[ovs-discuss] How does MTU work with tunnels?

Heim, Dennis Dennis.Heim at wwt.com
Tue Sep 10 14:07:18 UTC 2019


Your MTU on VXLAN needs to allow for give or take around 50 bytes. Either, you would need to move up to larger frames (9k frames) or decrease your MTU on the end stations. If I could I would have enabled Jumbo frames, but that wasn't an option for various reasons. My understanding is by default you get 1500 byte ethernet frames. Then, you add tunnel overhead ~50 bytes, which varies between VXLAN, GENEVE, GRE. Then you get a packet with 1550 bytes. Your underlay network (physical network), needs to support MTU of 1550 or greater. Otherwise you get fragmentation, and unexpected performance. I am not a network engineer, so this is mostly what I learned through sleepless nights.

Dennis Heim | Domain Architect (Collaboration Labs)
World Wide Technology, Inc. | +1 314-212-1814



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-----Original Message-----
From: Benjamin <benjamin.reis at vates.fr> 
Sent: Tuesday, September 10, 2019 9:49 AM
To: Heim, Dennis <Dennis.Heim at wwt.com>; ovs-discuss at openvswitch.org
Subject: Re: [ovs-discuss] How does MTU work with tunnels?

Thanks for your quick answer.

But i'm not sure I understand what you're saying: max MTU for VxLAN would be 1320?

Le 10/09/2019 à 15:26, Heim, Dennis a écrit :
> I run VXLAN over a DMVPN setup, as it is part of our Lab/PoC architecture (distributed). From an MTU perspective, Windows is happy with 1360, but some linux/apache servers require it set to 1320 to work properly.
>
> Dennis Heim | Domain Architect (Collaboration Labs) World Wide 
> Technology, Inc. | +1 314-212-1814
>
>
>
> "The most powerful person in the world is the story teller. The 
> storyteller sets the vision, values and agenda of an entire generation 
> that is to come" - Steve Jobs "Leadership isn't a different maker. It 
> is the difference maker" - Tim Kight "Leaders who don't listen will 
> eventually be surrounded by people who have nothing to say" --- Andy 
> Stanley "Worry less about who you might offend, and more about who you 
> might inspire" -- Tim Allen "Imagination is more important than 
> knowledge."  -- Albert Einstein "If you can raise the level of effort 
> and performance in those around you, you are officially a leader" - 
> Urban Meyer "The greatest danger for most of us is not that our aim is 
> too high and we miss it, but that it is too low and we reach it." -- 
> Michelangelo Buonarroti "Mediocore managers play checkers (assuming 
> everyone is the same). Great managers play chess (acknowledging that 
> everyone is unique)" - Marcus Buckingham "If you're not failing every 
> now and again, it's a sign you're not doing anything very innovative" 
> - Woody Allen
>
> Click here to join me in my Collaboration Meeting Room
>
> -----Original Message-----
> From: ovs-discuss-bounces at openvswitch.org 
> <ovs-discuss-bounces at openvswitch.org> On Behalf Of Benjamin
> Sent: Tuesday, September 10, 2019 9:04 AM
> To: ovs-discuss at openvswitch.org
> Subject: [ovs-discuss] How does MTU work with tunnels?
>
> Hi all,
>
> I working with openvswitch 2.5.3 (tried with 2.11.0 same behavior) on XCP-ng (XenServer fork).
>
> I'm trying to undersand how the mtu works with GRE and VxLAN tunnels.
> As for now when I create a GRE tunnel with any MTU, I think the MTU is not taken in consideration because i can ping -s 15000 on the corresponding interface.
> However, with VxLAN if I set a MTU greater than 1450 then the MTU is
> 1450 and if I set a MTU lower than 1450 I have same behavior as for GRE.
>
> So I'm a but confused.
>
> When I check the mtu value with : ovs-vsctl get int xapi0 mtu_request or ovs-vsctl get int xapi0 mtu the returned value is what I expect.
>
>
> Thanks in advance,
>
> Benjamin
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