[HECnet] Same MAC address on different nodes

[Johnny Billquist]

On 2014-05-26 02:52, Paul_Koning at Dell.com wrote:

On May 24, 2014, at 10:15 AM, Johnny Billquist <bqt at softjar.se> wrote:

...
Do I need to spell it out? :-)
The hardware address is the address the card have from factory. The physical address is the address the software have programmed the card to have. Since DECnet uses specific addresses, the address is changed from the hardware address, since you do not want/need that when running DECnet.

Not necessarily exactly that way.

Well, we are talking about old hardware here, Paul... :-)

The hardware address is the default physical address.   It is supposed to be globally unique (not just unique on each LAN).   If you have virtual devices, like in SIMH, chances are you   re responsible for this (you   re in essence the manufacturer).   Pedantically, if you administer MAC addresses, they should be from the    locally administered    address space, i.e., second bit set in the 1st byte.   In practice that doesn   t matter, but it avoids conflict with real hardware addresses.

Right. But you will be really unlucky if you manage to hit an address that you also happen to have some real hardware using, unless you explicitly sets it so. But even more, once DECnet starts up, it becomes irrelevant again, since DECnet changes the MAC address, and do not even consider retaining the ability to use the original MAC address.

DECnet Phase IV uses a physical address it supplies rather than the default.   Other protocols (including DECnet Phase V) don   t.   If your NIC type (or its driver) supports only a single physical address, the physical address changes for all protocols when you turn on DECnet Phase IV.   That   s why you have to turn on DECnet before LAT.

Right.

However... if your NIC and driver allow per-protocol physical address, then only DECnet Phase IV uses the aa-00-04-00 address and the others continue to use the hardware address.   For such systems, you have to be careful that the hardware address is unique even if DECnet is used.

DECnet on neither PDP-11 nor VAX tries any such tricks. They just assume you only will have one hardware address per interface, and sets it to what DECnet thinks it should be, and that's it. I have not checked Alpha, but I suspect it never do such a thing either. And I know that DECnet under Linux also don't play this way (or didn't last I looked). So while you are right that it could do this in theory, it is not done by anything as far as I know, and the additional complexity without any real gains outweight the potential use of such a behavior.

Most newer DEC NICs (Tulip and beyond) support multiple physical addresses, as does QNA.   UNA and LANCE do not.   Whether a particular OS/driver implements that is another matter.

simh and other things normally use libpcap, and that do not add extra addresses, but just gets the device into promiscuous mode, and then deals with it in software.

	Johnny

-- 
Johnny Billquist                                   || "I'm on a bus
                                                                  ||   on a psychedelic trip
email: bqt at softjar.se                         ||   Reading murder books
pdp is alive!                                         ||   tryin' to stay hip" - B. Idol