数到无穷大的信息给R1，以抑制住该故障路由扩散。
Which of the following are true regarding the command output shown in the display? (Choose two.)
A:There are at least two routers participating in the RIP process.
B:A ping to 192.168.168.2 will be successful.
C:A ping to 10.0.15.2 will be successful.
D:RtrA has three interfaces participating in the RIP process.
Correct Answers:A, C
因为RIP的metric是基于跳数的，在debug ip rip中RtrA发出的路由192.168.1.0的metric为2,所以在RIP进程下至少有两台路由器；而10.0.0.0 metric 1表示这个路由是自己直连的，显示收到来自10.0.15.2 on serical0/0的rip更新，所以Aping 10.0.15.2是能成功的。
Refer to the exhibit. What can be determined about routes that are learned from the router at IP address 190.171.23.12?
A: HQ_Router last received an update from 190.171.23.12 at 3:30 am.
B: If HQ_Router does not receive an update from 190.171.23.12 in 30 seconds, all routes from that source will be removed from the routing table.
C: If HQ_Router does not receive an update from 190.171.23.12 in 30 seconds, all routes from that source will be flagged with a hold-down timer.
D: 190.171.23.12 is expected to send an update to HQ_Router for network 190.172.0.0 in 3 minutes and 30 seconds.
Correct Answers:B
Routing information sources:
GatewayDistanceLast Update
190.171.23.1212000:03:30
在三条路由条目中进行比较，可以发现190.171.23.12这条路由一直未被更新，而RIP协议每隔30秒就会以广播的形式在网络中发送更新报文，该路由是一条故障路由。RIP对于故障路由的处理从图中可以看出：
sending updates every 30 seconds,
Invalid after 180 seconds,
hold down 180
flushed after 240
190.171.23.12这条路由已经在routing-table中存在3分30秒未得到更新，也就是210秒。根据flushed timer=240，可以推算出，在240-210=30秒后，该路由将会被移除。
Refer to the exhibit. Explain how the routes in the table are being affected by the status change on interface Ethernet0。
A: The router is requesting updates for these networks from routers that are connected to interface Ethernet1.
B: The router is poisoning the routes and broadcasting the new path costs via interface Ethernet1.
C: The router is receiving updates about unreachable networks from routers that are connected to interface Ethernet1.
D: The router is poisoning the routes and multicasting the new path costs via interface Ethernet1.
Correct Answers:D
当Ethernet0的链路down掉后，该路由器上的毒性路由就都从路由表中删除了，因为有接口出现故障了，所以选择路径也会发生变化，因此发送给Ethernet0口的路由的度量值也会相应地发生变化了。
Refer to the exhibit. After a RIP route is marked invalid on Router_1, how much time will elapse before that route is removed from the routing table?
A: 30 seconds
B: 60 seconds
C: 90 seconds
D: 180 seconds
E: 240 seconds
Correct Answers:B
根据“After a RIP route is marked invalid on Router_1”条件可以得知，该rip route处于hold down状态。默认情况下，路由处于hold down，需要等待60s后才会从路有表中被移除。（240-180=60）