Re: [Xen-users] MAC - unique?

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Nuno L. Ferreira wrote:

> I know that if I install a new domU and do not specify a MAC 
> address, a xen MAC wich is still not in use will be appointed  to 
> the new domU.
> If I have a pristine image (which already has a MAC) and want to 
> make several domU's from this one, I can I know apriori if a 
> certaoim MAC address is free?
Not unless you are managing them in some way. I assume that Xen will 
pick an address at random and check that it's not already on the 
network before allocating it to a client. It's got 16 million to go 
at so there shouldn't be too much risk of a clash as long as the 
randomisation is even half good.
Obviously the MAC address must be unique across any broadcast domain, 
and also across all networks that are directly connected to any 
individual router. If done right they should be globally unique which 
would satisfy both of those conditions, but with random generated 
ones they might not be.
If you have xen guests on either side of a router, then there is the 
potential for a clash that non of the xen hosts can detect - and I;d 
guess it would confuse the router which wouldn't know where that MAC 
address belonged.
Amusingly, a while ago I was talking to someone who was responsible 
for a major desktop refresh in a local university. They had some 
really strange problems with a handful of machines and it took them a 
long time to find out that instead of 256 machines having all 256 
possible values of the last octet, there were 257 machines with 2 of 
them sharing the same MAC address (I think both had the last octet as 
00).
The big name vendor had a slight "off by one" error in their process, 
but it never showed up until someone bought a large quantity in one 
batch. The customer wasn't overly happy !
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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