题目内容
传统的交换机作为第二层设备，只能识别并转发 (29) 地址，要支持VLAN间的通信只能借助于具有 (30) 功能的网络设备。具有这种功能的设备有路由器和三层交换机。当用路由器解决VLAN通信时，路由器得到一个VLAN包后，根据目的IP地址，获得目的MAC地址及相应的 (31) ，根据 (32) 将帧送往目的交换机端口。当用三层交换机解决VLAN通信时，由于交换机具有 (33) ，交换和路由速度可以达到线速。
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A good title for this selection is （）
With the start of BBC World Service Television, millions of viewers in Asia and America can now watch the Corporation’’s news coverage ,as well as listen to it. And of course in Britain listeners and viewers can tune in to two BBC television channels, five BBC national radio services and dozens of local radio stations. They are brought sport ,comedy ,drama, music, news and current affairs, education, religion, parliamentary coverage, children’’s programmes and films for an annual licenee fee of ￡ 83 per household. It is a remarkable record, stretching back over 70 years―yet the BBC’’s future is now in doubt. The Corporation will survive as a publicly-funded broadcasting organization, at least for the time being, but its role, its size and its programmes are now the subject of a nation-wide debate in Britain. The debate was launched by the Government, which invited anyone with an opinion of the BBC― including ordinary listeners and viewers―to say what was good or bad about the Corporation, and even whether they thought it was worth keeping. The reason for its inquiry is that the BBC’’s royal charter runs out in 1996 and it must decide whether to keep the organization as it is, or to make changes. Defenders of the Corporation―of whom there are many―are fond of quoting the American slogan "If it ain’’t broke, don’’t fix it. "The BBC " ain’’t broke" ,they say, by which they mean it is not broken (as distinct from the word ’’broke’’ ,meaning having no money), so why bother to change it Yet the BBC will have to change, because the broadcasting world around it is changing. The commercial TV channels―ITV and Channel 4―were required by the Thatcher Government’’s Broadcasting Act to become more commercial, competing with each other for advertisers, and cutting costs and jobs. But it is the arrival of new satellite channels―funded partly by advertising and partly by viewers’’ subscriptions―which will bring about the biggest changes in the long term. The world famous BBC now faces_________________.
Poor roads were probably so because of （）
过食甘味可致
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