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A switch doesn't alter the frame, and source/destination mac addresses remain how sending host specified them,so let me say it again Layer 2 Switching never changes the frames means no changes in MAC and in CRC values,switches either flood, forward, or filter MAC addresses. Since they do transparent bridging by nature!

Every single port on a switch has its own mac address then what are they used for?

there are some posts and discussion over this subject but there is no clear satisfying valid answer.

in this link here the guy says Switch changes the source mac address to its own port MAC addresses when want to flood or do any kinda transmission but provides no reference to that which kinda seems to be wrong!

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    identity of the port, layer-3 use of the port, ... a manufacturer could use them any way they want. Does a layer-2 port need a MAC? Of course not as nothing talks to the port itself. – Ricky Beam Sep 9 '14 at 19:50
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    1. identity of the port for what reason,sw gonna work transparent, 2.Layer3 usage is acceptable answer,3."manufacturer could use them any way they want" what are the examples?? – user7741 Sep 9 '14 at 20:40
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    Very good question. Simple switches don't need a MAC address for each interface. – Navin Mar 19 '16 at 8:39
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 10 '17 at 15:32
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One key use for per port MAC addresses on switches is for Spanning-tree BPDU's. These are Layer-2 multicasts with source MAC address of the egress switch port.

I would have to brush up on other Layer-2 protocols such as TRILL and SPB, but they might also take advantage of a per port MAC.

Does that help?

  • Actually i found some reasonable answer for my question gonna post by tomorrow – user7741 Sep 9 '14 at 22:18
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Okay, here is a more direct answer.

In the past implicit tagging required each port to have a MAC address. With 802.1Q, that is no longer necessary.

Transparent bridging does not alter the frame. Layer 2 switches do not need the base Ethernet MAC address of the device nor its switchport MAC addresses to operate. The source and destination MAC addresses of the incoming frame are examined, the first one being saved in the MAC address table along with the receiving port, while the destination MAC address being looked up in the MAC address table to see if there is an associated port. If there is the frame is forwarded out that port only, otherwise it gets broadcast out all ports, except the source port (split horizon rule).

During the whole process the frame remains completely unchanged (unless forwarded towards a VLAN trunk - then the frame gets tagged with VLAN ID).

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