When the router interfaces are added using pre-constructed neutron ports (with subnet gateway ips of course), and then when we associate such router to a L3 BGPVPN, L3 forwarding fails in datapath between VMs on the arms of such router.
Steps to create the problem:
1. Create network 1
2. Create a subnet1 on network1.
3. Boot VM1 on network 1
4. Create network 2
5. Create a subnet2 on network2.
6. Boot VM2 on network 2
7. Create a Router
8. Create a port using neutron-port-create
with name as say "port-router-net1-interface" and fixed-ip as subnet gateway ip of subnet1.
9. Create a port using neutron-port-create
with name as say "port-router-net2-interface" and fixed-ip as subnet gateway ip of subnet2.
10. Attach "port-router-net1-interface' to the router using
neutron router-interface-add Router port=port-router-net1-interface
11. Attach "port-router-net2-interface" to the router using
neutron router-interface-add Router port=port-router-net2-interface
12. Now make sure VM1 pings VM2. It may fail (or) succeed 
13. Create a L3 BGPVPN.
neutron bgpvpn-create --route-targets 40:4 --import-targets 40:1 --route-distinguishers '400:1' --tenant_id $(keystone tenant-list | grep '\sadmin' | awk '
14. Associate the router to this myBgpVpn1.
neutron bgpvpn-net-assoc-create myBgpVpn1 --network $(neutron router-list |grep '\sRouter' |awk '{print $2}
')
15. Now again ping from VM1 to VM2 and it FAILS.