问题 单项选择题

下列关于IEEE 802.1D生成树协议(STP)的描述中,错误的是______。

A.STP是一个数据链路层的管理协议
B.STP运行在网桥和交换机上,通过计算建立一个稳定、无回路的树状结构网络
C.网桥协议数据单元(BPDU)携带有Root ID、Root Path Cost、Bridge ID等信息
D.通知拓扑变化的BPDU长度不超过35B

答案

参考答案:D

解析: 生成树协议(Spaning Tree Protocol,STP)是一个数据链路层的管理协议。其基本工作原理是:通过在交换机之间传递网桥协议数据单元(Bridge Protocol Data Unit,BPDU),并用生成树算法 (Spanning Tree Algorithm,STA)对其进行比较计算。根据BPDU提供的参数和生成树计算,STP首先选定一个根(根网桥),根网桥(Root Bridge)是整个生成树拓扑结构的核心,所有的数据实际上都要通过根网桥。然后确定交换机冗余链路端口的工作状态,让一些端口进入阻塞模式,另一些端口进入转发模式。其中被阻塞的端口仍然是一个激活的端口,但它只能接收和读取BPDU,不能接收和转发数据流。用改变冗余端口的工作状态来阻断网络中的部分冗余路径,使其成为备份链路,以保证在任何两个终端站点之间只存在一条激活的路径,从而避免回路的产生。STP还计算从根到二层网络中所有交换机的最佳路径,并建立一个稳定、无环路的树状结构网络。
无论是根网桥的选定,还是树状结构的生成,主要依靠BPDU提供的信息。BPDU中携带了实现生成;时算法的有关信息,包括Root ID、Root Path Cost、Bridge ID、Port ID、Hello time、Max Age等。BPDU数据包有两种类型,一种是包含配置信息的配置BPDU(长度不超过35B),另一种是包含拓扑变化信息的通知BPDU(长度不超过4B)。

阅读理解

The new automobile fuel economy standards formally adopted by the Obama administration on Thursday will produce a series of benefits: reduced dependence on foreign oil, fewer greenhouse gas emissions(排放), and consumer savings at the pump.

This was truly a moment to celebrate. But it was tempered by the fact that some in Congress are trying to cancel the laws that made the new standards possible.

The standards will require automakers to build passenger cars, sport-utility vehicles and minivans that average 35.5 miles per gallon by 2016 — a 30 percent increase over today’s cars, and the biggest single jump in fuel economy since the original standards were adopted in the 1970s. Cars will cost more, but the government estimates that consumers will save an average of $3,000 in fuel over the life of a new vehicle.

The standards will also place the first-ever limits on automobile greenhouse gas emissions, and are expected to reduce emissions by 21 percent by 2030 compared with what the output would have been without the standards. Because emissions from passenger vehicles represent about one-fifth of America’s greenhouse gases, this is a step forward for the planet.

The automakers, who fought the rules until they went broke(破产), have come to accept this as a step forward as well. A single national standard provides regulatory certainty, and they’ve got to get more efficient to survive.

However, some in Congress seemed determined to roll back the laws that got us here. Senator Lisa Murkowski, a Republican from Alaska, and several other senators have added a challenge to the federal government’s authority to regulate greenhouse gases under the Clean Air Act — not just from automobiles but from other sources. The Supreme Court gave the Environmental Protection Agency that authority three years ago, and the new emissions standards would have been impossible without it.

There has also been talk in the Senate of eliminating California’s special authority under the Clean Air Act to set more aggressive motor vehicle standards than the federal limits. California used that authority to pass a law in 2002 setting greenhouse gas emissions limits for cars sold there. It was the first law of its kind in this country, and it provided the drive and the foundation for the new nationwide standards.

What all of these opponents mean to do is to roll back history and the hard-won environmental protections it has produced. That would be a huge mistake.

66. The following are the benefits of the new automobile economy standards EXCEPT ________.

A. reduced dependence on foreign oil

B. cancelling some of the laws

C. fewer greenhouse gas emissions

D. consumer savings at the pump

67. What goal is set for the year 2016?

A. Cars will cost more so fewer people will buy them.

B. There is a 30 percent increase in car manufacturing.

C. An average vehicle can go 35.5 miles with one gallon of gas.

D. Consumers will save an average of $3,000 in fuel per car.

68. The underlined word “it” in paragraph six refers to ________.

A. The Clean Air Act

B. The Supreme Court

C. The Environment Protection Agency

D. The federal government’s authority

69. What seems to be the root of the new automobile fuel economy standards?

A. California’s motor vehicle standards.

B. The Environment Protection Agency.

C. Some Senators, like Lisa Murkowski.

D. Greenhouse gas emissions.

70. According to the writer, the new automobile fuel economy standards will probably lead to the result that ________.

A. everyone wins

B. more cars will be sold

C. it would be a big mistake

D. nobody agrees

单项选择题