A: LAN Switching Logic
It seems simple… and then you’re sitting at the #CCENT or #CCNA exam. Where does that Ethernet frame really go? I posted a question about LAN Switching a few days back. Here are the answers! Post with any related clarifications or comments.
Several key concepts must be applied to answer this question. First, consider the frames. The first frames goes from A to C, and the second is a return frame from C to A. As worded, the question also implies that these frames both are addressed using the unicast MAC addresses of the two hosts, so neither are broadcast frames.
The second key concept is that by the time the first frame arrives at host C, both SW1 and SW2 will have built correct MAC table entries for host A’s MAC address. The reason is that switches learn MAC addresses based on the source MAC address, and the source MAC of the first frame is host A’s MAC address.
The second frame has a destination address of host A’s MAC. As a result, this second frame, sent from host C to host A, will not be flooded because both switches already know the best port out which to forward frames sent to host A’s MAC. This path will be: to first enter SW2’s F0/9 interface; exit SW2’s G0/2 interface; enter SW1’s G0/1 interface; then exit SW1’s F0/1 interface.
The wrong answers are wrong simply because they are not part of the forwarding path from SW2, to SW1, and on to host A, once the two switches have learned of host A’s MAC address.