[Bridge] VLAN tagged traffic and different behavior on two CentOS 6.6 boxes

Gionatan Danti g.danti at assyoma.it
Tue Feb 10 18:30:55 UTC 2015

Hi list,
I have a bridged KVM setup where I would trunk both tagged and untagged 
traffic. In other words, some VMs will reside on the untagged portion, 
while other on the tagged one.

I have two CentOS 6.6 boxes that behave quite differently each other and 
I can not understand why. On both boxes I have:
- an eth0 interface bridged to lanbr0
- an eth0.10 interface bridged to lanbr10

 From what I read here [1] and here [2], to have a working setup I need 
to create an ebtables rules similar to "ebtables -t broute -A BROUTING 
-p 802_1Q -i eth0 -j DROP". On the first system, until I create that 
rules I can not ping the lanbr10 interface (from another system).

But on the second system, after some 20-25 seconds (less if I disable 
STP on the bridge) I can ping the lanbr10 interface even _without_ the 
ebtables rules above.

How that is possibile? Why the second box seems to auto-configure it 
while the first one need the ebtables rule? I hope this is the right list...


[1] http://ebtables.netfilter.org/misc/brnf-faq.html
[2] http://www.rackspace.com/blog/vms-vlans-and-bridges-oh-my-part-2/

Additional informations:

[root at singularity ~]# ifconfig eth0.10
[root at singularity ~]# brctl addbr lanbr10
[root at singularity ~]# brctl addif lanbr10 eth0.10
[root at singularity ~]# ifconfig lanbr10 netmask
[root at singularity ~]# brctl show
bridge name	bridge id		STP enabled	interfaces
dmzbr0		8000.00a0d213ef5d	yes		eth1
lanbr0		8000.002522027e10	yes		eth0
lanbr10		8000.002522027e10	no		eth0.10
virbr0		8000.5254005a0aac	yes		virbr0-nic

[root at kvm-black ~]# brctl addbr lanbr10
[root at kvm-black ~]# brctl addif lanbr10 eth0.10
[root at kvm-black ~]# ifconfig lanbr10 netmask
[root at kvm-black ~]# brctl show
bridge name	bridge id		STP enabled	interfaces
lanbr		8000.0022196645d4	yes		eth0
lanbr10		8000.0022196645d4	no		eth0.10

Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
GPG public key ID: FF5F32A8

More information about the Bridge mailing list