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The TTL field was originally designed to hold a time stamp, which was decremented by each visited router. The datagram was (71 ) 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 ( 72) number of hops (routers) visited by the datagram. When a source host sends the datagram, it (73 ) 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 (74 ).This field limits the (75 ) of a datagram.

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如图所示，Switch A 通过 Switch B 和 NMS 跨网段相连并正常通信。SwitchA 与 Switch B 配 置相似，从给出的 Switch A 的配置文件可知该配置实现的是( )，验证配置结果的命令是( )。　　　　(68)

如下图所示，使用基本 ACL 限制 FTP 访间权限，从给出的 Switch 的配置文件判断可以实现 的策略是( )。　　①PC1 在任何时间都可以访问 FTP ②PC2 在 2018 年的周一不能访问 FTP ③PC2 在 2018 年的周六下午 3 点可以访问 FTP ④PC3 在任何时间不能访问 FTP　　(70)

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查看 OSPF 接口的开销、状态、类型、优先级等的命令是(66);查看 OSPF 在接收报文时出错 记录的命令是(67 )　　(66)