2. 计算机
  3. In which circumstance are multiple copies of the same unicast frame likely to be transmitted in the Ezonexam Switched LAN? A. Du

In which circumstance are multiple copies of the same unicast frame likely to be transmitted in the Ezonexam Switched LAN? A. Du

admin2009-05-19  16
问题 In which circumstance are multiple copies of the same unicast frame likely to be transmitted in the Ezonexam Switched LAN?
A. During high traffic periods
B. In an improperly implemented redundant topology
C. After broken links are re-established
D. When upper-layer protocols require high reliability
E. When a dual ring topology is in use
F. None of the above
选项
答案B
解析 Explanation:
A redundant topology eliminates single points of failure, but it also causes broadcast storms, multiple frame copies, and MAC address table instability problems. Multiple Frame Copies--when a new switch is added, the other switches may not have learned its correct MAC address. The host may send a unicast frame to the new switch. The frame is sent through several paths at the same time.

The new switch will receive several copies of the frame. This causes MAC database instability. MAC database instability results when multiple copies of a frame arrive on different ports of a switch. Layer 2 has no mechanism to stop the loop. This is the main reason for the Spanning Tree Protocol(STP) IEEE 802.1d which was developed to prevent routing loops.

If multiple connections between switches are created for redundancy purposes, network loops can occur iin an imprperly designed topology. Spanning Tree Protocol (STP) is used to stop network loops while still permitting redundancy.
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