I cannot reproduce this with neutron/master::

----

ubuntu@diag:~/devstack$ neutron net-show net1
+---------------------------+--------------------------------------+
| Field                     | Value                                |
+---------------------------+--------------------------------------+
| admin_state_up            | True                                 |
| availability_zone_hints   |                                      |
| availability_zones        |                                      |
| created_at                | 2017-01-12T14:12:57Z                 |
| description               |                                      |
| id                        | 0b60bfa7-08e6-4ce6-8a71-a51aa6065b4c |
| ipv4_address_scope        |                                      |
| ipv6_address_scope        |                                      |
| mtu                       | 1450                                 |
| name                      | net1                                 |
| port_security_enabled     | True                                 |
| project_id                | db997839c32842fab5fe7f390e8015c8     |
| provider:network_type     | vxlan                                |
| provider:physical_network |                                      |
| provider:segmentation_id  | 47                                   |
| revision_number           | 3                                    |
| router:external           | False                                |
| shared                    | False                                |
| status                    | ACTIVE                               |
| subnets                   |                                      |
| tags                      |                                      |
| tenant_id                 | db997839c32842fab5fe7f390e8015c8     |
| updated_at                | 2017-01-12T14:12:57Z                 |
+---------------------------+--------------------------------------+
ubuntu@diag:~/devstack$ neutron net-update net1 --router:external=True
Updated network: net1
ubuntu@diag:~/devstack$ neutron net-show net1
+---------------------------+--------------------------------------+
| Field                     | Value                                |
+---------------------------+--------------------------------------+
| admin_state_up            | True                                 |
| availability_zone_hints   |                                      |
| availability_zones        |                                      |
| created_at                | 2017-01-12T14:12:57Z                 |
| description               |                                      |
| id                        | 0b60bfa7-08e6-4ce6-8a71-a51aa6065b4c |
| ipv4_address_scope        |                                      |
| ipv6_address_scope        |                                      |
| is_default                | False                                |
| mtu                       | 1450                                 |
| name                      | net1                                 |
| port_security_enabled     | True                                 |
| project_id                | db997839c32842fab5fe7f390e8015c8     |
| provider:network_type     | vxlan                                |
| provider:physical_network |                                      |
| provider:segmentation_id  | 47                                   |
| revision_number           | 4                                    |
| router:external           | True                                 |
| shared                    | False                                |
| status                    | ACTIVE                               |
| subnets                   |                                      |
| tags                      |                                      |
| tenant_id                 | db997839c32842fab5fe7f390e8015c8     |
| updated_at                | 2017-01-12T14:13:21Z                 |
+---------------------------+--------------------------------------+
ubuntu@diag:~/devstack$


----

I'll have to retry this with newton