I learned that router change mac address of packet with that one of its own outgoing interface, while this doesn't happen with bridges. I find it difficult to understand the logic under this implementation ,so i hope you can help me.

Thank you so much!

  • 1
    If you had a router that didn't change the MAC address, switches on the next segment wouldn't be happy.
    – Joshua
    Jun 23 '21 at 21:16

Routers do not change the MAC address, they completely strip off the layer-2 frame, losing any layer-2 information, e.g. MAC addressing. A router then routes the layer-3 packet that was encapsulated in the lost frame to a different interface, and it builds a new frame for the new interface.

Remember that not all layer-2 protocols use MAC addressing, only the IEEE protocols (ethernet, token ring, Wi-fi, etc.) do.. For example, frame relay uses DLCI, ATM uses VPI/VCI, and PPP does not use addressing. If the next interface is connected with one of those protocols, the frame the router builds will not have MAC addressing. It is common fot things like xDSL to use PPP and ATM, so there is no MAC addressing on the WAN interface.

On the other hand, switches are transparent devices that do not change the frame, so any MAC addressing is not disturbed.

  • effectively i was not considering the replacement of layer 2 with a completely new one binded to the specific interface. Thank you much !
    – loremas89
    Jun 23 '21 at 18:32

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