VLAN Frame Flooding and the Ingress-Port Exception

200-301 By islever Editorial Last updated

Describe switching concepts Configure and verify VLANs (normal range) spanning multiple switches

Short answer

C During frame flooding, the switch forwards the frame to every port in the same VLAN except the originating port.

Question

What occurs to frames during the process of frame flooding?
  1. Frames are sent to all ports, including those that are assigned to other VLANs.
  2. Frames are sent to every port on the switch that has a matching entry in MAC address table.
  3. Frames are sent to every port on the switch in the same VLAN except from the originating port. Correct Answer
  4. Frames are sent to every port on the switch in the same VLAN.

Key Takeaways

This item tests VLAN-scoped flooding and its ingress-port exception; the main trap is choosing the VLAN-wide option that omits the originating-port exclusion.

VLAN frame flooding replicates a frame to every switch port in the same VLAN except the ingress port; the correct option is (C).

Common mistake: Option D is the most plausible wrong answer: it correctly keeps flooding inside the VLAN but forgets that a switch never sends the flooded frame back out the port on which it arrived.

Expert Analysis

The boundary: VLAN scope versus ingress exclusion Frame flooding is a forwarding action, not a destination type. When a switch receives a frame and has no specific MAC-table entry for the destination, or when the frame is broadcast or multicast, it replicates the frame to every port in the same VLAN except the port on which the frame arrived. That ingress-port exception is the operational detail this question is built around. Option C captures both the VLAN scope and the exclusion; Option D captures the VLAN scope but omits the exception.

Why VLAN crossing and known-unicast forwarding are wrong Option A crosses a VLAN boundary. A VLAN is a separate broadcast domain, so a switch does not flood a VLAN 10 frame out access ports assigned to VLAN 20. Option B describes normal known-unicast forwarding after a successful MAC address table lookup, not flooding.

What the discussion clarifies The comments repeatedly focus on the phrase about excluding the incoming port, and one correctly notes that the question concerns Layer 2 frames and MAC addresses rather than Layer 3 IP multicast packets. That distinction matters because flooding can be triggered by unknown unicast, broadcast, or multicast frames, but the switch behavior remains scoped to the VLAN and excludes the ingress port.

Correct choice The correct answer is C. The key boundary is not merely “all ports in the VLAN” versus “all switch ports”; it is “all ports in the VLAN except the originating port.”

Official Reference

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

This link opens the official vendor documentation in a new tab.

Exam Strategy

For switch-forwarding questions, first identify whether the frame is unknown unicast, broadcast, or multicast, then identify the VLAN and eligible ports. Eliminate answers that cross VLAN boundaries or describe known-unicast forwarding after a MAC-table lookup. When two options differ only by an “except the originating port” clause, prefer the one that states the ingress exclusion, because switches do not flood a frame back out the receiving port in the same VLAN.

Frequently Asked Questions

Does a switch flood a frame to ports in other VLANs?

No. Flooding is scoped to the VLAN where the frame belongs; ports in other VLANs are separate broadcast domains and do not receive that flooded frame.

Why is option D wrong if flooding goes to every port in the VLAN?

Option D omits the ingress-port exception. A switch floods to every other port in the VLAN, not back out the port where the frame was received.

Is frame flooding the same as broadcasting?

No. Flooding is the switch forwarding behavior; it can be triggered by unknown unicast, broadcast, or multicast frames. Broadcast is one destination type that causes flooding.

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Discussion 12 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 12

  1. ZUMY 👍 19
    Given answer C is correct
  2. nav2802 👍 4
    Ans is C
  3. Jay1324 👍 3
    No C is correct, you provided IPV^ multicast addresses which operate at layer 3 and are known as packets. The question states layer 2 frames meaning mac addresses.
  4. kminh9999 👍 3 Answered C
    The switch floods all ports except the incoming port. In case of VLAN, it floods all ports of that VLAN except the incoming port.
  5. Wong93 👍 3
    C is correct
Show 7 more comments
  1. yonachimodzi 👍 2 Answered C
    During the process of frame flooding, frames are replicated and forwarded to all ports on a network switch, except the port from which the frame was received. This technique is commonly used in Ethernet networks when the destination MAC address of a frame is unknown or if the frame needs to be broadcasted to multiple devices on the network. Therefore answer is C.
  2. zbednar4 👍 1 Answered C
    This is the only option that mentioned anything about "except the incoming port"
  3. Shamwedge 👍 1
    D is correct. FF00::/8 and FF00::/10 are both multicast addresses.
  4. duongccna 👍 1
    C, all port except original port
  5. 2b081f1 👍 1 Answered C
    This one is obvious.
  6. JhinTonic 👍 1 Answered C
    C is correct.
  7. bymrdas 👍 1 Answered C
    Correct

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