Hello, If you want to configure jumbo frames for you tenants and you are using OVN, you must ensure that the physical network(s) on which the tenant's routers are attached is(are) also in jumbo frame +50 (9050). Because OVN does not yet know how to correctly manage fragmentation. Thus, if a tenant is in mtu 1500 and the OVN router receives a packet larger than 1500 on its external link to the tenant, it will drop it. Similarly, if it receives a packet of size 9000 on the tenant's link and the external link has an MTU of 1500, it will silently drop it. If you respect this constraint, fragmentation or defragmentation will be performed on the physical router managing the north/south network. An MTU size of 9000 allows, during large north/south data transfers, to limit the number of packets to be sent or received (6 times less). Each packet must be reprocessed to add or remove the vxlan header, and these actions are performed by default by the server's CPU. Make sure you have properly set the load-balancing of the irq on all the cores or you may experience a bottleneck at a core level during heavy network traffic. There are others solutions to offload the server's CPU from network management (DPDK, ASAP2 for Mellanox NIC, SRIOV, vDPA, etc.), but these solutions are not easy to implement and bring other constraints. Vincent Le jeu. 31 juil. 2025 à 01:28, Karl Kloppenborg < kkloppenborg@resetdata.com.au> a écrit :
Hi Neutrinos,
I was discussing with a colleague about our MTU settings within our Openstack regions, generally speaking we have a physnet_global_mtu of 1550 with tenancy networks automatically provisioned as 1500 VXLAN.
This ends up with VM’s receiving DHCP MTU settings of 1500 which is fine, however reading about yesterday’s discussion on OVN performance, Nguyen’s recommendation was to enable Jumboframes.
This got me thinking, what do you set your tenancy and phynet MTU’s to?
Very interested to hear about your experiences with running 9k MTU’s on the VM networks etc, what impact this has on OVS/OVN routers and northbound connectivity being 1500.
Thanks, Karl.
Karl Kloppenborg
Chief Technology Officer
m: *+61 437 239 565* *resetdata.com <https://resetdata.com/>*
[image: reset.png]
*ResetData supports Mandatory Client Related Financial Disclosures – Scope 3 Emissions Reporting*For more information on the phasing of these requirements for business please visit; *https://treasury.gov.au/sites/default/files/2024-01/c2024-466491-policy-stat... <https://treasury.gov.au/sites/default/files/2024-01/c2024-466491-policy-state.pdf>*
This email transmission is intended only for the addressee / person responsible for delivery of the message to such person and may contain confidential or privileged information. Confidentiality and legal privilege are not waived or lost by reason of mistaken delivery to you, nor may you use, review, disclose, disseminate or copy any information contained in or attached to it. Whilst this email has been checked for viruses, the sender does not warrant that any attachments are free from viruses or other defects. You assume all liability for any loss, damage or other consequences which may arise from opening or using the attachments. If you received this e-mail in error please delete it and any attachments and kindly notify us by immediately sending an email to *contact@resetdata.com.au <contact@resetdata.com.au>*
