Subject: Re: [PATCH] macvlan: Set IFF_UNICAST_FLT flag to prevent unnecessary promisc mode.

From: Vlad Yasevich <[email-masked]>
Date: Thu, 07 Mar 2013 15:43:43 -0500

On 03/07/2013 03:32 PM, Vlad Yasevich wrote:
> On 03/07/2013 03:27 PM, Ben Greear wrote:
>> On 03/07/2013 12:21 PM, Vlad Yasevich wrote:
>>> Macvlan already supports hw address filters.  Set the IFF_UNICAST_FLT
>>> so that it doesn't needlesly enter PROMISC mode when macvlans are
>>> stacked.
>>
>> Out of curiosity, how can stacking mac-vlans be useful?
>>
>> (You are stacking more macvlans on a base mac-vlan?)
>
> Yep, and macvlans allows, but it looks like it only tries to detect a 2
> level stack.  I am not sure what the reason is.  Might be realated to
> vlan-to-vlan communication.

Aha.  The reason is explained in this commit.

commit b0832a2961022a076c812384435b5f0290b3fc91
Author: Eric Biederman <[email-masked]>
Date:   Fri Mar 13 13:15:37 2009 -0700

     macvlan: Support creating macvlans from macvlans

     When running in a network namespace whose only link to
     the outside world is a macvlan device, not being
     able to create another macvlan is a real pain.

     So modify macvlan creation to allow automatically forward
     a creation of a macvlan on a macvlan to become a creation
     of a macvlan on the underlying network device.

I don't know if it's useful to support more then 2 levels.

-vlad

> -vlad
>> Thanks,
>> Ben