Switching process

Hi,

I am a little confused, as to why the switch is doing this.

For example:

Let's say there are 3 hosts, PC1,PC2, and PC3. And, let's say they are all connected to the same switch via fa0/1 for PC1, FA0/2 for PC2, and FA0/3, for PC 3. The switch has just been powered on, so there's not mac address in he mac table of the switch. So, PC1 sends a frame to PC2, what will happen to this frame? I think The switch will add the source mac of PC1 and at the same time send a broadcast frame out all other ports except the one it came.

So now, let's say PC3 is connected off of FA0/3 and sends a frame to PC2. What will happen to this frame, I know it will get forwarded to PC2 because the mac for PC2 is there, but why will this frame be also sent out of port fa0/1? Is it because PC3 checked its arp table and does't see an entry for PC2, so arps for the mac address of PC2, and thereby the frame also going out interface fa0/1 to PC1?

If someone can explain this to me that would be great.

Thanks.

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Comments

james43026

You never stated that PC2 ever sent any frames. So how would the switch know the MAC address of PC2, and what port it is off of? It simply doesn't, which is why the frame sent from PC3 is unicast flooded. You keep saying that the packets are broadcasted, they are not, they are unicast flooded. There is a difference between them, mainly with the destination address of layer 3 and layer 2.

Koolster

Thank you James!

I should have explained it better. PC1 has sent a frame destined for PC2, and PC2 was never going to reply back because it was just a one way frame destined for PC2. So, you're right, PC2's mac adderss will not be learned by the switch because it has not sent a reply, back to PC1. Therefore PC 3 will flood a frame going to PC2 because the mac address entry for PC2 is not in the mac address table. I was getting confused about.