Re: [RRG] Re: Should the identifier be used as local locator

["William Herrin" <bill@herrin.us>]

On Mon, Jul 7, 2008 at 10:04 PM, Brian E Carpenter
<brian.e.carpenter@gmail.com> wrote:
> On 2008-07-05 13:38, William Herrin wrote:
>> The MAC address is strictly a locater. Depending on which interface
>> you approach the host from it usually has a different MAC addresses.
>> The routing system doesn't care. On the last hop, it uses the layer 3
>> locater as a key to find a layer-2 locater.
>
> The way I read the Ethernet specification (or Token Ring or FDDI for
> that matter), the MAC address is strictly an identifier - it's when
> a station sees its own MAC address in a frame that it knows the frame
> is for itself.

Brian,

You're right. I had completely forgotten that Ethernet was originally
a broadcast network. Every station saw every packet. The card's job
was to filter the packets so that the CPU only saw the ones that bore
its ID or another ID of interest such as the all-stations ID.

Do they even make gig-e and ten-gig-e hubs or has that been completely
abandoned in favor of switches? The coaxial bus is, of course, long
gone in favor of hub-and-spoke technologies like twisted pair and
fiber.

At any rate, with the exception of -optional- stateless
autoconfiguration in IPv6, the layer 2 address is not used as an
identifier by anything in layer-3 or above. As a result, layer 2 -can-
use it's address strictly as a locater and change it willy-nilly if it
wishes to, without disturbing anything above it.

Regards,
Bill Herrin





-- 
William D. Herrin ................ herrin@dirtside.com bill@herrin.us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004

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