Open Stack

Hi,
You can set MTU on the network level, see:
https://docs.openstack.org/api-ref/network/v2/index.html?expanded=create-network-detail#create-network

Lajos Katona (lajoskatona)

shubjero <shubjero at gmail.com> ezt írta (időpont: 2021. szept. 13., H,
16:28):

> 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
>
>
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