The OVS mechanism driver automatically adds implicit rules that allow egress to the DHCP server even if you deleted the default egress rule.

The OVN mechanism driver does not add these implicit rules. If you deleted the default egress rule in your OVS environment, your target OVN environment does not provide rules that allow DHCP and metadata traffic to reach the DHCP server. In this case, the instance would not get an IP address

We need to document how to add rules to restore DHCP access in such cases.