When creating a NAD with topologies "localnet" or "ipBlock" no validation is done over the spec fields.
When creating a NAD with the wrong spec field of value, the NAD is created successfully but nothing is really changing in the configuration on the cluster due to the faulty definition. This can cause a lot time waste on debugging from the user side.

4.15.0-rc.5

100%

1. Create a namespace:
oc create new-project overlay-default-brex

2. Create a NAD with a wrong spec field (wither the key or the value) - here it is the topology value:
cat << EOF | oc create -f -
apiVersion: k8s.cni.cncf.io/v1
kind: NetworkAttachmentDefinition
metadata:
  name: localnet-network-brex
  namespace: overlay-default-brex
spec:
  config: |
    {
      "cniVersion": "0.3.1",
      "name": "localnet-network-brex",
      "type": "ovn-k8s-cni-overlay",
      "topology":"overlay",
      "mtu": 1500,
      "netAttachDefName": "overlay-default-brex/localnet-network-brex"
    }     

The NAD is created without a warning or error, but nothing changed in the actual network configurations - no new network was created.

Some validation and prevention mechanism the will show an error message and won't create the NAD.