How to add additional routers for different external networks into one private network

Issue

• How to add additional routers for different external networks into one private network
  An image of the goal in this solution is below.

                     [Instance1]
                          |
                          |
  private1  -------+------+------------+-------------
                   |                   |
                [router1]          [router2]
                   |                   |
  external1 -------+------             |
                                       |
  external2                    --------+-------------
  

• Following steps were conducted but the mapping second router to external2 external network always failed.

  1. Create networks including private and external networks.
  2. Create a subnet for each network.
  3. Create two routers.
  4. Create a router for external1 with 'neutron router-interface-add router1 external1' then it succeeded.

  5. Create a router for external2 with 'neutron router-interface-add router2 external2 ' then it failed. 'neutron router-interface-add ' always returns 'the address is in use error' if the private network already has a router for an external network.

     # neutron router-interface-add router2 private1
     Unable to complete operation for network 1ecc7ee2-457c-42ad-a8af-47defc48976b. The IP address 10.0.0.1 is in use.