Subject: [rbridge] Should layer 2 or layer 3 IP Multicast addresses beadvertised?

From: jjoslin at belairnetworks.com (Jeff Joslin)
Date: Fri, 5 May 2006 10:43:50 -0400

On 2006-05-04, Derek Fawcus wrote:
> 
> On Wed, May 03, 2006 at 11:19:08AM -0400, Gray, Eric wrote:
> >
> > These are fair assumptions, at this time.  In fact, I would go a
> > bit further and speculate that nobody is using any MAC multicast
> > address except in a relatively small set of well-known multicast
> > addresses and address ranges - including the IP multicast MAC
> > multicast address range.
> 
> Well for IPv4 maybe,  but for IPv6 one may have a problem.
 
Eric claims "nobody is using any MAC multicast address..." but
I disagree. For example, 802.11 Access Points generate Station
Announce messages to a multicast address, and a new client's MAC
as the SA. These multicasts flood the bridged network and cause
bridge forwarding tables to be updated to the current location
of the client.

I also see network management multicasts using vendor-assigned
addresses. 

(Aside 1: the original 802.11 specification did not describe how
inter-AP mobility was supposed to work, but the consensus 
solution is to implement the multicast messages as described
above. Each vendor uses a multicast address with their own OUI
as the DA. The 802.11F Inter-AP Protocol "Best Practice" specified
the use of an ADD-Notify message using IP multicast address
224.0.1.178. But 802.11F was never widely implemented and is now
deprecated. I had a quick look at the draft mobility spec, 11r,
and could not find an equivalent message.)

(Aside 2: the IEEE 802 11s Task Group is standardizing a WLAN
mesh networking protocol. They are creating a framework to
support choices of forwarding algorithms. RBridging could be
one of those, but it has not been seriously proposed as far as
I know. Last I checked, a version of AODV was to be the
mandatory-to-support option.)

Jeff