Ethernet – Why don’t switches rewrite mac-addresses

Since we use Ethernet mostly only as a means to transport IP, is there any reason to have that extra layer of L2 overhead?

Naming a few common protocols or features that require L2 overhead such as Ethernet:

• Spanning-Tree (requires 802.2 LLC)
• ISIS (requires 802.2 LLC)
• Vlans
• ARP (which is not just for ethernet)
• Choosing between IPv4 and IPv6
• IEEE 802.11 Wifi (which shares many basic functions with 802.3 Ethernet, but is fundamentally a different protocol)

Ethernet has effectively turned into a point-to-point interconnect between an endpoint and a switch, which switches the packet based on the MAC destination. Is there any compelling argument for keeping things the way they are?

If you say Ethernet is only for addressing and point-to-point connections, you are over-simplifying the protocol. IEEE 802.3 also covers the physical layer: various forms of copper and fiber media, encoding on the wire, error recovery, line conditioning, and so on. If you add all these functions directly on IPv4, you duplicate many of Ethernet's functions and what have you really saved? This also ignores the monumental standardization and engineering effort to build these functions directly into IPv4 and IPv6. My brain hurts to think how this would work at a practical level anyway.

In the end, the argument is economics. The entire planet has engineered servers / switches / operating systems / etc around the assumption of a link-layer between IP and the signal encoding on the wire (or wifi-radio). Ethernet does a lot for us, and it's extremely cheap because it's the de facto interconnect technology for most computers on the planet. Replacing Ethernet is somewhat akin to replacing the US Congress as a governing body. It may not be perfect, but it's inconceivable to do anything else at this point.

Putting multiple IP addresses on a single interface (and thus a single MAC address) is quite common for servers.

For example: if a server has multiple roles and services running on it, it might be a good idea to give each role/service its own IP address. When you later want to move a role/service to a different machine you can do so without disrupting the other roles/services on the first server.