[ovs-dev] Odd gre_sys MTU setting (2.6.x vs 2.8.x)

James Page james.page at ubuntu.com
Mon Jan 15 15:17:28 UTC 2018


Hi Eric

On Mon, 15 Jan 2018 at 16:54 Eric Garver <e at erig.me> wrote:

> On Sat, Jan 13, 2018 at 03:57:08PM +0000, James Page wrote:
> > Hi Ben s
> >
> > On Sat, 13 Jan 2018 at 14:55 James Page <james.page at ubuntu.com> wrote:
> >
> > > OK, I sent a patch:
> > >>         https://patchwork.ozlabs.org/patch/860192/
> > >
> > >
> > > Thanks Ben - I'll pull this patch into the Ubuntu packages and test
> this
> > > weekend.
> > >
> >
> >  I pulled your patch in ontop of our current 2.8.1 packages and
> re-tested;
> > I'm not seeing a difference in behaviour with the patch in place. I
> removed
> > the vport_gre and ip_gre kernel modules to force a re-creation of the
> > device on restart of OVS as well as trying a reboot of the test machine:
> >
> > 7: gre0 at NONE: <NOARP> mtu 1476 qdisc noop state DOWN mode DEFAULT group
> > default qlen 1
> >     link/gre 0.0.0.0 brd 0.0.0.0
> > 8: gretap0 at NONE: <BROADCAST,MULTICAST> mtu 1462 qdisc noop state DOWN
> mode
> > DEFAULT group default qlen 1000
> >     link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
> > 9: gre_sys at NONE: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1472 qdisc
> > pfifo_fast master ovs-system state UNKNOWN mode DEFAULT group default
> qlen
> > 1000
> >     link/ether 52:ad:6d:89:bb:54 brd ff:ff:ff:ff:ff:ff
>
> Are there any errors is dmesg? It's possible setlink won't accept
> UINT16_MAX. grep for "Invalid MTU".
>

Not under 4.4 but I do under 4.13:

[    6.794079] gre: GRE over IPv4 demultiplexor driver
[    6.797621] ip_gre: GRE over IPv4 tunneling driver
[    6.798326] gre_sys: Invalid MTU 65535 requested, hw max 1500

Looks like the kernel is limiting to 1500.


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