[openstack-dev] [Nova][Neutron] Linuxbridge as the default in DevStack [was: Status of the nova-network to Neutron migration work]

Assaf Muller amuller at redhat.com
Fri Apr 10 17:46:00 UTC 2015

----- Original Message -----
> On Fri, Apr 10, 2015, at 12:31 AM, Kevin Benton wrote:
> > I mentioned this in my email in the previous thread, but I am also
> > concerned about the use of the Linux bridge plugin as the default for
> > devstack.
> > 
> > It will reflect poorly on the Neutron project because we are defaulting
> > to
> > something that gets almost no development effort and that is not even
> > gated
> > on (other than unit tests). This is a risky move that can damage
> > first-time
> > users' opinions of the viability of OpenStack. I wouldn't feel confident
> > about something that has defaults that could be broken at any time...
> > even
> > during a release.
> > 
> > Can someone point me to the list of complaints about OVS? I would rather
> > invest time in addressing those issues rather than ignoring everything a
> > good chunk of the neutron community has spent significant time on.
> As someone that just spent a large chunk of this week debugging OVS in
> order to get multinode testing of Neutron with DVR [0] running without
> breaking existing multinode testing of nova net with multihost here is
> what I have learned.
> OVS documentation is terrible. There is no cohesive documentation that
> explains the operation and use of OVS to the user. There are dev docs
> [1], there are man pages for a large number of ovs-* commands [2], there
> are "cookbooks" [3], and there are blog posts from luckier souls than me
> that actually got OVS to work [4]. What I would really like to see is a
> cohesive set of docs that explains the operation of OVS (please do point
> me at them if they exist I just haven't found them).
> There are a large number of commands, so far I have found ovs-appctl,
> ovs-dpctl, ovs-ofctl, and ovs-vsctl. Unfortunately, as a user I am
> unsure what specifically each command exists for. I know they are ovs
> related and they control things. Even reading the man pages I am not
> quite sure what I would use each of them for. I think this is largely
> due to the lack of cohesive documentation that explains things like
> datapaths and how ovs-dpctl might be useful to control them.
> tcpdump does not work on OVS interfaces. I just about gave up at this
> point. To get packet captures I have to set up a veth, add it to the OVS
> bridge, then mirror the the OVS interface onto this new port and tcpdump
> the other end. This is non trivial [5] and given the lack of docs I
> wouldn't expect users to just know how to do this. Also, this isn't a
> physical switch in a datacenter. I shouldn't have to pretend I am moving
> cables around and setting up a sniffer device. I am running linux I
> should be able to just tcpdump a device.
> ARP stops working [6]. OVS appears to be intentionally dropping arp who
> has requests. A switch without ARP is not useful. (The setup here is an
> OVS bridge on one VM connected to an OVS bridge on another VM via GRE
> similar to [3], one bridge has ip address and the other
> has Pinging from does not work
> because ARP does not work).

The only ARP-drop flow I can think of is when DVR installs flows that drop
ARP requests to the DVR routers interfaces on br-tun. This is because the
local router is supposed to answer ARP requests to its interfaces.
You shouldn't have any other flows that drop ARP messages. That being said,
I just got devstack multinode DVR working. The configuration is detailed at:

I did not run in to any issues apart from a bug I reported which was since

> This is basically where I am stuck. Armax has been a tremendous amount
> of help, but we still don't have this working. I think I would be much
> less frustrated if the linux bridge setup that we already have for nova
> network testing with multiple routers didn't just work for nova net. Its
> easy to understand, you can tcpdump the interfaces, and ARP who has is
> properly broadcast to all interfaces on the same l2.
> Thats my rant. I do want to stress that I am very interested in making
> Neutron DVR testing on multiple nodes work upstream, and I understand
> that OVS is likely the way to do this (so please help). I have less of a
> fight in the discussion of devstack defaults, but infra is looking at
> building a cloud and based on my experiences this week I am a giant -1
> on using OVS anywhere in that deployment.
> > 
> > If OVS really is a non-starter because of lack of tooling and lack of
> > deployer experience, we (the neutron community) will need to put
> > significantly more efforts into automated testing and feature parity
> > (e.g.
> > re-implement DVR).
> [0] https://review.openstack.org/#/c/171809/
> [1] https://github.com/openvswitch/ovs/blob/master/README.md
> [2] http://openvswitch.org/support/dist-docs/ovs-dpctl.8.txt
> [3] http://openvswitch.org/support/config-cookbooks/port-tunneling/
> [4]
> http://blog.scottlowe.org/2013/05/07/using-gre-tunnels-with-open-vswitch/
> [5] http://www.yet.org/2014/09/openvswitch-troubleshooting/
> [6] http://paste.openstack.org/show/201479/
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