All things related to multicast

[Bill Nickless <>]

At 12:15 AM 8/10/2002 +0200, Havard Eidnes wrote:

> I wonder if it wouldn't be an architecturally better idea (and
> something which is likely to produce a better longer-term stable
> solution) to standardize a protocol which the L3 devices (routers)
> could use to speak to the L2 devices to control the distribution of
> multicast traffic whithin the L2 network.  That way, if one decide to
> tweak the L3 protocol, the L3-to-L2 protocol would not necessarily
> need to change.

I think you're referring to the IEEE GARP/GMRP protocol.  This protocol 
operates entirely at L2, communicating the desire of devices to receive or 
not receive frames with certain multicast MAC addresses.

See http://www.ieee802.org/1/mirror/8021/tobagi/garp-gmrp-timers.pdf for a 
basic introduction to that protocol.  The rest of this note is an excerpt 
from that document.  Note that the "multicast group" in this context is the 
IEEE 802 MAC multicast group, not the IP multicast group.

II. Description of GARP/GMRP

GARP/GMRP is a scheme defined in IEEE802.1p which allows stations to 
dynamically register/deregister for a multicast group. The design 
objectives behind GARP/GMRP include simplicity, robustness and scalability, 
consistently
with the design objectives of transparent bridges; furthermore, its design 
takes advantage of the broadcast nature of LANs whenever possible, and 
guarantees the propagation of information about the whereabouts of 
destinations throughout the entire network along the spanning tree.

The basic goal of the scheme is to determine at any point in time if a port 
in a bridge is to forward the traffic for a particular multicast group out 
on that port; this would be the case if some stations reachable by that 
port are
interested in receiving that traffic; by following this rule for all ports 
in all bridges, the interested stations would be in a position to receive 
the appropriate multicast traffic, regardless of where the sources for 
these multicast groups happen to be. To achieve this goal, the scheme makes 
use of two control messages; namely:

1.the JOIN message which is sent by a station to express its interest in 
receiving traffic destined to a group;

2. the LEAVE message which is sent by a station to express its interest in 
leaving a group.


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Bill Nickless    http://www.mcs.anl.gov/people/nickless      +1 630 252 7390
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