Switch Frame Forwarding: Flooding Unknown Unicast Destinations

Describe switching concepts
Answer Correct answer: A — A switch floods a frame with an unknown destination MAC out every port except the incoming port until the destination replies and the MAC is learned.

How does frame switching function on a switch?

  1. floods unknown destinations to all ports except the receiving port Correct Answer
  2. modifies frames that contain a known source VLAN
  3. rewrites the source and destination MAC address
  4. buffers and forwards frames with less than 5 CRCs

Community Votes

A
78%
D
22%

78% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Community Insight

Flooding is the default behavior for unknown unicast and broadcast frames; it is how a switch discovers where devices live, not an error condition.

When a switch receives a frame whose destination MAC is not in its MAC address table, it does not know the egress port. It forwards a copy out every port in the same VLAN except the ingress port. Once the destination device responds, the switch learns the source MAC and stops flooding to that address.

Confusing flooding (unknown destination) with forwarding (known destination). A Layer 2 switch does not rewrite MAC addresses or route; it only switches inside a VLAN using the learned MAC table.

Community Discussion (8 comments)

lmmujsi 👍 1 Selected: A
A. floods unknown destinations to all ports except the receiving port
lmmujsi 👍 1 Selected: A
A. floods unknown destinations to all ports except the receiving port
Jhonesy18 👍 1
Answer is C
Starlord2535 👍 1 Selected: A
answer is correct
a67c04a 👍 1 Selected: A
It´s A
f50fd1d 👍 1 Selected: A
Should be A
Boulet_Dude 👍 2 Selected: D
I’m going to guess D
hainsberg 👍 1
agree?

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Expert Analysis

Why the Answer Is Correct

A is correct. A switch builds its MAC address table by inspecting the source MAC of each frame. When the destination MAC is absent from that table (unknown unicast), the switch floods the frame out all ports in the VLAN except the receiving port, guaranteeing delivery while it learns the path back. This is exactly what option A describes.

Why the Other Options Are Wrong

B is wrong: a switch does not modify a frame's VLAN tag based on the source MAC during ordinary switching. C is wrong: a switch never rewrites source or destination MAC addresses — that is router/NAT behavior, not Layer 2 switching. D is wrong and nonsensical: frames with CRC errors are dropped, not "buffered and forwarded with less than 5 CRCs."

Community Comment Notes

The examtopics discussion shows A with 78 votes versus 22 for D, and several commenters restate the unknown-unicast flooding rule. The dominant community pick matches the switching-concept definition.

Official Reference

https://learningnetwork.cisco.com/s/ccna-exam-topics

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