The ping was not success after connect 2nd router with external network in Multiple VLAN external network.

Issue Scenario:

Creating a router and connecting an external network. Verified that an external VLAN network flows are properly installed. And the ping was success. Creating a second router and connecting a same external network, that time an external VLAN network flows are override, and due to this ping was not succeed.

Steps to reproduce:

1. Create an external network
neutron net-create provider-vlan100 --provider:network_type vlan --router:external true --provider:physical_network physnet1 --provider:segmentation_id 100
neutron subnet-create --name subnet-provider-100 provider-vlan100 192.165.6.0/24 --enable_dhcp=False --allocation_pool start=192.165.6.25,end=192.165.6.45 --gateway 192.165.6.17
2. Create a router on tenant A and connect it to the external network
neutron router-create router1
neutron router-gateway-set router1 provider-vlan100

3. Create another router on tenant B and connect it to the external network too
neutron router-create router2
neutron router-gateway-set router2 provider-vlan100