Switch MAC learning

Unanswered Question
Oct 15th, 2007
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Hi All,

When switch gets on, it's doesn't have any information in MAC-table. When one PC in that switch transfer any data to other PC in the same switch, it will broadcast that Frame to all ports in that switch. Normally Frame has a field of "Destination MAC Address" in it. When initially MAC-table is empty, how it gets the Destination MAC address of that PC? How it works,explain me.........

Will all the PC's MAC Address will be stored in Interface buffer?



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Overall Rating: 3.5 (2 ratings)
michal.grzelak Mon, 10/15/2007 - 23:11
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When PC1 (IP wants to send a frame to PC2 ( on the same switch and in the same Vlan, source PC1 will broadcast arp packet asking to get MAC od PC1.It will send packet with destination MAC of broadcast FFFF and IP od PC1 . Every station in this Vlan will analyze that, but only station with IP of PC1 will answer to that request.

That is how PC knows what is MAC address of other computers.

Hope that helped. If yes, please rate my answer.



Jon Marshall Mon, 10/15/2007 - 23:22
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Hi Michal

Just to clarify for the original poster. When PC1 wants to send a frame to PC2 ( PC1 will broadcast arp packet asking to get Mac address of PC2 (not PC1) ie broadcast Mac and IP of PC2.

I fully appreciate you know this, just a typo.


michal.grzelak Mon, 10/15/2007 - 23:44
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Yes, that was typo, it sends ARP packet with destination MAC=FFFF and destination IP=PC2.

nalini1023 Tue, 10/16/2007 - 04:06
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Hi michal,

Thanks for ur reply. Still I have questions on:

1)But when a broadcast happens, will ARP packet be sent to learn the MAC addres or the frame is sent?

2)Can you give an example for a broadcast structure?

3)Where ARP table is maintained?

4)How ARP updates MAC-table?

If u know any link that solves all my questions please refer.



sandy3006 Mon, 10/15/2007 - 23:21
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Hi nalini,

do u know somthn called BPDU and do u know dat every switch port has one ASIC chip where actually the mac address are stored and the MAC table is created when there is a first time communication that time a broadcast takes place with this procedure as per my knowledge.



lgijssel Mon, 10/15/2007 - 23:24
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Your question is: how does the mac-table get populated?

Answer: The switch is learning the location (port or interface) of mac adresses by looking at the source adresses of all packets that are received.

When observing a new source adress, an entry in table is created. This is called learning.

After not observing a mac-adress for a predetermined time, it is removed from the table. This is called aging.




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