Which of the following descriptions about the working principle of a Layer 2 switch is correct?

Question

Options

• AUnicast forward all broadcast packets after receiving them
• BMulticast sending after receiving the multicast message
• CRouting and forwarding after receiving the unicast message
• DAfter receiving broadcast, multicast and unknown unicast, it will be sent out from all other ports in

• A
  14% (3)
• B
  5% (1)
• C
  5% (1)
• D
  77% (17)

Explanation

Option D correctly describes the fundamental flooding behavior of a Layer 2 switch: when it receives a broadcast (e.g., ARP request), a multicast frame, or a unicast frame whose destination MAC address is not in its MAC address table (unknown unicast), it floods the frame out all ports except the one it arrived on - this is called BUM flooding.

Why the distractors are wrong:

• A is wrong because a switch does not unicast-forward broadcasts; it floods them to all other ports, which is the opposite of unicast delivery.
• B is wrong because "multicast sending" implies selective delivery, but a basic Layer 2 switch without IGMP snooping simply floods multicast out all ports just like a broadcast.
• C is wrong on two counts: routing is a Layer 3 function (performed by routers/L3 switches using IP), and a known unicast frame is forwarded out a single specific port based on the MAC table - not "routed."

Memory tip: Remember the acronym BUM - Broadcast, Unknown unicast, Multicast - all three get flooded (sent out all other ports). Only known unicast gets intelligently forwarded to one port. If you remember "BUM traffic gets flooded," option D will always be obvious.

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