OVN implements a native DHCPv4 support which caters to the common use case of providing an IP address to a booting instance by providing stateless replies to DHCPv4 requests based on statically configured address mappings. To do this it allows a short list of DHCPv4 options to be configured and applied at each compute host running ovn-controller.
OVN ML2 driver uses this feature if the config option ‘ovn_native_dhcp’ is set to True.
OVN northbound db provides a table ‘DHCP_Options’ to store the DHCP options. Logical switch port has a reference to this table.
When a subnet is created and enable_dhcp is True, a new entry is created in this table. The ‘options’ column stores the DHCPv4 options. These DHCPv4 options are included in the DHCPv4 reply by the ovn-controller when the VIF attached to the logical switch port sends a DHCPv4 request.
In order to map the DHCP_Options row with the subnet, the OVN ML2 driver stores the subnet id in the ‘external_ids’ column.
When a new port is created, the ‘dhcpv4_options’ column of the logical switch port refers to the DHCP_Options row created for the subnet of the port. If the port has multiple IPv4 subnets, then the first subnet in the ‘fixed_ips’ is used.
If the port has extra DHCPv4 options defined, then a new entry is created in the DHCP_Options table for the port. The default DHCP options are obtained from the subnet DHCP_Options table and the extra DHCPv4 options of the port are overriden. In order to map the port DHCP_Options row with the port, the OVN ML2 driver stores both the subnet id and port id in the ‘external_ids’ column.
If admin wants to disable native OVN DHCPv4 for any particular port, then the admin needs to define the ‘dhcp_disabled’ with the value ‘true’ in the extra DHCP options.
Ex. neutron port-update <PORT_ID> –extra-dhcp-opt ip_version=4, opt_name=dhcp_disabled, opt_value=false