netdev - Multicast GRE changed behaviour

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Message-ID: <003301d04a4f$8e1691e0$aa43b5a0$@canadiansignals.com>
Date:	Mon, 16 Feb 2015 20:17:53 -0500
From:	"Marc St-Onge" <marc.st-onge@...adiansignals.com>
To:	<netdev@...r.kernel.org>
Subject: Multicast GRE changed behaviour

I'm not 100% sure if the issue we are facing is related to recent changes in
ip_gre.c but while I was reading through the code I came about the comment
on line 341 and suspect some changes may have been introduced in ip_gre.c.
We have been using Multicast GRE tunnels in our organisation (we actually
call it Broadcast GRE and have given it the BGRE acronym) but essentially,
we successfully setup the tunnels exactly as described in the example.

The recent Ubuntu upgrade has caused an backward compatibility issue
somewhere between kernel 3.8 and 3.13. The previous kernel would use
Multicast to do the ARP and discovery as well as encapsulation of Multicast
user traffic and would use the end-point Unicast address to encapsulate the
Unicast user traffic between the remote nodes.  In Kernel 3.13 we are
observing that the Unicast traffic is also encapsulated in a Multicast
datagram.

Marc St-Onge

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