1. What is the problem
Our current mechanism to handle dhcp port creation is that we first
create the top dhcp port, then use the allocated ip to create the
bottom dhcp port. If we get an "IpAddressInUse" error, meaning that
bottom dhcp agent has already created the dhcp port with the same
ip, we just reuse the port; otherwise, we successfully create the
bottom dhcp port, we remove other dhcp ports with different ips, so
dhcp agent can be notified and use the bottom dhcp port we create.
However, we find that this mechanism doesn't work in the following
situation. Dhcp agent may create the bottom dhcp port after we do
the check about whether other dhcp ports exist, so we don't remove
the dhcp port created by dhcp agent, thus we end up with two bottom
dhcp ports, one is created by dhcp agent, one is created by us.
2. What is the solution to the problem
Create bottom subnet with dhcp disabled, so dhcp agent will not be
scheduled, then create bottom dhcp port and update bottom subnet
to enable dhcp. Finding that there is already a reserved dhcp port,
dhcp agent will not create another dhcp port and directly use the
existing one.
3. What the features need to be implemented to the Tricircle
to realize the solution
No new feature introuduced.
Change-Id: I8bb8622b34b709edef230d1f1c985e3fabd5adb0
5f0978ef7b