Ethernet address of second board on Sun

[Daniel R. Ehrlich]

Hans van Staveren <cs.vu.nl!sater at rutgers.uucp> writes:
>Contrary to every law in the Ethernet world Sun has decided to give
>both boards the same Ethernet address. Although this is illegal I have
>not been able yet to think of a scenario where this would lead to problems.

Try this one.  We have a single coax on which we run two different class B
networks, 128.118 and 130.203.  As far as I know this is perfectly OK.
Here is were the trouble starts.  Assume I have a Sun with three ethernet
interfaces that I would like to use as a gateway between the two class B
nets (remember these are both on the same physical wire) and a third class
B net.  Common sense would seem to indicate that I could connect two
interfaces to the same coax as long as they had different IP network
numbers (possibly even the same network number if subnetting is used).
>From the way I understand the ARP protocol to work, it expects a UNIQUE
mapping between IP addresses and Ethernet physical addresses.  Well, with
Sun setting all three interfaces to the same Ethernet address this
assumption is not valid.

Next, some other machine on the net issues an ARP broadcast for one the
the IP addresses.  Someone answers with an Ethernet address.  The
requesting machine stuffs this into the destination address of the
Ethernet packet and shoves the packet out onto the wire.  Things should
get interesting here.  As there are two interfaces listening for the same
Ethernet physical address, both should recieve the packet.  They both try
to process it.  I haven't taken the time yet to peruse the code but I do
not believe that this would do good things inside the kernel.

Can someone at Sun address this?  I spoke with someone from the hotline
(it only took ten days) <pravin at sun.com> and he was sort of in agreement
that this scenario may not work.

>If anyone can think of one we can have them sued or so :-)

Unfortunately it isn't clear from reading "The Ethernet - A Local Area
Network Data Link Layer and Physical Layer Specifications, Version 2.0,
Nov 1982" if setting all interfaces to the same address is a violation of
the specification.

The specification speaks (in section 6.2.1) of a `station' having a
uniquie 48 bit Ethernet address.  The glossary in Appendix A defines a
`station' as: "A single addressable site on the Ethernet, generally
implemented as a computer and appropriate peripherals, and connected to
the Ethernet via a controller and a tranciever."

Appendix A defines a `controller' as: "The implementation unit which
connects a station to the Ethernet, typically comprising part of the
Physical Layer, much or all of the Data Link Layer, and appropriate
electronics for interfacing the station."  

But, also in Appendix A is the following definition of a `physical
address': "The unique address value associated with a given station on the
network.  An Ethernet physical address is defined to be distinct from all
other physical addresses on all Ethernets."

So it is not at all clear that what Sun is doing violates the letter of
the specification.  It appears to me that they are violating the spirit of
the specification though.

Dan Ehrlich <ehrlich at shire.cs.psu.edu>
The Pennsylvania State University     
Department of Computer Science        
University Park, PA   16802