Computer Network Homework3’ s hard question

1. Which kind of protocol does CSMA belong to?

A. Random Access Protocol

B. Take Turns Protocol

C. Centralized Protocol

(2 points)

Answer:

A

6. Which provides connection-oriented service?

A. LLC1

B. LLC2

C. LLC3

(2 points)

Answer:

B

17. A host connected by hub works on which method?

A. full duplex

B. half duplex

C. simplex

(2 points)

Answer:

B

Explanation:

集线器(hub)用电子线路模拟总线方式，每次只允许一个站点发送。

18. In an Ethernet, four stations have one longest frame (1520 bytes) to send when the channel is busy. When all these frames are sent successfully, how many collisions will happen at least?

A. 1

B. 2

C. 3

D. 4

(2 points)

Answer:

C

Explanation:

第一次是在当前站点发送完毕后四个站点同时发送会产生冲突。然后，一个选择0t，另三个选择1t，结果冲突一次，然后这三个冲突站点分别选择从四个时间长度选三个，无论如何还会冲突一次，因为如果无冲突，则有一个站点先发送（帧很长），另外两个站点都会检测到信道忙，结果信道一空闲，它们立即发送，造成冲突。

19. Which of the following Ethernet LAN has most collision domains?

A. all the hosts are connected to one switch by UTP cables

B. all the hosts are connected to one hub by UTP cables

C. all the hosts are connected by thin coax (10base-2)

(2 points)

Answer:

A

Explanation:

交换机每个接口一个形成冲突域。不过只有在这个接口接hub时才会有冲突发生，接口全双工工作时不会冲突。

20. Which field does not belong to an Ethernet Frame?

A. destination MAC address

B. source MAC address

C. type

D. frame check sequence

E. control

(2 points)

Answer:

E

DIXv2  -- type >=1536

802.3(original)  -- length

802.3 (1997 edition) -- type/length

21. What is the maximum length of the data field of an Ethernet Frame?

A. 46

B. 512

C. 1000

D. 1500

(2 points)

Answer:

D

23. When is a SNAP frame required in Ethernet?

A. The header of an Ethernet frame lacks length.

B. The header of an Ethernet frame lacks premeable.

C. The header of an Ethernet frame lacks source address.

D. The length of the playload field of an Ethernet frame is less than 46 bytes.

Answer:

D

Explanation:

使用长度字段主要是当Data中有填充字节时确定Data中payload的长度。

24. The Ethernet (802.3), fast Ethernet (802.3u) and Gigabit Ethernet standard are different mainly in the layer of OSI.

A. Physical

B. data link

C. network

D. transport

(2 points)

Answer:

A

Explanation:

10M/100M/1000Mbps以太网主要不同点是在物理层：物理介质不同、调制和编码方法不同、速率不同。

29. In spanning tree algorithm，how many bridges will be designated for a LAN segment?

A. 1

B. 2

C. 3

D. varied

(2 points)

Answer:

A

33. In spanning tree algorithm，which port will be selected as the root port for a bridge if two shortest paths from the bridge to the root bridge have the same neighboring bridge on the paths?

A. the port of the bridge with the highest port ID

B. the port of the bridge with the lowest port ID

C. the port of the neighboring bridge with the highest port ID

D. the port of the neighboring bridge with the lowest port ID

(2 points)

Answer:

D

Explanation:

不采用自己的端口号而采用邻居的端口号打破平衡。

35. In spanning tree algorithm，a data frame cannot be forwarded between which port pair of a bridge?

A. the designated port and root port

B. the designated port and the designated port

C. the blocked port and the root port

(2 points)

Answer:

C

37. What is one benefit of PVST+?

A. PVST+ supports Layer 3 load balancing without loops.

B. PVST+ reduces the CPU cycles for all the switches in the network.

C. PVST+ allows the root switch location to be optimized per VLAN.

D. PVST+ automatically selects the root bridge location, to provide optimized bandwidth usage.

(2 points)

Answer:

C

38. If a switch (802.1Q) receives a frame without VLAN ID tagged from trunk port, what will it do?

A. regard it as default VLAN

B. regard it as native VLAN

C. drop it

(2 points)

Answer:

B

40. Which of the following VLANs is static?

A. based on MAC address

B. based on subnets

C. based on network protocol

D. based on ports

(2 points)

Answer:

D

42. Which of the following spanning trees is most flexible for load balancing and computational complex?

A. PVST

B. CST

C. MST

(2 points)

Answer:

C

Explanation:

MST为多生成树算法，可以设置生成树的个数和每个VLAN采用哪颗生成树。

46. How does a switch differ from a hub?

A. A switch does not induce any latency into the frame transfer time.

B. A switch tracks MAC addresses of directly-connected devices.

C. A switch operates at a lower, more efficient layer of the OSI model.

D. A switch decreases the number of broadcast domains.

E. A switch decreases the number of collision domains.

(2 points)

Answer:

B

Explanation:

使用交换机会增加冲突域的数量

48. If host A sends an IP packet to host B, what will the source physical address be in the frame when it reaches host B?

The connections are: [host A]--[Router R1]--[Router R2]--[host B]

A. host A's MAC Address

B. host B's MAC Address

C. R1's MAC Address

D. R2's MAC Address

Answer:

D

Explanation:

在这里，每对路由器和主机之间采用一种数据链层协议，通过每一跳的源MAC地址和目的MAC都不同。IP分组被封装在帧中，通过最后一段数据链路层协议时，源MAC为路由器R2的，目的MAC地址为主机B的。

8. How many collision domains are there in the following Ethernet?  How many broadcast domains?

Answer:

7,1

Explanation:

Switch每个端口一个冲突域（只有接hub时会冲突）。集线器所有端口位于同一个冲突域。一个广播帧可以到达的地方属于同一个广播域。

12. In the following figure, S1~S3 are switches, A~E are hosts,which of following statements will be true? (select two)

A. If host A sends a frame to host D, D will receive it.

B. If host B sends a frame to host D, D will receive it.

C. If host D sends a frame to host A, A will receive it.

D. If host D sends a frame to host B, B will receive it.

E. If host E sends a frame to host A, A will receive it.

F. If host E sends a frame to host B, B will receive it.

G. If host A sends a frame to host E, E will receive it.

Answer:
EG