In previous OSP releases, the recommended configuration is to configure either Controller or Dedicated Networker nodes as OVN gateways. See [1], basically need to configure external bridge, exteranl_ids:ovn-bridge-mappings and external_ids:ovn-cms-options=enable-chassis-as-gw configured by THT parameters.

With RHOSP 18 Dev Preview documentation [2] we not have any instruction on how to configure this. Which leads to undesired behavior where compute nodes nodes acts as gateway chassis. This happens as there is no other chassis explicitly configured as gateway chassis so as a fallback compute nodes acts as gateway chassis as they are the only chassis with bridge mappings defined. The fallback behavior is being fixed as part of [3] , But we still need to document on how to configure ovnController for SNAT or centralized traffics.

For this we require a dedicated NIC for external traffic on OCP worker nodes and this NIC will be plugged into OVN Controller pods using nicMappings: {"<physnet_name>": "<dedicated NIC on OCP worker>"}, with this configuration ovn-operator will take care of:-

• creating network-attach-ment-definitions
• adding it to the pods
• creating external bridge
• configuring external-ids:ovn-bridge-mappings
• external-ids:ovn-cms-options=enable-chassis-as-gw is configured by default.

We also have an option to use dedicated Networker Node for hosting these gateway chassis, so need to document that as well describe use cases for that, can be taken care as part of this story or can have dedicated story for that.

 [1] https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/17.1/html-single/configuring_red_hat_openstack_platform_networking/index#create-custom-network-role-ovn_work-ovn

 [2] https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/18.0-dev-preview/html-single/deploying_red_hat_openstack_platform_18.0_development_preview_3_on_red_hat_openshift_container_platform/index#ref_default-RHOSP-networks_preparing

 [3] https://bugs.launchpad.net/neutron/+bug/2019217