[Bridge] bridge doesn't broadcast properly

Harald Sauff 2gr84u at gmail.com
Tue Nov 16 12:08:53 PST 2004

Hello list,

I'm setting up a virtual network consisting of several UMLs (User Mode
Linux) connected with the virtual bridge.
Communications works quite well between the host and the virtual machines.

2 UMLs connected via tap-devices to the bridge (tap1,tap2).
The host uses the bridge-device br0 to talk to the clients.

Now I want to tcpdump the traffic between host and UML1 on virtual machine UML2.

With the standard-settings UML2 only catches the initial
ARP-broadcasts (but that's the intended behaviour of the
bridge/switch). :-)

Now I set "brctl setageingtime br0 0", so that the bridge behaves like
a hub (it forgets the MAC-addresses immediately) and broadcasts all
packets on all connected ports. Checking this with "brctl showmacs
br0" shows, that this is working, the MAC-table is always empty.

BUT: on UML2 (the listening machine) I see only the complete traffic
coming from the host (via br0 into the bridge), but the answers from
UML2 to the host (entering the bridge on tap2) are hidden (only
broadcasts can be seen).

Why does the broadcasting of all packets only work from the bridge
directly (br0) and not from the connected tap-devices?

Could this have to do with the fact, that with version 1.0.4 of the
bridge-utils (Debian 3.1, testing) I can't control the GC-time of the

In the mailing-list archive I found a thread, where someone wanted the
bridge to be configured as a hub, but the advice to remove some lines
of code from the bridge-code didn't work for me, the bridge is
unusable after removing the code.

Can someone give me a hint, how to get the desired behaviour? An
option for switching between hub- and switch-behaviour would be
nice... ;-)

thanx in advance,
 Harald Sauff

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