netdev - Re: GRE devices and MULTICAST flag.

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Date:	Fri, 23 Sep 2011 23:16:25 +0200
From:	David Lamparter <equinox@...c24.net>
To:	Ben Greear <greearb@...delatech.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: GRE devices and MULTICAST flag.

On Fri, Sep 23, 2011 at 01:42:53PM -0700, Ben Greear wrote:
> On 09/23/2011 12:56 PM, Ben Greear wrote:
> > I was under the impression that GRE tunnels could support MULTICAST
> > traffic, but the MULTICAST flag is not set. This is making the xorp
> > router crap out because it checks the MULTICAST flag and will
> > not start multicast on the interface if it's not set....
> >
> > Is it supposed to be set?
> >
> > 7: gre0: <NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN
> > link/gre 0.0.0.0 brd 0.0.0.0
> > 8: netb: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN
> > link/gre 192.168.100.223 peer 192.168.100.138
> > inet 10.0.1.1/32 scope global netb
> 
> Well, ifconfig netb multicast seems to add the flag,
> so maybe all is as it should be.

I distinctively remember that the MULTICAST flag is supposed to be an
input to the kernel, not a feature-indication flag output. I'm quite
unsure where I picked this up, but I remember using it in that way that
xorp does and then finding out it's wrong.

-David

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