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From Marcus <>
Subject Re: vxlan - mtu inside VM problem
Date Sun, 26 Oct 2014 15:52:44 GMT
Just want to clarify, with our 9216 MTU we easily run 9000 on our VMS. You
may want to set MTU to 1550 or greater on your KVM host interface used for
vxlan, and adjust the network accordingly. Most vxlan documentation for
network equipment should mention the 50 byte overhead and how to adjust for
it (if necessary).
On Oct 26, 2014 9:45 AM, "Marcus" <> wrote:

> You should instead increase MTU on the host interface to accommodate. For
> example, we used jumbo frames with an MTU of 9216 for the host interface. I
> think it wasn't mentioned because its largely assumed in CS documentation
> that the admin understands the network design they are using, i.e. you'd
> run into the same issue even without cloudstack handling orchestration if
> you were manually adding VMs to a vxlan-uplinked bridge.
> There is not much network documentation aside from the cloudstack
> tunables.  I agree though that there would be no harm in putting a note
> somewhere reminding the admin of vxlan encapsulation requirements. You can
> submit a patch for the docs to reviewboard.
> On Oct 26, 2014 7:39 AM, "Andrija Panic" <> wrote:
>> Hi folks,
>> I'm trying to figure out - why is there no documentation on the NEED to
>> configure MTU inside VM, when using vxlan as guest isolation method ?
>> Right now, by defaut/design, traffic/MTU goes like this:
>> eth0 inside VM is by default 1500 bytes --> vnetY mtu1450 --> virbrX
>> mtu1450--> vxlan mtu1450--> ethX mtu1500--> physical network (in this case
>> I use ethX as traffic label instead of bridge, vxlan interface is created
>> on top of ethX interface)
>> Inside VM, I can get IP address via DHCP, use ping, because those generate
>> packet less than 1500 bytes.
>> From within VM - i.e. SSH/SCP login works, but SCP data transfer fails,
>> yum
>> update fails, etc -
>> Any other traffic from VM to outside does not work and no other
>> connectivity, until I configure MTU inside VM to be less than 1500...
>> What is the recommended way to configure vxlan - documentation is just
>> asking for supported kernel and iproute2 versions, use ethX or bridgeX as
>> traffic label, give it IP - and that's it.
>> There must be some clear decision on how to make this works:
>> 1) eather - don't bother client configuring MTU inside VM/template, and
>> make MTU on vxlan and vnet interfaces 1500 bytes - but ask Administrator
>> to
>> increase mtu to 1600 on physical interface ethX or bridgeX
>> 2) as it currently is the case, use 1450 MTU on vnet,vxlan, and make
>> trouble for user to configure MTU for each of his VMs/templates.
>> Am I missing something here perhaps ?
>> Is there any more complete documentation on this ?
>> Best
>> --
>> Andrija Panić

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