A number of files got shuffled around in Mountain Lion Server. One is the swupd.plist file used to daisy chain multiple servers so they act as a cascade of software update servers. The new path for the property list is /Library/Server/Software Update/Config/swupd.plist. Here, the metaIndexURL key is sill the location that points to an internal Software Update Server that the server you are editing should look to for updates.
To set a server to look at another internal server for software updates, edit the metaIndexURL key in the /Library/Server/Software Update/Config/swupd.plist file to include the path to the new server. The path should always have /content/meta/mirror-config-1.plist after the FQDN of the host name. So if your internal software update server was called daneel.foundation.lan the command to set that as the upstream software update server would be:
defaults write /Library/Server/Software Update/Config/swupd metaiIndexURL "http://daneel.foundation.lan/content/meta/mirror-config-1.plist"
This is a minor change, but one that might be frustrating if you were still trying to cascade updates the old way. If you’re new to cascading updates, this is a pretty straight forward configuration change, run from a Terminal command. It’s also worth noting that there are a few other settings in this file that could come in handy. You can limit bandwidth using the limitBandwidth key, purge any old updates using the PurgeUnused key, set a max download speed using the maxDownloadSpeed key, configure the Software Update Server TCP port using the portToUse key (automatically set to 8088), change the path to the updates (e.g. if you mv them and then want to repoint to the new location without downloading them all again) using the updatesDocRoot key, etc. Overall, the settings align with the old settings, but just in a new place.
Note: The keys above correspond to settings found in the following command:
serveradmin settings swupdate