第55題： IP數(shù)據(jù)報首部中IHL (Internet首部長度)字段的最小值為(   )。
A.5
B.20
C.32
D.128
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第56題： 查看OSPF接口的開銷、狀態(tài)、類型、優(yōu)先級等的命令是( 1 )；查看OSPF在接收報文時出錯記錄的命令是( 2 )。
（1）A.display ospf
B.display ospf error
C.display ospf interface
D.display ospf neighbor
（2）A.display ospf
B.display ospf error
C.display ospf interface
D.display ospf neighbor
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第57題： 如圖所示，Switch A通過Switch B和NMS跨網(wǎng)段相連并正常通信。SwitchA 與Switch B配置相似，從給出的Switch A的配置文件可知該配置實現(xiàn)的是( 1 )，驗證配置結(jié)果的命令是( 2 )。

（1）A．實現(xiàn)毫秒級鏈路故障感知并刷新路由表
B.能夠感知鏈路故障并進行鏈路切換
C.將感知到的鏈路故障通知NMS
D.自動關(guān)閉故障鏈路接口并刷新路由表
（2）A.display nqa results
B.display bfd session all
C.display efm session all
D.display current-configuration|include nqa
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第59題： The TTL field was originally designed to hold a time stamp, which was decremented by each visited router. The datagram was ( 1 ) when the value became zero. However, for this scheme, all the machines must have synchronized clocks and must know how long it takes for a datagram to go from one machine to another. Today, this field is used mostly to control the ( 2 ) number of hops (routers) visited by the datagram. When a source host sends the datagram, it ( 3 ) a number in this field. Each router that processes the datagram decrements this number by 1. If this value, after being decremented, is zero, the router discards the datagram.This field is needed because routing tables in the Internet can become corrupted.A datagram may travel between two or more routers for a long time without ever getting delivered to the ( 4 ).This field limits the ( 5 ) of a datagram.
（1）A.received
B.discarded
C.rejected
D.tansferred
（2）A.maximum
B.minimum
C.exact
D.certain
（3）A.controls
B.transmits
C.stores
D.receives
（4）A.switch
B.router
C.source host
D.destination host
（5）A.lifetime
B.moving time
C.receiving time
D.transmitting time
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