Unique MAC Addresses?!?!?!?!?!


I work with networked medical equipment and there are several companies tha t manufacture devices that are dual honed, where the device is specifically designed to have each NIC connected to a separate VLAN. So the manufactur ers actually assign the same MAC address to each NIC. Duh?! In response t o this, I just finished reading ISO/IEC 15802-1, and strangely, I didn't se e anything that stated, or implied that MAC addresses have to be unique.

What's up with this?

Thanks, Ed

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Yes. As well as I understand it, when the standard was made that was one of the choices. The only one I ever knew to do it was Sun.

Specifically, the address was in ROM on the CPU board, and was used by all ethernet ports on the machine.

Then again, that was before VLANs were invented.

Well, yes, they have to be unique, but is it by port or by host. Both are allowed.

Works fine unless you want two (or more) in the same LAN.

-- glen

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glen herrmannsfeldt

Some protocols encode layer 3 info into the MAC address - main one was Decnet phase 4.

A Decnet router would end up with the same MAC address on each interface on the same logical area. AFAIR XNS was similar.

Routers which also handle other protocols and alloow this stuff to be configured dynamically such as a Cisco would have multiple MACs per port - 1 native + 1 for a Decnet config on the port and so on.

A lot of VLAN capable bridging kit now operates with MAC table per VLAN (or the VLAN tag is part of the look up table)

You need this even with unique MAC addresses if 2 VLANs on the same switch are linked via layer 2 bridging elsewhere, since your unique MAC will end up being known in both VLANs.

Since that bit works, same MAC on multiple interfaces from a host will work as well by side effect

- but bridging between VLANs at that point generates huge confusion as the apparent ports for the traffic source switch around as packets arrive from different sources

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Hi Ed,

The U/L bit in the MAC address (U for universal and L for local) is never checked in bridges. It's up to the user to take care for having a unique MAC address in the VLAN domain.

There is no problem to exchange data between 2 equipments in 2 different VLAN with the same MAC address. Each VLAN domain is associated to a distinct IP subnet.

Best regards, MIchelot

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