Current status and considerations:

Following the solution https://access.redhat.com/solutions/5876771, the custome define several static routes to reach common services (e.g.: DNS servers) and apply them to the Master nodes [1] of first OCP clusters deployed in 4.12 (if not in a more old version).

The UPI OCP clusters node were based on SDN newtorking with IPv4 address and default gateway statically assigned (no DHCP) and static routes attached to the exiting network interface: ens192.

In OCP 4.16, migrating from SDN (ip address assigned to ens192) to OVN (ip address moved to br-ex) the static routes have been lost because not more applicable to the ens192 because this interface was changed to unnumbered.

$ oc patch Network.operator.openshift.io cluster --type=merge \
>   --patch '{
>     "spec":{
>       "defaultNetwork":{
>         "ovnKubernetesConfig":{
>           "gatewayConfig":

}}}}'

To solve this issue, 
the Customer is available to replace the current IPv4 default gateway associated to the br-ex with a new one capable to reach all remote IP targets.

But, there isn't a clear vision of how to replace the default gateway IPv4 address with a new one in an OCP 4 environment based on OVN networking.

On this topic there are two solutions: 

• https://access.redhat.com/solutions/7025487 
  This solution is probably based on an SDN environment (network interface ens32)

• https://access.redhat.com/solutions/7014876 
  This solution seems not covering the Customer case where, without changing the br-ex interface, only the IPv4 of default gateway is changed (e.g.: from 26.0.164.1 to 26.0.164.254).

Theoretically, the nmcli dev modify br-ex could be also used to update the IPv4 default gateway (e.g.: nmcli command applied via MachineConfig).

OCPBUG scope:
Identify the right approach and syntax (e.g.: using the Wants, After, Before, and WantedBy arguments of the service invoked via MachineConfig) to correctly replace the IPv4 default gateway in an OCP 4.16+ environment based on OVN networking, ensuring proper operation during day two management.