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  From: Matthew Tippett <mtippett@ticons.com.au>
  To  : Richard Sharpe <sharpe@ns.aus.com>
  Date: Wed, 7 Apr 1999 02:09:44 +0930 (CST)

Re: Duplicate MAC addresses

> 2. Duplicate MAC addresses on different segments separated by a router:
> 
>     11:22:33:44:55:66                      11:22:33:44:55:66
>            -----------------[Router]------------------ 
> 

As it turns out, dual homed Sun systems do exactly this.  Present the
MAC address to be *precisely* the same on either network.  This is 
irrespective of whether the machine is packet forwarding or not.

Taken from 'arp -a' from a Sun box at work.

hme0   gols              255.255.255.255 SP    08:00:20:95:e3:ed
hme1   gols-dmz          255.255.255.255 SP    08:00:20:95:e3:ed
hme0   10.55.32.2           255.255.255.255       00:e0:1e:34:b1:f1


Two different interfaces on different networks, with the *same* MAC
address.

The interesting thing is how IP on Ethernet routes.  The IP layer
and above stays almost exactly the same no matter where on the network
the two communicating hosts are.

When the routing logic decides that the remote host is not local (and
hence doesn't have a local MAC address) it uses the gateway machines MAC
address in the packet, effectively presenting the gateway with a packet
that contains the IP information for a remote host, but addressed MAC wise 
to the gateway.  

If the router finds that the host is on one of the joining networks, it will
then place the hosts MAC address in the packet and send it to the recepient
host.

Using a worked example... I will use the following definitions for the hosts.

	Host A - hme0 - 10.0.0.2 - 08:00:20:95:e3:ed
	       - hme1 - 10.0.1.2 - 08:00:20:95:e3:ed
	Host B - hme0 - 10.0.0.3 - 08:00:20:95:e3:ef
	       - hme1 - 10.0.1.3 - 08:00:20:95:e3:ef
	Router - if0  - 10.0.0.1 - 08:00:20:95:e3:ee
	       - if1  - 10.0.1.1 - 08:00:20:95:e3:ee

A packet will be simplified as IP/MAC as 0.2/e3:ed.  Shorthand for 
host/interface will be A0 = Host A, hme0

The router is present on both networks.  Both networks have the same 
interface names on them.

a0 talking to b0

1) a0 is on the same network as b0, so the packet becomes

	Source = 0.2/e3:ed  Destination - 0.3/e3:ef

	One hop!

a0 talking to b1

1) a0 not on b1's network.  hence a0 must use the router's if0.

	Source = 0.2/e3:ed Destination - 1.3/e3:ee

2) router gets the packet and then forwards it to b1 through if1.

	Source = 0.2/e3:ee Destination - 1.3/e3:ef

So as you can see the MAC address changes, while the IP address stays the same,
depending on the interface that the devices are talking to they are still 
distinct hosts and addresses.  

Probably clear as mud now :)

Regards,

Matt 
-- 
Matthew Tippett          - +61 416 006 047     - mtippett@ticons.com.au
Linux Support Initiative - BETA Testers Wanted - http://lsi.ticons.com.au/
Tippett Information Consulting Pty Ltd -       - http://www.ticons.com.au/

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