[ovs-discuss] SSH over GRE tunnel, MTU 1500 devices in VMs on same physical host

[Orabuntu-LXC]

So just to update this in case it helps anyone.  The reason MTU 1500 was
working in this setup is because the traffic wasn't going over the GRE
tunnel which as Sherlock Holmes might have said "once you have eliminated
the impossible, whatever remains must be the truth" which is to say it's
impossible for packets of MTU 1500 to traverse a GRE tunnel,
unencapsulated, no? so then it followed that the traffic wasn't using the
tunnel.  So I just took away the tunnel entirely and found that all the LXC
containers on all the VM's on this host indeed could talk to each other
ssh, etc. just fine at MTU 1500 with no GRE tunnel needed.

This setup is VirtualBox VM's which are on the same OpenvSwitch network as
the LXC containers, with everything on the same physical host.  It was
quite simple to prove to myself simply by taking away the GRE tunnel and
observing that containers in different VM's on the same phyiscal host could
still talk with each other just fine.  I'm not a networking person
originally I've just added on iptables, switches, routers as needed to
support my Orabuntu-LXC work, so I must confess I still don't quite
understand why the containers on the VM's on same physical host can talk at
MTU 1500 without a GRE tunnel while the LXC containers in VM's or physical
hosts on different physical hosts required a GRE tunnel and of course MTU
1420, but I'm looking into it.  I'm sure it'll be a d'oh! revelation!

On Fri, Dec 1, 2017 at 1:44 PM, Gregory Rose <gvrose8192 at gmail.com> wrote:

>
> On 11/29/2017 12:12 PM, Orabuntu-LXC wrote:
>
> Hi,
>
> I have what is probably a dumb question so it should be an easy one for
> gurus.
>
> I built two VM's on VirtualBox using my Orabuntu-LXC software.  The VM
> VNIC's are ports on OvS sw1 on each VM.  The VM's are on the same physical
> host.  I have LXC containers on the sw1 switch also.  What has surprised me
> with this setup is that I can ssh between containers that are on different
> VMs and all the network devices, the VNIC's and the OvS switches, and the
> physical interface on the host, are all set to MTU 1500.  Not anywhere in
> this setup is MTU 1420 used.  My understanding was, and what I have found
> in all previous cases, was that I had to use MTU 1420 for ssh over a GRE
> tunnel to allow for encapsulation, so my question is I am wondering how can
> ssh be working over this GRE tunnel when all the MTU of all devices is set
> to 1500?
>
>
> ssh will use smaller packets  for most terminal oriented applications.
> Perhaps you're not exchanging traffic with larger packet sizes.
>
> Try iperf or something like that which will use maximum size MTUs
>
> - Greg
>
>
> TIA
>
> --
> Gilbert Standen
> Creator Orabuntu-LXC
> 914-261-4594 <(914)%20261-4594>
> gilbert at orabuntu-lxc.com
>
>
>
> _______________________________________________
> discuss mailing listdiscuss at openvswitch.orghttps://mail.openvswitch.org/mailman/listinfo/ovs-discuss
>
>
>


-- 
Gilbert Standen
Creator Orabuntu-LXC
914-261-4594
gilbert at orabuntu-lxc.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.openvswitch.org/pipermail/ovs-discuss/attachments/20171208/cced18c4/attachment-0001.html>