Subject: Re: [PATCH v3 net-next 2/2] netem: add cell concept to simulate special MAC behavior

From: Rick Jones <[email-masked]>
Date: Thu, 01 Dec 2011 10:25:28 -0800

On 12/01/2011 08:57 AM, Stephen Hemminger wrote:
> On Thu, 1 Dec 2011 16:38:51 -0000
> "David Laight"<[email-masked]>  wrote:
>
>>
>>> One idea to do small delays at higher speed is to insert
>>> dummy pad frames into the device.
>>> It would mean generating garbage, but would allow for
>>> highly accurate fine grain delay.
>>
>> Not a good idea.
>> They would have to be sent to a known MAC address
>> otherwise all the ethernet switches would forward them
>> on all output ports.
>>
>> 	David
>>
>>
>
> Yes it would have to be a constant destination, not sure if there
> is a discard value in Ethernet protocol spec.

Aren't there special addresses that aren't supposed to be forwarded by 
(intelligent) switches?  IIRC LLDP uses such things.  Though the IEEE 
may take a dim view of using it for such a purpose, and knuth only knows 
what switch bugs would be uncovered that way...

http://standards.ieee.org/develop/regauth/grpmac/public.html
http://en.wikipedia.org/wiki/Link_Layer_Discovery_Protocol

rick jones