[Bridge] Are multicast Ethernet frames supported in Linux bridge?

Stephen Hemminger stephen at networkplumber.org
Wed Mar 25 17:59:03 UTC 2015

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: >  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: >  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.

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