I have been working on a Lync project. Instead of using topology builder to design the topology directly, I used a tool named Lync 2010, planning tool. This tool is not only more user friendly but also provides prescriptive guidance.
Here is how I use Lync planning tool to design Lync in my sandbox environment.
Part one:
1) Create a new design, tick the features that you are going to deploy in your environment.
2) Type in the internal SIP domain.
3) Enable virtualization if you are going to deploy Lync on virtual machines.
4) Conference settings. I leave the settings as default.
5) Enable ‘External User Access’ if you want user to be able to access Lync outside the internal network. Deploying a director role server is recommended if you allow the external access.
6) Check the servers that will be deployed as virtual machines.
7) Click yes if you have more than one site.
8) Click draw to start to draw the topology.
Part Two:
1) Double click ‘sandbox’ the site I just built.
2) Click ‘Site Topology’ tab to check the topology diagram.
Double click each object to read the details.
firewall settings
3) Click ‘Edge Network Diagram’ tab, then double click each object and edit your settings.
I have made the changes according to my environment
4) Click ‘Edge Admin Report’ to read detailed reports.
5) Click ‘Site Summary’ tab and read the hints in the Topology Considerations.
Part Three:
1) Export the draw to a Topology Builder file.
2) Launch Topology Builder and import the file. Type in the missed FQDN.
When you changed your ips from different subnets to the same subnet have you had any issues with performance on audio and video conversations?
I have not seen any issues. I guess it might be caused by a high bandwidth utilization in your LAN, check your network monitoring to see what is happening.