switch filter a frame

Question

I was reading a text book, it says suppose a host A sends a frame with destination address DD-DD-DD-DD-DD-DD arrives at the switch on interface x. The switch indexes its table with the MAC address DD-DD-DD-DD-DD-DD, and if There is an entry in the table, associating DD-DD-DD-DD-DD-DD with interface x. In this case, the frame is coming from a LAN segment that contains adapter DD-DD-DD-DD-DD-DD. There being no need to forward the frame to any of the other interfaces, the switch performs the filtering function by discarding the frame.

I have a question in regards to this case:

1. if A's frame arrives at interface x which also associates with A's MAC address, which means A is sending a frame to itself, why a host need to send frames to itself?

Answer 1

A switch - or a MAC bridge - doesn't know what exactly is connected to any of its ports. This may be an end node (single MAC source), another switch/bridge (multiple MAC sources) or even a repeater hub (also multiple MAC sources).

The essential difference between a downstream switch or a hub is that a switch won't forward frames destined for another one of its ports to the upstream switch - but a hub will. A hub doesn't make any forwarding decision, it just repeats the data stream to all other ports. Accordingly, a hub with port 1 connected to the upstream switch repeats a frame received from port 2 and destined for port 3 to all of its ports, including port 1.

If the upstream switch would reflect the frame back via the source port, the destination node would receive two copies of the same frame and the source would get an echoed copy of its own frame. Therefore, the upstream switch simply drops the frame.