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New Member

Switch learning mac addresses

In a video that I watched a few days ago someone explained a basic process of booting up a switch and how a switch learns mac addresses. He said something that I would like to discuss. I know... it is not important but want to clarify :)

 

PC1---SW1----PC2

 

PC1 wants to send sth to PC2. In the video it was said:

'a frame arrives at SW1 and SW1 learns the mac address of pc1 but it does not know the mac address of pc2 so it will flood this frame to all ports'

 

My uderstanding is that it all starts with an arp message: pc1 does not know the mac address and sends an arp and it will allow the switch to learn both mac addresses: pc1 and pc2. I am too lazy to do it in wireshark but did that in PT and that's what I saw as well. After the arp - switch learnt both macs and did not flood the frame.

 

Am I correct? I know it is not important but... ;-)

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

It may be possible that there

It may be possible that there was some aspect of the switch environment in the video that would change the behavior (perhaps something like a long timer for the ARP cache in the PC and a short MAC ageing timer on the switch). But in general you are correct. PC1 would send an ARP request as a broadcast, the switch would learn the MAC of PC1 and forward the ARP request. When PC2 sends its response to the ARP request the switch would learn the MAC of PC2 and forward the ARP response. So the switch should have both MAC addresses when data traffic begins to flow.

 

HTH

 

Rick

2 REPLIES
Hall of Fame Super Silver

It may be possible that there

It may be possible that there was some aspect of the switch environment in the video that would change the behavior (perhaps something like a long timer for the ARP cache in the PC and a short MAC ageing timer on the switch). But in general you are correct. PC1 would send an ARP request as a broadcast, the switch would learn the MAC of PC1 and forward the ARP request. When PC2 sends its response to the ARP request the switch would learn the MAC of PC2 and forward the ARP response. So the switch should have both MAC addresses when data traffic begins to flow.

 

HTH

 

Rick

New Member

Thanks Rick.

Thanks Rick.

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