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If PC1 sends an Ethernet frame to Switch1 which forwards the frame to Switch2 which in turn forwards the frame to PC2, does any decapsulation happen that changes the source MAC address?

Ron Maupin
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Rana Mallah
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    Ron is correct. [Switches do not change MAC addresses](http://www.practicalnetworking.net/packet-traveling/host-to-host-through-a-switch/). The L2 header only changes when a packet crosses a router. In fact, the L2 header is how a PC gets the packet to a router, so that the [router can forward](http://www.practicalnetworking.net/packet-traveling/host-to-host-through-a-router/) it to the next router (or final destination). – Eddie Mar 01 '16 at 21:43
  • Does this answer your question? [Why don't switches rewrite mac-addresses?](https://networkengineering.stackexchange.com/questions/5337/why-dont-switches-rewrite-mac-addresses) – Stefan van den Akker Jan 19 '20 at 12:43

2 Answers2

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No. If all the switches are layer-2 switches, the frames are switched without any changes.

Only with routers, including layer-3 switches where the packets need to cross to other VLANs, will the frames be stripped and rewritten for the new network or VLAN.

Ron Maupin
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The MAC address is a unique identifier which is used by hosts on the local network.

If the switch were to change the destination MAC, the frame would not get delivered to the appropriate host. In the cases that it would, for example if the frame gets flooded, the destination host would drop it because it would no longer be destined for the host.

If the switch were to change the source MAC address, the destination host would use this MAC address for any responses (including updating any ARP entries with bad data). This would result in the same situation I already described, just for all return traffic.

Could mechanisms be developed to do this? I am sure they could. But there is no reason to do so at this point and this would only complicate networking and add unnecessary processing. We are not close to exhausting the available MAC address pool so there is no need for something like MAT (don't know if the concept of MAC address translation even exists anywhere so maybe I just coined a term?).

Answered by @YLearn Why don't switches rewrite mac-addresses?

Community
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R__raki__
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  • "don't know if the concept of MAC address translation even exists anywhere so maybe I just coined a term?" it exists, google "ebtables MAC nat". I would only reccomend using it in exceptional circumstances though. – Peter Green Sep 21 '16 at 19:34