[计算机类试卷]思科认证640-802模拟试卷4及答案与解析.doc

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1、思科认证 640-802模拟试卷 4及答案与解析 1 The corporate head office of Ezonexam has a teleconferencing system that uses VOIP (voice over IP) technology. This system uses UDP as the transport for the data transmissions. If these UDP datagrams arrive at their destination out of sequence, what will happen? （ A） UDP w

2、ill send an ICMP Information Request to the source host. （ B） UDP will pass the information in the datagrams up to the next OSI layer in the order that they arrive. （ C） UDP will drop the datagrams. （ D） UDP will use the sequence numbers in the datagram headers to reassemble the data in the correct

3、order. （ E） UDP will not acknowledge the datagrams and wait for a retransmission of the datagrams. 2 The Ezonexam network administrator has just issued the “switchport trunk native vlan 998“ command on switch VE2. What is the function of this command? （ A） It designates VLAN 998 as the default for a

4、ll unknown tagged traffic （ B） It designates VLAN 998 for untagged traffic （ C） It blocks VLAN 998 traffic from passing on the trunk （ D） It creates a VLAN 998 interface （ E） None of the above 3 An administrator issues the show ip interface s0/0 command and the output displays that interface Serial0

5、/0 is up, line protocol is up. What does “line protocol is up“ specifically indicate about the interface? （ A） The cable is attached properly. （ B） CDP has discovered the connected device. （ C） Keepalives are being received on the interface. （ D） A carrier detect signal has been received from the co

6、nnected device. （ E） IP is correctly configured on the interface. 4 You have just installed a new web server on the Ezonexam network. You are required to ensure that the web server is accessible from the Internet. The network uses private addressing, so an IP-to-registered address mapping is require

7、d. To do this, you enter the following command: Ezonexam(config)# ip nat inside source static 192.168.2.1 198.18.1.254 You unsuccessfully try to ping the Internet from a PC host on the LAN. During the troubleshooting process, you enter the “show ip nat translations“ command but the output is blank.

8、What is the most likely cause of the problem? （ A） The keyword overload is missing from the command. （ B） The NAT pool must be defined first. （ C） An access list must be defined to create static NAT translations. （ D） The interfaces must be configured for NAT. （ E） None of the above 5 When is a swit

9、ched network that is running the Spanning Tree Protocol considered to be fully converged? （ A） when all switches have the same BID as the root switch （ B） when all switches have received the MAC address of each neighboring switch （ C） when every enabled switch port has been assigned a unique identif

10、ier （ D） when every operating switch port is in either the blocking or forwarding state （ E） when all switches are in either client, server, or transparent mode 6 What is the maximum number of IP addresses that can be assigned to hosts on a Ezonexam subnet that uses the 255.255.255.224 subnet mask?

11、A. 14 B. 15 C. 16 D. 30 E. 31 F. 32 7 Which of the following statements correctly describe the differences between half-duplex and full-duplex Ethernet? (Select two answer choices.) （ A） Full-duplex Ethernet uses CSMA/CD to prevent collisions. （ B） Half-duplex Ethernet uses a loopback circuit to det

12、ect collisions. （ C） A full-duplex Ethernet card allows 20Mbps for data transmission. （ D） Full-duplex Ethernet makes use of two pairs of wires for data. （ E） An Ethernet hub can operate both half and full duplex simultaneously. 8 The Ezonexam network administrator wants to ensure that only a single

13、 web server can connect to pot Fa0/1 on a catalyst switch. The server is plugged into the switchs Fast Eth. 0/1 port and the network administrator is about to bring the server online. What can the administrator do to ensure that only the MAC address of this server is allowed by switch port Fa0/1? (C

14、hoose two) （ A） Configure port Fa0/1 to accept connections only from the static IP address of the server （ B） Configure the MAC address of the server as a static entry associated with port Fa0/1 （ C） Employ a proprietary connector type on Fa0/1 that is incomputable with other host connectors （ D） Co

15、nfigure port security on Fa0/1 to reject traffic with a source MAC address other than that of the server （ E） Bind the IP address of the server to its MAC address on the switch to prevent other hosts from spoofing the server IP address 9 The LAN needs are expanding at the Ezonexam corporate office,

16、which is quickly growing. You are instructed to enlarge the area covered by a single LAN segment on the Ezonexam network. Which of the following are layer 1 devices that you can use? (Choose all that apply.) （ A） A switch （ B） A router （ C） A network adapter card （ D） A hub （ E） A repeater 10 Which

17、of the following components must be elected before the Spanning Tree Protocol can converge in a switched LAN? A. Designated ports. B. Duplex operating mode. C. Fast mode ports. D. Root bridge. E. Root ports. F. BDPU priority. 思科认证 640-802模拟试卷 4答案与解析 1 【正确答案】 B 【试题解析】 Explanation: VOIP systems utiliz

18、e UDP because it is faster and uses less overhead. In addition, the reliable transport mechanism used in TCP is useless to VOIP because if a packet gets dropped and needs to be resent, it will be already too late. UDP provides a service for applications to exchange messages. Unlike TCP, UDP is conne

19、ctionless and provides no reliability, no windowing, and no reordering of the received data. However, UDP provides some functions of TCP , such as data transfer, segmentation, and multiplexing using port numbers, and it does so with fewer bytes of overhead and with less processing required. UDP data

20、 transfer differs from TCP data transfer in that no reordering or recovery is accomplished. Applications that use UDP are tolerant of lost data, or they have some application mechanism to recover data loss. 2 【正确答案】 B 3 【正确答案】 C 【试题解析】 Explanation: Serial 0/0 is up, line protocol is up is the proper

21、 status line condition. Line serial0/0 is up: This statement represents that the physical connection is good. Line protocol is up: This represents that the data link layer is OK, meaning that layer 2 keepalives are being received on the interface. When the line protocol goes down, a possible problem

22、 is keep-alives not being sent by remote router. 4 【正确答案】 D 【试题解析】 Explanation: In order to successfully configure a static NAT translation, the interfaces must be configured for NAT, in addition to the global NAT command that was entered. The router interface that lies on the inside part of the net

23、work must be defined using the “ip nat inside“ command. Similarly, the WAN interface that is being used for the Internet connection must be defined using the “ip nat outside“ command. Incorrect Answers: A. In order to make an internal server reachable from the Internet, a static one to one NAT entry

24、 must be configured for the server. The keyword “overload“ is used to configure many to one NAT, or PAT. B, C. This need not be done in order to create a static NAT entry. These steps are typically done in setting up NAT so that inside LAN users can access the Internet via NAT. 5 【正确答案】 D 【试题解析】 Exp

25、lanation: States of the Spanning Tree Protocol: Blocking A blocked port wont forward frames; it just listens to BPDUs. All ports are in blocking state by default when the switch is powered up. The purpose of the blocking state is to prevent the use of looped paths. Listening The port listens to BPDU

26、s to make sure no loops occur on the network before passing data frames. A port in listening state prepares to forward data frames without populating the MAC address table. Learning The switch port listens to BPDUs and learns all the paths in the switched network. A port in learning state populates

27、the MAC address table but doesnt forward data frames. Forwarding The port sends and receives all data frames on the bridged port. Disabled A port in the disabled state does not participate in the frame forwarding or STP. A port in the disabled state is virtually nonoperational. In Blocking States, a

28、ll ports are in blocking state. The purpose of the blocking state is to prevent the use of looped paths. The forward state sends and receives the all data frames in a loop free network. So these two states are considered to be fully converged. 6 【正确答案】 D 【试题解析】 Explanation: The subnet mask 255.255.2

29、55.224 means that there are 27 network bits. The remaining 5 bits are the host bits. The maximum possible combinations with 5 bits are 25 = 32. As all zeros and all ones hosts are not allowed so, maximum number of valid hosts with the mask 255.255.255.224 are 25 -2 =32-2 = 30 Hosts 7 【正确答案】 B,D 【试题解

30、析】 Explanation: Half-duplex Ethernet send and receives on the same line, so a loopback needs to be set to detect collisions. Alternatively, full-duple Ethernet doesnt have to because it uses two pairs of wire, one to send and the other to receive. Collisions are not possible on full duplex Ethernet

31、networks. Incorrect Answers: A. Full duplex uses both pairs of wires, so transmissions are sent on the first pair, and data that is received come in on the other pair. This prevents collisions. C. Full duplex allows for data to be sent and received at the same time. It will not double the amount of

32、bandwidth at any given time. The speed of the Ethernet link will remain at 10/100. E. Hubs are shared devices and can only support one mode, unlike switches. 8 【正确答案】 B,D 【试题解析】 Explanation: You can use port security to block input to an Ethernet, Fast Ethernet, or Gigabit Ethernet port when the MAC

33、 address of the station attempting to access the port is different from any of the MAC addresses specified for that port. When a secure port receives a packet, the source MAC address of the packet is compared to the list of secure source addresses that were manually configured or autoconfigured (lea

34、rned) on the port. If a MAC address of a device attached to the port differs from the list of secure addresses, the port either shuts down permanently (default mode), shuts down for the time you have specified, or drops incoming packets from the insecure host. The ports behavior depends on how you c

35、onfigure it to respond to a security violation. When a security violation occurs, the Link LED for that port turns orange, and a link-down trap is sent to the Simple Network Management Protocol (SNMP) manager. An SNMP trap is not sent if you configure the port for restrictive violation mode. A trap

36、is sent only if you configure the port to shut down during a security violation. 9 【正确答案】 D,E 【试题解析】 Explanation: A hub simply repeats the electrical signal and makes no attempt to interpret the electrical signal (layer 1) as a LAN frame (Layer 2). So, a hub actually performs OSI layer 1 functions,

37、repeating an electrical signal, whereas a switch performs OSI layer 2 functions, actually interpreting Ethernet header information, particularly addresses, to make forwarding decisions. Hubs can be used to increase the number of stations that can be supported on a LAN. Because the repeater does not

38、interpret what the bits mean, but does examine and generate electrical signals, a repeater is considered to operate at Layer 1. Repeaters can be used to physically extend the LAN to greater distances. 10 【正确答案】 A,D,E 【试题解析】 Explanation: Designated port: Either a root port or a port that has been det

39、ermined as having the best (lower) cost-a designated port will be marked as a forwarding port. Root bridge: The root bridge is the bridge with the best bridge ID. With STP, the key is for all the switches in the network to elect a root bridge that becomes the focal point in the network. All other de

40、cisions in the network-like which port is to be blocked and which port is to be put in forwarding mode-are made from the perspective of this root bridge. Root port: Always the link directly connected to the root bridge, or the shortest path to the root bridge. If more than one link connects to the root bridge, then a port cost is determined by checking the bandwidth of each link. The lowest cost port becomes the root port. So these three components must be elected before the spanning tree protocol can converge in a switched LAN