9000MTU on bridge with some VMs running on default? (1500)

daemonix

Member
Dec 12, 2021
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Hi,

I have a 10Gbe Unifi network linking proxmox's (one DAC SPF working as bridge - vlan aware) TrueNAS, pfsense and some other VMs with the important workstations.
With some initial testing I had a throughput limit with the default MTU and truenas (SSD vDevs) so I enabled jumbo frames in the network and on the bridge interface plus the truenas VM.

All all other VMs/pfsense is on 1500 p.size.

Is there an issue (from proxmox's point of view) if you run 9000 on the bridge and some VMs but not all?

Thanks
 
Last edited:
Some systems set the "do not fragment" with MTU 9000. That means that any networking-equipment in between is not allowed to split packets into several 1500 packets
= everything needs to support and have enabled jumbo frames

In case the "do not fragment" bit is not set, fragments may be split to accomodate the networking infrastructure in between, but that would lead to even worse performance (additional work to split the 9000er packets).
So in some cases MTU 9000 doesn't yield a performance gain - if it works for you, nice!

-> https://en.wikipedia.org/wiki/IP_fragmentation


But no, it should be no problem if some VMs still use 1500. Jumboframes is like a 5-lane highway, and now your other VMs still only use two - it works.
 
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Some systems set the "do not fragment" with MTU 9000. That means that any networking-equipment in between is not allowed to split packets into several 1500 packets
= everything needs to support and have enabled jumbo frames

In case the "do not fragment" bit is not set, fragments may be split to accomodate the networking infrastructure in between, but that would lead to even worse performance (additional work to split the 9000er packets).
So in some cases MTU 9000 doesn't yield a performance gain - if it works for you, nice!

-> https://en.wikipedia.org/wiki/IP_fragmentation


But no, it should be no problem if some VMs still use 1500. Jumboframes is like a 5-lane highway, and now your other VMs still only use two - it works.
true. to be honest Im still not 100% sure I cant live without them... Ill internal network is ok with 9000 (hopefully). Its all unifi. from vbr0(+vlans) for the pfsense and out its 1500. So I was thinking that this might not be the best idea. Ill re-test.
 

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