Does a MAC Table Hit Always Forward the Frame?
Short answer
B A MAC table hit does not guarantee forwarding, since same-port filtering, STP blocking states or a down port can stop the frame.
Question
Key Takeaways
The trap is the word "definitely" — a MAC table hit only selects a candidate egress port; same-port filtering, STP blocking/discarding state and port availability can still prevent transmission.
This H12-811 question tests known-unicast forwarding on an Ethernet switch: a destination MAC found in the MAC address table does not always mean the frame is forwarded, so the correct answer is B (FALSE).
Common mistake: Choosing A (TRUE) by assuming MAC table lookup alone determines forwarding; this ignores same-port filtering and Spanning Tree port states, which stop the frame even on a table hit.
Expert Analysis
The governing principle
Switch forwarding is governed by MAC address table lookup, but a table hit only tells the switch which port is associated with the destination MAC — it does not guarantee that a frame will be transmitted out of that port. Forwarding is a two-step decision: select the egress port, then verify that the frame is actually allowed to leave on it.
Why "definitely" makes the statement false
Several documented behaviours break the guarantee:
- Same-port filtering. If the destination MAC was learned on the same port the frame arrived on (typical when a hub or another switch sits behind one port), the switch filters the frame instead of sending it back out that port. This is normal unicast forwarding behaviour, not an error.
- Spanning Tree state. Even with a table hit, a port in a blocking or discarding state under STP will not forward the frame; the frame is dropped on that candidate port.
- Port availability. A port that is administratively shut down or otherwise not in a forwarding condition will not carry the frame.
The community comments capture exactly this: one notes that when source and destination MAC addresses map to the same port, the switch filters rather than forwards; another stresses that the outbound port must actually be in a forwarding state. A single dissenting vote chose TRUE, but the absolute word "definitely" is what invalidates the claim.
Conclusion
The correct choice is B (FALSE). A MAC table hit is necessary for known-unicast forwarding, but it is not sufficient — ingress/egress port identity, STP port state, and port administrative state all intervene.
Exam Strategy
Watch for absolute words such as "definitely", "always" or "guaranteed" in TRUE/FALSE items; they usually signal a documented exception that flips the statement to FALSE. Then ask yourself the standard exception set for known-unicast forwarding: same-port filtering, STP port state, and port administrative state. Apply this pattern to any switching question that equates a lookup result with an actual forwarding action.
Frequently Asked Questions
Why is a unicast frame filtered instead of forwarded when the MAC table has the entry?
Can a known-unicast frame be dropped even though the MAC address table lookup succeeds?
What does a MAC address table hit actually tell the switch?
Community Votes
75% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 4 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 4
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konyuz 👍 2 Answered BTricky question. A key exception is when the source and destination MAC addresses are associated with the same port. This scenario is common when multiple devices are connected to a single switch port via a hub or another switch. If a frame arrives on a port and the switch's MAC address table indicates that the destination MAC address is also reachable through the same port, the switch will filter or discard the frame. This is known as loop prevention and is a crucial function of a switch. There would be no point in forwarding the frame back out the same port it just came in on, as the destination device has already received it.
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e9b92a4 👍 1 Answered B❌ B. FALSE Explanation Even if a switch finds the destination MAC address in its MAC address table, the data frame is not always guaranteed to be forwarded through the corresponding port. Here’s why: ✅ The switch locates the outbound port using the MAC address table. ❌ However, the port must be in a forwarding state to actually transmit the frame. There are several situations where the frame will not be forwarded despite a valid MAC entry: The port is blocked by STP (for example, in Blocking or Discarding state). The port is administratively down. The port is in a non-forwarding VLAN state. Security policies (port security, ACLs, storm control, etc.) prevent forwarding.
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4243643 👍 1 Answered BWhile switches typically forward unicast frames to the port associated with the destination MAC address in their MAC table, this is not guaranteed in all cases. If the frame arrives on the same port where the destination MAC is learned, the switch will filter (drop) the frame rather than forward it back out the same port. This filtering behavior prevents loops and unnecessary traffic.
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ButcherCapybara 👍 1 Answered ATrue, The switch will forward the unicast frame through the port with the learned MAC address
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