late collision相关问题

[ccie_010101]

What are two reasons that cause late collisions to increment on an Ethernet interface?(choose two)
A. when the sending device waits 15 seconds before sending the frame again
B. when the cable length limits are exceeded
C. when one side of the connection is configured for half-duplex
D. when Carner Sense Multiple Access/Collision Detection is used
E. when a collision occurs after the 32nd byte of a frame has been transmitted

如果两个站点都在某一时间检测到信道是空闲的，并且同时开始传送数据，则它们几乎立刻就会检测到有冲突发生。如果发生冲突，信道上可以检测到超过发送站点本身发送的载波信号幅度的电磁波，由此判断出冲突的存在。一旦检测到冲突，发送站点就立即停止发送，并向总线上发一串阻塞信号，用以通知总线上通信的对方站点，快速地终止被破坏的帧，可以节省时间和宽带要求站点在发送数据过程中进行冲突检测，而一旦检测到冲突立即停止发送数据。这样的协议被称为带冲突检测的载波监听多路访问协议，即 CSMA/CD

我觉得应该选BC不是很理解答案选D

我的理解就是半双工条件下，线路过长就会发生延迟碰撞。

[huoran1234]

看起来是题库啊，不太建议发在这儿。。

回到问题本身，个人感觉应该是选BC，而且是有一端是半双工或线路过长这个说法更合适一些。我认为之所以有些答案会给到BD是因为把collision和late collision混在一起了。在CSMA/CD的情况下，有collision是正常的，但会立刻发送jam signal，这跟late collision还是有一些区别的。

[Rps-Cheers]

First, Section D "Carner Sense" should be "Carrier Sense"

According to the instructions in CISCO CCNA 200-301, the correct answer should be:
B. when the cable length limits are exceeded
C. when one side of the connection is configured for half-duplex

Explanation:
A late collision is defined as any collision that occurs after the first 512 bits (or 64th byte) of the frame have been transmitted. The usual possible causes are full-duplex/half-duplex mismatch, exceeded Ethernet cable length limits, or defective hardware such as incorrect cabling, non-compliant number of hubs in the network, or a bad NIC. Late collisions should never occur in a properly designed Ethernet network. They usually occur when Ethernet cables are too long or when there are too many repeaters in the network.

Thx

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