第54题： 以下关于三层交换机的叙述中，正确的是(   )。
A.三层交换机包括二层交换和三层转发，二层交换由硬件实现，三层转发采用软件实现
B.三层交换机仅实现三层转发功能
C.通常路由器用在单位内部，三层交换机放置在出口
D.三层交换机除了存储转发外，还可以采用直通交换技术
答案解析与讨论：www.cnitpm.com/st/404761213.html

第55题： IP数据报首部中IHL (Internet首部长度)字段的最小值为(   )。
A.5
B.20
C.32
D.128
答案解析与讨论：www.cnitpm.com/st/4047728459.html

第56题： 查看OSPF接口的开销、状态、类型、优先级等的命令是( 1 )；查看OSPF在接收报文时出错记录的命令是( 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
答案解析与讨论：www.cnitpm.com/st/4047919870.html

第58题： 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

A.①②③④
B.①②④
C.②③
D.①③④
答案解析与讨论：www.cnitpm.com/st/4048529892.html

第59题： 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
答案解析与讨论：www.cnitpm.com/st/4048627556.html