netdev - Re: multicast with multiple devices

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Message-ID: <OF839A1C25.E7A3F5EA-ON8825745F.005E9988-8825745F.005FB2EC@us.ibm.com>
Date:	Thu, 5 Jun 2008 10:25:18 -0700
From:	David Stevens <dlstevens@...ibm.com>
To:	Jan Engelhardt <jengelh@...ozas.de>
Cc:	netdev@...r.kernel.org
Subject: Re: multicast with multiple devices

netdev-owner@...r.kernel.org wrote on 06/05/2008 02:54:54 AM:

> But what if you do not have an application to begin with?
> Does not the kernel do autonomic forwarding of multicast
> like it does for unicast?

        I'm not sure I understand your question, but if I
do, then "no." The kernel uses multicasting itself on
particular interfaces (e.g., with IPv6 neighbor discovery).
Multicast packets are not forwarded unless the system
is configured as a multicast router. Being a unicast
router does not cause forwarding of multicast packets,
and multicast routers maintain a completely separate
routing table from the unicast routing table. Multicast
routers and unicast routers need not be the same set
of machines.
        Really, multicast routing has nothing to do with
unicast routing, and relying on the unicast routing
table for multicast interface selection is usually
just a source of application failures.

                                                +-DLS

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