Cisco Certified Network Associate (CCNA) Practice Exam

What determines the port from which a frame will be flooded if the destination MAC is not known?

1. The source MAC address's VLAN

2. The destination VLAN configured on the switch

3. The switch's overall configuration

4. The last learned MAC address of any port

The correct answer is: The source MAC address's VLAN

When a switch receives a frame with a destination MAC address that it does not recognize, it floods the frame out to all ports within the same VLAN as the source MAC address. This behavior is based on the concept of VLAN segmentation and ensuring that the broadcast domain is respected. In this context, the source MAC address of the incoming frame is crucial because it indicates the VLAN to which the frame belongs. By flooding the frame only to those ports associated with the same VLAN, the switch avoids unnecessary traffic on other VLANs and maintains the integrity of network segmentation. If the source MAC address were to be in VLAN 10, for instance, the switch would only forward the frame to ports in VLAN 10. Other VLANs would not see the broadcast, which optimally regulates traffic and enhances security within the network. The role of the destination VLAN and the switch's overall configuration, while important for general operations in networking, do not directly determine the specific flooding behavior in response to an unknown destination MAC address. The last learned MAC address of any port is irrelevant in this scenario, as it does not contribute to configuring the flood behavior specific to unrecognized destinations.