Inside the WSUS 3.0 SP2 Operations Guide document in Appendix E (p123) I found this very cool nugget. This applies to WSUS deployments where updates are not downloaded, stored, and distributed locally.
Assuming all the clients are online and scheduled to query and sync for Windows Updates (WSUS, WU, MU) together you could greatly benefit from only downloading a patch once. For my peers in Australia who pay for bandwidth used this may be quite valuable.
Peer caching is a new feature of BITS 3.0 that allows peers (computers within the same subnet of a network that have the peer caching feature enabled) to share files. If peer caching is enabled on a computer, the Automatic Update agent instructs BITS to make downloaded files available to that computer’s peers as well.
When the files have been downloaded, BITS caches them. When another (peer caching-enabled) computer tries to download the same update, BITS on that computer sends a multicast request to all of that computer’s peers. If one or more of the peers responds to the request, BITS will download the file from the first computer to respond. If the download from the peer fails or take too long, BITS continues the download from the WSUS server or Microsoft Update.
This feature of BITS can optimize the bandwidth used by WSUS in several ways.
1. Peer caching decreases the amount of data transferred from the WSUS server to its clients, because computers in the same subnet will usually download the updates from each other.
2. Peer caching decreases the amount of data transferred across the WAN when some or all of the clients of a WSUS server are located in different locations.
3. Peer caching decreases the amount of data transferred across the Internet if WSUS clients in the same subnet are configured to download updates from Microsoft Update.
BITS peer caching requires computers to be running Windows Vista or Windows Server 2008, and to be part of an Active Directory Domain.
To enable peer caching (on Windows Vista)
1. Start the Group Policy Object Editor (click Start, click Run, and then type gpedit.msc).
2. Expand Computer Configuration, then Administrative Templates, then Network, then Background Intelligent Transfer Service.
3. Enable the Allow BITS Peercaching setting.
4. Enable the Maximum network bandwidth used for Peercaching setting, and set the maximum bandwidth in bits per second (the default is 104857), then click OK.
5. Enable the Limit the BITS Peercache size setting, and set the percentage of disk space to be used for the peer cache (the default is 5 percent), and then click OK.
6. Enable the Limit age of items in the BITs Peercache setting, and set the number of days (the default is 90), and then click OK.
You must be an administrator to perform this procedure.
**author note** besides modifying the applicable GPO on Vista you could also do this from a Server 2008 server. Vista is the first client OS to provide domain scope GPO management.