Description of problem:
When tries to create IPv6 SLAAC subnet in multisegment network it raises the FixedIpsSubnetsNotOnSameSegment error. But the subnet is actually created.


Version-Release number of selected component (if applicable):
RHOS-16.2-RHEL-8-20210216.n.2


How reproducible:
The issue is replicated in the multisegment environment (OVN).
The issue is NOT replicated if there is only one segment has been created for the multisegment network.
The issue is NOT replicated if there are no ipv4 subnets created in BOTH available segments.



Steps to Reproduce:
openstack network create --share --provider-physical-network leaf0 --provider-network-type vlan --provider-segment 777 test_net
SEGMENT=`openstack network segment list --network test_net | awk '/777/ {print $2}'`
openstack network segment set --name segment777 $SEGMENT
openstack network segment create --physical-network leaf1 --network-type vlan --segment 778 --network test_net segment778
openstack subnet create --network test_net --network-segment segment777 --ip-version 4 --subnet-range 10.77.7.0/24 --dhcp segment777-v4
openstack subnet create --network test_net --network-segment segment778 --ip-version 4 --subnet-range 10.77.8.0/24 --dhcp segment778-v4
openstack subnet create --network test_net --network-segment segment777 --ip-version 6 --subnet-range 2001:10:77:7::/64 --dhcp --ipv6-address-mode slaac segment777-v6


Actual results:

REQ: curl -g -i -X POST http://10.10.10.147:9696/v2.0/subnets -H "Content-Type: application/json" -H "User-Agent: openstacksdk/0.36.4 keystoneauth1/3.17.3 python-requests/2.20.0 CPython/3.6.8" -H "X-Auth-Token:
{SHA256}3b20e2e77647bb4d2a8e3a1de7fe7591a966d2d6d5fdc598ac9f640bf359bf02" -d '{"subnet": {"ip_version": 6, "ipv6_address_mode": "slaac", "cidr": "2001:10:77:7::/64", "segment_id": "34a289c0-2d5f-46ab-a469-353c7e
1a998d", "enable_dhcp": true, "network_id": "e646f378-0ad1-47ff-bb49-60543874b53f", "name": "segment777-v6"}}'

          
RESP: [400] Content-Length: 140 Content-Type: application/json Date: Tue, 16 Mar 2021 16:31:56 GMT X-Openstack-Request-Id: req-b6c10a3b-d1f1-4441-9db4-7172452bebc7
RESP BODY: {"NeutronError": {"type": "FixedIpsSubnetsNotOnSameSegment", "message": "Cannot allocate addresses from different segments.", "detail": ""}}