[ovs-discuss] OVS select group bug(?)

[Ben Pfaff]

On Thu, Feb 11, 2016 at 11:56:46AM +0100, Balázs Brunáczky wrote:
> Dear All,
> 
> I've spent my last days with playing group tables, and I have a feeling that
> the select group isn't working properly or I do something wrong.
> 
> I defined the group table with
> 
> "ovs-ofctl -O OpenFlow13 add-group br0
> group_id=1,type=select,bucket=output:3,bucket=output:4"
> 
> command and added the flow with this one:
> 
> ovs-ofctl -O OpenFlow13 add-flow br0 ip,nw_dst=1.2.3.2,actions=group:1
> 
> Then I ping'ed the 1.2.3.2, but all packets go out on the 4th port instead
> of round robining between 3 and 4.
> 
> ARP requests are handled correctly, both sides know the MAC addresses.
> 
> Could you please help what I do wrong?

Select groups don't do round robin.

The FAQ says:

### Q: How does OVS divide flows among buckets in an OpenFlow "select" group?

A: In Open vSwitch 2.3 and earlier, Open vSwitch used the destination
   Ethernet address to choose a bucket in a select group.

   Open vSwitch 2.4 and later by default hashes the source and
   destination Ethernet address, VLAN ID, Ethernet type, IPv4/v6
   source and destination address and protocol, and for TCP and SCTP
   only, the source and destination ports.  The hash is "symmetric",
   meaning that exchanging source and destination addresses does not
   change the bucket selection.

   Select groups in Open vSwitch 2.4 and later can be configured to
   use a different hash function, using a Netronome extension to the
   OpenFlow 1.5+ group_mod message.  For more information, see
   Documentation/group-selection-method-property.txt in the Open
   vSwitch source tree.  (OpenFlow 1.5 support in Open vSwitch is still
   experimental.)