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If I read the IEEE 802.3 spec. correctly, there has no protocol between the MAC layer and PHY layer. There's only a few signals such as signaling whether PHY is ready for transmission and whether MAC is ready for reception from the MAC client.

Is it true that there has no protocol or the likes between them?

I want to do "MAC layer"-level redundancy (i.e., multiple MACs and a single PHY), and I'm evaluating its feasibility. If there has no protocol, it would be much easier.

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  • "I want to do "MAC layer"-level redundancy (i.e., multiple MACs and a single PHY)," That will be vendor-dependent. The MAC address is "burned into" the hardware. The hardware will have space to overwrite (but not replace) the MAC address. Much is now handled in the hardware that would be done by software in the old days. How much and what is vendor dependent.
    – Ron Maupin
    Dec 13, 2023 at 16:27

2 Answers 2

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If I read the IEEE 802.3 spec. correctly, there has no protocol between the MAC layer and PHY layer. There's a few signals such as signaling whether PHY is ready for transmission and whether MAC is ready for reception from the MAC client.

That's correct: there's no protocol between any layers, each protocols serves or works on a single layer (with more or less interaction with the lower layers).

There's a service interface between the physical layer and the data link layer in Ethernet - depending on the generation, that interface is AUI, MII, GMII etc. and their derived variants. But I don't think that's what you’re asking about.

I want to do "MAC layer"-level redundancy (i.e., multiple MACs and a single PHY), and I'm evaluating its feasibility. If there has no protocol, it would be much easier.

You seem to be asking about using a virtual MAC address that can move from a master to a standby router like with VRRP, HSRP or CARP. You don't need to care about the physical layer for that.

A similar scheme can also be used with hosts but those are explicitly off topic here.

In a nutshell: don't worry about the physical layer or the intricacies of the data link layer, just configure the NIC with the virtual MAC and do your redundancy scheme on top of the NICs.

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There are protocols such as HSRP, VRRP, and GLBP which work similar in which multiple devices share a virtual MAC address and IP address. When a host attempts to reach out to the default gateway, one of the Routers will reply with the virtual MAC address. The host will send any traffic destined outside the network to that virtual MAC address. If that Router should fail, the the host will continue to send to the same virtual MAC address, but it will be handled by the newly active Router.

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