Subject: Question about RFC 3306

From: "Mario Goebbels" <[email-masked]>
Date: Tue, 29 Apr 2003 13:08:46 +0200

I have a question about those unicast-prefix based multicast addresses. Since the whole prefix is embedded in the multicast address, are those multicast groups globally reachable from whatever other subnet? Let's say if on prefix 3FFE:FFFF:1::/48 I generate the multicast prefix FF3E:0030:3FFE:FFFF:0001::/96, and thus let's assume want to send a packet to all nodes on that prefix, e.g. address FF3E:0030:3FFE:FFFF:0001::1 (assuming it's the same as in e.g. FF02::1), would this work?

Thanks for any further information.

-mg

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From: Brian Haberman <[email-masked]>
Date: Tue, 29 Apr 2003 07:54:23 -0400

Mario Goebbels wrote:
> I have a question about those unicast-prefix based multicast
> addresses. Since the whole prefix is embedded in the multicast
> address, are those multicast groups globally reachable from whatever
> other subnet? Let's say if on prefix 3FFE:FFFF:1::/48 I generate the
> multicast prefix FF3E:0030:3FFE:FFFF:0001::/96, and thus let's assume
> want to send a packet to all nodes on that prefix, e.g. address
> FF3E:0030:3FFE:FFFF:0001::1 (assuming it's the same as in e.g.
> FF02::1), would this work?

The reachability of the multicast address is determined by the
scope field in the multicast prefix.  In your case, FF3E is a
globably scoped address and could be routed as such.

However, nodes are not required to join ANY multicast address
generated using this mechanism.  So, you would not be able to
reach all nodes in the 3FFE:FFFF:1::/48 prefix by sending a
multicast message to FF3E:0030:3FFE:FFFF:1::1.

Regards,
Brian