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[mininet-discuss] More than 256 ovs switches

Gary Brown uvugdbrown at
Tue Dec 3 21:45:56 PST 2013

Thank you to everyone who replied to me privately.
I have not run a controller, so I assume the default controller is
So far, all I had done was define a 3D torus topology in Mininet using a
topo module I wrote, which worked fine for a smaller network as long as the
quantity of vSwitches does not seem to exceed 128-132 or so.
However, when running Mininet from a putty window and while it was setting
the ARP table for the hosts (one host per vSwitch) to prevent an ARP storm,
my putty windows (I had two open) would get a "PuTTY Fatal Error" message
of "Network error: Software caused connection abort".  At that point they
were completely non-responsive and I had to close them.
When I tried to reconnect with a new putty window, I got a "Network error:
Connection timed out" error and could only use the VM console.
Through the console I later found Mininet (mn) was still running along with
576 "bash" processes (one per 288 hosts and 288 vSwitches) and according to
top, ovs-vswitchd was consuming ~75% of the CPU and ovsdb-server was
consuming the remaining 25%, which probably explained the putty windows
I found when I ran Mininet in the VM console window, that I had no problem
creating many switches.  It appears when creating the static ARP tables
after creating the hosts and then creating the switches and their links
that the OVS daemons mentioned above basically kill the VM's performance
and I lose the non-console windows, which was not apparent until I
attempted to hit a key (e.g. ctrl-c) in a window and it timed out.
Mininet is creating the large number of switches I wanted; I just cannot do
it from any window excepts the VM's console window as the other windows do
not get any CPU cycles in a timely manner.
After discovering this problem and finding I have to use the VM's console,
I have been able to create large numbers of switches.  To use other windows
to the VM, I just have to wait until after Mininet has finished creating
the hosts and switches and linked everything up before I can use
non-console windows successfully.  The fact the windows became unresponsive
had appeared to me as a Mininet failure, when in fact, it had not failed at
all, just the windows had.
Well, just another odd set of facts learned!

Gary D. Brown

On Tue, Dec 3, 2013 at 8:10 PM, Brian O'Connor <bocon at>wrote:

> Sorry, fixLimits is actually called the first time a Mininet object is
> instantiated (either using "mn" or in a custom script).
> If you are using a custom script and you are using a Mininet object, you
> should be able to create 500+ switches (although it might take awhile).
> If you are using the lower level API (creating Host and Link objects
> directly), then you may need to call fixLimits to limit the file descriptor
> limit.
> - Brian
> On Tue, Dec 3, 2013 at 7:04 PM, Brian O'Connor <bocon at>wrote:
>> Hi Gary,
>> The limit appears to be related to the file descriptor limit.
>> When you invoke "mn" from the command line, we run util.fixLimits to
>> increase the number of allowed file descriptors.
>> If you are running a custom Python script, you can try:
>> from mininet.util import fixLimits
>> fixLimits()
>> - Brian
>> On Thu, Nov 28, 2013 at 7:19 AM, Gary Brown <uvugdbrown at> wrote:
>>> I am trying to recreate an actual network topology experiment I did this
>>> past summer that requires 288 vSwitches, but when I use Mininet to create
>>> the network, I get an exception at about 256 switches (highly suspicious)
>>> and Mininet cleans everything up and quits.
>>> I have seen posts on this mailing list indicating people are creating
>>> experiments with more than 256 switches, up to 1024 and even 1200.  My
>>> online searching has indicated ovs switches have a limit of 256 switches
>>> (see Limits at bottom of webpage at
>>> ).
>>> Has anyone been able to create more than 256 ovs switches and if so, how
>>> did you do so, or at least how did you raise the limit?
>>> I am using the standard Mininet 2.1.0 VM running Ubuntu 13.04 inside
>>> VirtualBox 4.2.18 r88780 on a Windows 7 laptop with Core i7 (Ivy Bridge)
>>> and 32 GB of memory.
>>> --
>>> Gary
>>> M.S. student, University of Utah
>>> _______________________________________________
>>> mininet-discuss mailing list
>>> mininet-discuss at
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