The Un-Official CCNA guide #16: How ethernet frames are forwarded

In this diagram, we have 3 PCs connected to a switch. PC0 wants to communicate with PC2. We will assume that all PCs and switches know each others’ MAC addresses.

1. PC0 will send an ethernet frame to switch 0.
2. The ethernet frame has a source MAC address of PC0 and destination MAC address of PC2
3. Since the switch knows the MAC address of PC2, it will forward the frames directly to PC2. This type of frame is known as known unicast frame.
4. PC1 will drop the packet because the destination MAC address of the frame is not that of PC1.

Let’s say that the PCs knows each other MAC addresses, but the switch does not know the MAC address of the PCs.

1. PC0 will send an ethernet frame to switch 0.
2. The ethernet frame contains the source MAC address of PC0 and the destination MAC address of PC2.
3. Since the switch does not know the MAC address of PC2, it has no choice but to “flood” the packet. This packet is known as a unknown unicast frame.
4. “Flooding” a packet means forwarding the packet out of all interfaces, except the interface the switch receives the packet from.
5. An important thing to note is the switch learns to associate one of the interface with the MAC address of PC0. Switch learns about MAC addresses through the source MAC address of an ethernet frame.
6. PC1 will still drop the packet because the destination MAC address of the frame is not that of PC1