Linux Ethernet Bridging - Mar 2015 - [Bridge] Are multicast Ethernet frames supported in Linux bridge?

Hi,

I have a network topology where in "Dell PE860" runs a Linux
virtual-switch br0:

http://s24.postimg.org/j1un2gs9h/virtual_bridge.png

Now if I send an Ethernet frame to broadcast address from "IBM
ThinkCentre":

17:10:23.569021 00:a1:ff:01:02:05 > ff:ff:ff:ff:ff:ff, ethertype IPv4
(0x0800), length 34: 127.0.0.1 > 127.0.0.1:  ip-proto-0 0

..then I see this frame in both virtual-machines as I should. If I
send an Ethernet frame to MAC address which is not known in br0 MAC
address table, then the br0 also behaves correctly and floods the
frame to all ports expect to one where the frame came in(eth1 in this
example). However, if I send a multicast frame from "IBM ThinkCentre":

17:17:05.513283 00:a1:ff:01:02:05 > 01:33:44:55:66:77, ethertype IPv4
(0x0800), length 34: 127.0.0.1 > 127.0.0.1:  ip-proto-0 0

..then for some reason Linux virtual-switch does not flood it to all
the ports(except the one where the frame came in from). Why is that
so? I would expect that switch handles multicast frames exactly like
broadcast frames.


thanks,
Martin

On Wed, 25 Mar 2015 11:01:57 +0200
Martin T <m4rtntns at gmail.com> wrote:
> Hi,
> 
> I have a network topology where in "Dell PE860" runs a Linux
virtual-switch br0:
> 
> http://s24.postimg.org/j1un2gs9h/virtual_bridge.png
> 
> Now if I send an Ethernet frame to broadcast address from "IBM
ThinkCentre":
> 
> 17:10:23.569021 00:a1:ff:01:02:05 > ff:ff:ff:ff:ff:ff, ethertype IPv4
> (0x0800), length 34: 127.0.0.1 > 127.0.0.1:  ip-proto-0 0
> 
> ..then I see this frame in both virtual-machines as I should. If I
> send an Ethernet frame to MAC address which is not known in br0 MAC
> address table, then the br0 also behaves correctly and floods the
> frame to all ports expect to one where the frame came in(eth1 in this
> example). However, if I send a multicast frame from "IBM
ThinkCentre":
> 
> 17:17:05.513283 00:a1:ff:01:02:05 > 01:33:44:55:66:77, ethertype IPv4
> (0x0800), length 34: 127.0.0.1 > 127.0.0.1:  ip-proto-0 0
> 
> ..then for some reason Linux virtual-switch does not flood it to all
> the ports(except the one where the frame came in from). Why is that
> so? I would expect that switch handles multicast frames exactly like
> broadcast frames.
> 
> 
> thanks,
> Martin
Linux bridge does IGMP snooping (by default).
Therefore you need an application that uses IGMP advertisements
or manually disable this via sysfs.