Before deleting external network in newton PUT request is sent for neutron northbound API deleting most of the provider network info.

2016-12-26 13:41:47.053 13369 DEBUG networking_odl.common.client [-] Sending METHOD (put) URL (http://10.0.97.169:8080/controller/nb/v2/neutron/networks/05ed22ed-969b-4cd9-82f5-dc5f38fa2b12) JSON ({
  "network": {
    "provider:physical_network": null, 
    "ipv6_address_scope": null, 
    "revision_number": 6, 
    "port_security_enabled": true, 
    "provider:network_type": null, 
    "router:external": true, 
    "availability_zone_hints": [], 
    "availability_zones": [
      "nova"
    ], 
    "ipv4_address_scope": null, 
    "shared": false, 
    "description": "", 
    "tags": [], 
    "updated_at": "2016-12-26T11:41:46Z", 
    "is_default": false, 
    "provider:se

The actual delete of provider port will have to take place at this phase.
Curently the L3 models are not deleted on update and can't be deleted afterwords since L2 models are already gone