netdev - Re:

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Date:	Mon, 27 Sep 2010 13:54:36 -0700
From:	Bob Arendt <rda@...con.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	David Stevens <dlstevens@...ibm.com>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"netdev-owner@...r.kernel.org" <netdev-owner@...r.kernel.org>
Subject: Re:

On 09/27/10 13:23, Christoph Lameter wrote:
> On Mon, 27 Sep 2010, David Stevens wrote:
>
>>          Because a querier can set the robustness value and
>> query interval to anything you want. In the original report,
>> he's not running a querier. The fact that it's a new group
>> doesn't matter -- these are per-interface.
>
> The per interface settings are used to force an IGMP version overriding
> any information by the queriers. You would not want to enable that because
> it disables support for other IGMP versions. Without the override
> different version of IGMP can be handled per MC group.
>
If a network vlan has IGMPv3 capability, then it should be able
to support both v2 and v3 Joins (clients).  But if the vlan is
IGMPv2 only, then an initial Join from a Linux client might go out
as v3 (if it hasn't seen a query yet) and be ignored.  I believe
this is the case that force_igmp_version really addresses.

And it turns out that  force_igmp_version=2 doesn't fully work.
If the host sees a IGMPv3 query, it still responds with a v3 Join
despite the flag.  Bug report and candidate patch here:
https://bugzilla.kernel.org/show_bug.cgi?id=18212
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