L2 switch MAC learning procedure

Unanswered Question
Oct 31st, 2008


Wondering if ethernet frames with destination MAC broadcast are used in the MAC learning procedure within a L2 switch. For example I have heard that ARP requests (having broadcast destination MAC) are not used to populate the table containg the MAC address associated to a given port. Is that true? Actually I have not found anything in IEEE 802.1 on this regard.



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satish_zanjurne Fri, 10/31/2008 - 06:40

Even i could not find anything such !!

Actually MAC-ADDRESS-LEARNING process must not bother about ARP broadcast address.

It just has to see the source MAC address of the frame & add it into mac-address-table.

pkaretnikov Fri, 10/31/2008 - 07:41

Something came up locally that completely surprised me. An ARP response is layer 3 concept, as in there is a mac-address in the payload that does not have to match the sending mac-address.

ARP was developed to support more than just IP, so the message contains a protocol field.

To more directly answer your question, switches only learn based on what source mac-address they see coming from what port. A destination of broadcast still has a source of the actual sender so the switch will learn from that frame.

A frame that contains an ARP will have a source address and the switch will learn that source mac, HOWEVER this can actually be different information than what is contained in the ARP reply. It's a little devious but it serves a purpose in some setups like GLBP.

Take a look at the wiki, for more detailed info


Hope that helps you out.


arun-naiyar Fri, 10/31/2008 - 16:00

The layer 2 switch learning process is very simple. If a switch generates a frame with any unknown unicast or even a broadcast address, the switch only keeps the source port and MAC address info in the switching table. Now try relating this logic with your statement about ARP and you will get your answer.


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