Hyper-V how to create a firewall between two virtual switches?

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http://serverfault.com – We are having a site wide networking problem that seems to be related to running a large number of VMs under Hyper-V. We have some tests that start up ~40 VMs (Windows 7 and Server 2008 machines) When all the machines are online the main network becomes unusable. Wireshark captures indicate a huge number of netbios broadcasts. As soon as the VMs are shutdown the network becomes useable again. I was thinking I should be able to better isolate the VMs from the main network. Currently they are configured to use a "External" connection and are bound to one of the adapters on the Hyper-V host. (HowTos)