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  3. A Ezonexam Ethernet switch receives a unicast frame with a destination MAC that is listed in the switch table. What will this sw

A Ezonexam Ethernet switch receives a unicast frame with a destination MAC that is listed in the switch table. What will this sw

admin2009-05-19  57
问题 A Ezonexam Ethernet switch receives a unicast frame with a destination MAC that is listed in the switch table. What will this switch do with the frame?
A. The switch will forward the frame to a specific port
B. The switch will forward the frame to all ports except the port on which it was received
C. The switch will return a copy of the frame out the source port
D. The switch will not forward unicast frames
E. The switch will remove the destination MAC from the switch table
F. None of the above.
选项
答案A
解析 Explanation:
How Does the Switch Find Host MACs?
Let’s use the diagram below to help us understand how address learning process takes place.


Let’s assume that we have just powered on the switch. It has nothing in its MAC table. We connect the cables from the hosts on the switch interfaces as shown in the diagram. Host A initiates a connection towards Host D, and the following takes place:
1. Host A (interface fe0/0) sends a frame to Host D (MAC address:0000.43c5.334c).

2. The switch inspects the Source Address in the frame and notes in its table the MAC address of Host A along with the Interface number from which the frame originated.

3. The switch inspects the Destination Address in the frame. Since it does not have Hosts D MAC address in its table, it constructs a broadcast frame and forwards out all interfaces except the interface from where the original frame arrived.

4. Host D identifies itself as the expected recipient and responds back to Host A. The switch receives the respond frame on interface fe0/11 and places the SA in its table along with the interface number where the frame came from.

5. From now on, further communication between the two hosts will be switched to the appropriate interfaces based on the MAC tables entries.

This process takes place every time a new host is attached on the switch and initiates traffic. The switch tries to keep its MAC table up-to-date, therefore if some hosts do not initiate traffic for a certain amount of time, the switch removes them from its table and reinserts them when they begin sending traffic.
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