Subject: (IPng 7584) Re: Multicast Listener Discovery timing question

From: Cyndi Jung <[email-masked]>
Date: Wed, 02 Jun 1999 11:16:31 -0700

At 10:25 AM 6/2/99 -0700, Richard Draves wrote:
>> As I said in a previous mail, I suppose the rationale for using MLD
>> on sollicited node multicast groups is that even though 
>> multicast addresses
>> whose scope is 2 should not be routed, there can be level 2 
>> bridges/switches
>> which are performing multicast selective forwarding, and thus 
>> engage in the
>> mld protocol.
>
>Gosh, I thought that MLD should *not* be used for link-local multicast
>addresses. Our implementation does not send MLD messages when it joins
>solicited-node multicast groups.
>
>Rich

This has been the source of problems in the IPv4 multicasting.  The
IGMP specs - right from the beginning - have stated that the membership
report be issued for all but the 224.0.0.1 address, even though any
address 224.0.0.X is not to be forwarded off the local subnet.  The
intent was to facilitate efficient forwarding - and filtering - by 
the layer 2 devices that comprise the local subnet, though this was
not explicitly mentioned in the spec.  Unfortunately, some people
felt it was unnecessary to send membership reports out for those
other 224.0.0.X group addresses, and others read the spec and believed
everyone else was implementing per the spec, so some of the recent
switches with IGMP "snooping" features don't work for some of the 
routing protocols that use these 224.0.0.X protocols (OSPF, RIPv2, PIM!),
because they expect to see a membership report before they forward the
packets anywhere.

I'm sure the intelligent "layer 3 aware" layer 2 devices will be with
us for a long time, and the Users really like the effect of IGMP snooping
switches in the IPv4 networks.  If the MLD protocol is even *better* at
keeping the layer 2 devices informed of group membership - even of
groups constrained to the local subnet - it would be an improvement on
the current relationship that these devices have multicast in the IPv4 
environment.

Cyndi

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