Hey Openstackers, I am trying to determine how to configure/manipulate an OpenStack instances MTU configuration upon deployment/creation. I know that in neutron.conf you can set the 'global_physnet_mtu = value' and 'path_mtu = value' in ml2_conf.ini. In our case we are setting these to 9000 as the physical interfaces of our control plane and compute nodes are all using 9000 mtu however the bonded interfaces for our various networks are 8900. With this mtu configured, we've noticed that our OpenStack neutron networks within a tenancy are auto-selecting a mtu of 8958 as from what I've read it will take the 9000 mtu and minus the approriate overhead to pass the underlying physical network properly but I think because our bonded interfaces are 8900 this has started to introduce some problems in instances with operating systems that hard code the MTU from Neutron networks. When an instance gets deployed it may (Ubuntu 20.04) or may not (Ubuntu 18.04) retrieve the network mtu from neutron metadata service and builds it in to the netplan configuration as provided below. My question is, where in Neutron could I configure a lower MTU for the instances? I am not sure if I should change my global_physnet_mtu and path_mtu to 8900 OR some other configuration option? Thanks all! ---- # curl --silent http://169.254.169.254/openstack/2018-08-27/network_data.json | jq . { "links": [ { "id": "tap4318b50a-76", "vif_id": "4318b50a-7685-4777-917f-eea947aa1cb6", "type": "ovs", "mtu": 8958, <--- Matches the MTU of the underlying OpenStack network attached to this instance "ethernet_mac_address": "fa:16:3e:a4:19:44" } ], "networks": [ { "id": "network0", "type": "ipv4_dhcp", "link": "tap4318b50a-76", "network_id": "4acb077c-0656-4731-ba42-031a22af7931" } ], "services": [] } ---- # cat /etc/netplan/50-cloud-init.yaml # This file is generated from information provided by the datasource. Changes # to it will not persist across an instance reboot. To disable cloud-init's # network configuration capabilities, write a file # /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following: # network: {config: disabled} network: version: 2 ethernets: ens3: dhcp4: true match: macaddress: fa:16:3e:a4:19:44 mtu: 8958 <--- MTU gets hard-coded in the config for Ubuntu 20.04 but not Ubuntu 18.04. set-name: ens3