Hello Brian, my version of openstack is Branch: statble-rocky I create an IPv6 network ipv6_net which there are two subnets A:2010:1111:2222:3:/64 and B:2011:4444:5555:6:/64 [root@ ~]# openstack network show ipv6_network +---------------------------+----------------------------------------------------------------------------+ | Field | Value | +---------------------------+----------------------------------------------------------------------------+ | admin_state_up | UP | | availability_zone_hints | nova | | availability_zones | nova | | created_at | 2019-06-05T09:08:27Z | | description | | | dns_domain | None | | id | 47da9430-d385-4369-ae8b-4fc2b8443fff | | ipv4_address_scope | None | | ipv6_address_scope | None | | is_default | None | | is_vlan_transparent | None | | mtu | 1450 | | name | ipv6_network | | port_security_enabled | True | | project_id | 5010e5f5083140e8a9e98922623d0218 | | provider:network_type | vxlan | | provider:physical_network | None | | provider:segmentation_id | 1099 | | qos_policy_id | None | | revision_number | 4 | | router:external | Internal | | segments | None | | shared | False | | status | ACTIVE | | subnets | 23366735-3f85-45cb-8320-f05048cba3ef, aa4f1fe4-bd77-468e-bc65-d867b10aedf9 | | tags | | | updated_at | 2019-06-05T09:10:05Z | +---------------------------+----------------------------------------------------------------------------+ [root@ ~]# openstack subnet show ipv6_subnet_A +-------------------+----------------------------------------------------------+ | Field | Value | +-------------------+----------------------------------------------------------+ | allocation_pools | 2010:1111:2222:3::2-2010:1111:2222:3:ffff:ffff:ffff:ffff | | cidr | 2010:1111:2222:3::/64 | | created_at | 2019-06-06T01:09:37Z | | description | | | dns_nameservers | | | enable_dhcp | True | | gateway_ip | 2010:1111:2222:3::1 | | host_routes | | | id | 70e3e0dc-bedf-4229-b60b-efa4afda0694 | | ip_version | 6 | | ipv6_address_mode | None | | ipv6_ra_mode | None | | name | ipv6_subnet_A | | network_id | 47da9430-d385-4369-ae8b-4fc2b8443fff | | project_id | 5010e5f5083140e8a9e98922623d0218 | | revision_number | 0 | | segment_id | None | | service_types | | | subnetpool_id | None | | tags | | | updated_at | 2019-06-06T01:09:37Z | +-------------------+----------------------------------------------------------+ [root@ ~]# openstack subnet show ipv6_subnet_B +-------------------+----------------------------------------------------------+ | Field | Value | +-------------------+----------------------------------------------------------+ | allocation_pools | 2011:4444:5555:6::2-2011:4444:5555:6:ffff:ffff:ffff:ffff | | cidr | 2011:4444:5555:6::/64 | | created_at | 2019-06-06T01:10:25Z | | description | | | dns_nameservers | | | enable_dhcp | True | | gateway_ip | 2011:4444:5555:6::1 | | host_routes | | | id | 7266ece7-c515-4bbe-90ff-f7cfd5d52826 | | ip_version | 6 | | ipv6_address_mode | None | | ipv6_ra_mode | None | | name | ipv6_subnet_B | | network_id | 47da9430-d385-4369-ae8b-4fc2b8443fff | | project_id | 5010e5f5083140e8a9e98922623d0218 | | revision_number | 0 | | segment_id | None | | service_types | | | subnetpool_id | None | | tags | | | updated_at | 2019-06-06T01:10:25Z | +-------------------+----------------------------------------------------------+ Now, I add subnet A to router ipv6_router. Instead of using default gateway address, I use custom address 2010:1111:2222:3:9. The command is as follows: [root@ ~]# neutron port-create --fixed-ip subnet_id=70e3e0dc-bedf-4229-b60b-efa4afda0694,ip_address=2010:1111:2222:3::9 47da9430-d385-4369-ae8b-4fc2b8443fff neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead. Created a new port: +-----------------------+--------------------------------------------------------------------------------------------+ | Field | Value | +-----------------------+--------------------------------------------------------------------------------------------+ | admin_state_up | True | | allowed_address_pairs | | | binding:host_id | | | binding:profile | {} | | binding:vif_details | {} | | binding:vif_type | unbound | | binding:vnic_type | normal | | created_at | 2019-06-06T01:14:12Z | | description | | | device_id | | | device_owner | | | extra_dhcp_opts | | | fixed_ips | {"subnet_id": "70e3e0dc-bedf-4229-b60b-efa4afda0694", "ip_address": "2010:1111:2222:3::9"} | | id | 444a8244-264b-412f-b0e7-c69472cc799c | | mac_address | fa:16:3e:71:48:81 | | name | | | network_id | 47da9430-d385-4369-ae8b-4fc2b8443fff | | port_security_enabled | True | | project_id | 5010e5f5083140e8a9e98922623d0218 | | qos_policy_id | | | revision_number | 2 | | security_groups | d0e077a8-e710-48d2-9cb1-94eb33c4273d | | status | DOWN | | tags | | | tenant_id | 5010e5f5083140e8a9e98922623d0218 | | updated_at | 2019-06-06T01:14:12Z | +-----------------------+--------------------------------------------------------------------------------------------+ [root@ ~]# neutron router-interface-add ipv6_router port=444a8244-264b-412f-b0e7-c69472cc799c neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead. Added interface 444a8244-264b-412f-b0e7-c69472cc799c to router ipv6_router. [root@ ~]# neutron router-port-list ipv6_router neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead. +--------------------------------------+-------------------------------------------------+----------------------------------+-------------------+--------------------------------------------------------------------------------------------+ | id | name | tenant_id | mac_address | fixed_ips | +--------------------------------------+-------------------------------------------------+----------------------------------+-------------------+--------------------------------------------------------------------------------------------+ | | 444a8244-264b-412f-b0e7-c69472cc799c | | 5010e5f5083140e8a9e98922623d0218 | fa:16:3e:71:48:81 | {"subnet_id": "70e3e0dc-bedf-4229-b60b-efa4afda0694", "ip_address": "2010:1111:2222:3::9"} | +--------------------------------------+-------------------------------------------------+----------------------------------+-------------------+--------------------------------------------------------------------------------------------+ So far, it's normal, and then I want to add subnet B to router ipv6_router. Instead of default gateway address, I use address 2011:4444:5555:6:9 as gateway address. [root@ ~]# neutron port-update --fixed-ip subnet_id=7266ece7-c515-4bbe-90ff-f7cfd5d52826,ip_address=2011:4444:5555:6::9 444a8244-264b-412f-b0e7-c69472cc799c neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead. Updated port: 444a8244-264b-412f-b0e7-c69472cc799c [root@ ~]# neutron port-show 444a8244-264b-412f-b0e7-c69472cc799c neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead. +-----------------------+--------------------------------------------------------------------------------------------+ | Field | Value | +-----------------------+--------------------------------------------------------------------------------------------+ | admin_state_up | True | | allowed_address_pairs | | | binding:host_id | node32 | | binding:profile | {} | | binding:vif_details | {"port_filter": true, "datapath_type": "system", "ovs_hybrid_plug": true} | | binding:vif_type | ovs | | binding:vnic_type | normal | | created_at | 2019-06-06T01:14:12Z | | description | | | device_id | bc177df9-18d3-4d5a-855c-c0244cf28f43 | | device_owner | network:ha_router_replicated_interface | | extra_dhcp_opts | | | fixed_ips | {"subnet_id": "7266ece7-c515-4bbe-90ff-f7cfd5d52826", "ip_address": "2011:4444:5555:6::9"} | | id | 444a8244-264b-412f-b0e7-c69472cc799c | | mac_address | fa:16:3e:71:48:81 | | name | | | network_id | 47da9430-d385-4369-ae8b-4fc2b8443fff | | port_security_enabled | True | | project_id | 5010e5f5083140e8a9e98922623d0218 | | qos_policy_id | | | revision_number | 10 | | security_groups | d0e077a8-e710-48d2-9cb1-94eb33c4273d | | status | ACTIVE | | tags | | | tenant_id | 5010e5f5083140e8a9e98922623d0218 | | updated_at | 2019-06-06T01:23:28Z | +-----------------------+--------------------------------------------------------------------------------------------+ [root@ ~]# neutron router-port-list ipv6_router neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead. +--------------------------------------+-------------------------------------------------+----------------------------------+-------------------+--------------------------------------------------------------------------------------------+ | id | name | tenant_id | mac_address | fixed_ips | +--------------------------------------+-------------------------------------------------+----------------------------------+-------------------+--------------------------------------------------------------------------------------------+ | 444a8244-264b-412f-b0e7-c69472cc799c | | 5010e5f5083140e8a9e98922623d0218 | fa:16:3e:71:48:81 | {"subnet_id": "7266ece7-c515-4bbe-90ff-f7cfd5d52826", "ip_address": "2011:4444:5555:6::9"} | +--------------------------------------+-------------------------------------------------+----------------------------------+-------------------+--------------------------------------------------------------------------------------------+ The result shows that the gateway IP address of subnet A is covered by gateway IP of subnet B. But I can append the gateway address of subnet B to the same interface with the "openstack port set " command. [root@ ~]# openstack port set --fixed-ip subnet=ipv6_subnet_A,ip-address=2010:1111:2222:3::9 444a8244-264b-412f-b0e7-c69472cc799c [root@ ~]# [root@~]# neutron port-show 444a8244-264b-412f-b0e7-c69472cc799c neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead. +-----------------------+--------------------------------------------------------------------------------------------+ | Field | Value | +-----------------------+--------------------------------------------------------------------------------------------+ | admin_state_up | True | | allowed_address_pairs | | | binding:host_id | node32 | | binding:profile | {} | | binding:vif_details | {"port_filter": true, "datapath_type": "system", "ovs_hybrid_plug": true} | | binding:vif_type | ovs | | binding:vnic_type | normal | | created_at | 2019-06-06T01:14:12Z | | description | | | device_id | bc177df9-18d3-4d5a-855c-c0244cf28f43 | | device_owner | network:ha_router_replicated_interface | | extra_dhcp_opts | | | fixed_ips | {"subnet_id": "70e3e0dc-bedf-4229-b60b-efa4afda0694", "ip_address": "2010:1111:2222:3::9"} | | | {"subnet_id": "7266ece7-c515-4bbe-90ff-f7cfd5d52826", "ip_address": "2011:4444:5555:6::9"} | | id | 444a8244-264b-412f-b0e7-c69472cc799c | | mac_address | fa:16:3e:71:48:81 | | name | | | network_id | 47da9430-d385-4369-ae8b-4fc2b8443fff | | port_security_enabled | True | | project_id | 5010e5f5083140e8a9e98922623d0218 | | qos_policy_id | | | revision_number | 11 | | security_groups | d0e077a8-e710-48d2-9cb1-94eb33c4273d | | status | ACTIVE | | tags | | | tenant_id | 5010e5f5083140e8a9e98922623d0218 | | updated_at | 2019-06-06T01:31:41Z | +-----------------------+--------------------------------------------------------------------------------------------+ [root@ ~]# neutron router-port-list ipv6_router neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead. +--------------------------------------+-------------------------------------------------+----------------------------------+-------------------+--------------------------------------------------------------------------------------------+ | id | name | tenant_id | mac_address | fixed_ips | +--------------------------------------+-------------------------------------------------+----------------------------------+-------------------+--------------------------------------------------------------------------------------------+ | 444a8244-264b-412f-b0e7-c69472cc799c | | 5010e5f5083140e8a9e98922623d0218 | fa:16:3e:71:48:81 | {"subnet_id": "70e3e0dc-bedf-4229-b60b-efa4afda0694", "ip_address": "2010:1111:2222:3::9"} | | | | | | {"subnet_id": "7266ece7-c515-4bbe-90ff-f7cfd5d52826", "ip_address": "2011:4444:5555:6::9"} | +--------------------------------------+-------------------------------------------------+----------------------------------+-------------------+--------------------------------------------------------------------------------------------+ Normally, multiple IPv6 subnetworks of the same network share a common routing internal interface