Multicast vs. Unicast performance

Recently at a customer site, we decided to compare the performance of multicast and unicast.  The environment for the test consisted of the following pieces:

• Windows Deployment Services (WDS) server running on Windows Server 2008 R2.
• 24 Dell Precision T3400 client computers.
• A 10/100 switched network, with all computers using 1GB network adapters.
• PXE boot.
• Windows 7 Enterprise image.

The situation?  A lot of machines need to be reimaged soon.  The team will reimage about 20 or 30 machines at a time.  We wanted to determine the fastest way to accomplish the imaging.

Results
Multicast - 41 minutes
Unicast - 9.5 minutes

We monitored the network performance and WDS showed quite a bit more network usage during the multicast than during the unicast deployments.  So, based on this customer's configuration and imaging process, Unicast is the preferred choice.

There are important considerations though:

1. Is the imaging taking place during business hours?
2. Is the imaging limited to 50 computers or less at one time?
3. Does the network support multicasting?

I think there is a breakeven point, at which multicast becomes superior to unicast (even when the imaging is taking place after business hours) - but I have a feeling it is when imaging is taking place on more than 100 computers at a time.