Way back in 2011 I wrote the post: Managing Apple Software Update Server Across Multiple Servers 10.5/10.6/10.7. This itself was based off of an Apple KB article that seems to have disappeared, but the method outlined in the post above is alive & well, just needs some tweaking for 10.8+.

Changes?

This is due to that with Server.app 2.+ Apple have started to move Servers config files into /Library/Server.

Ok, so now what?

Therefore, to point an ASUS  to an internal ASUS (also known as cascading).. follow the below;

# Backup the original swupd.plist
sudo cp /Library/Server/Software\ Update/Config/swupd.plist /Library/Server/Software\ Update/Config/swupd.plist.bak

# Change the MetaIndexURL to point to your own SUS (replace swupdate.example.com with your SUS's FQDN).
sudo /usr/libexec/PlistBuddy -c 'set metaIndexURL http://swupdate.example.com:8088/catalogs.sucatalog' /Library/Server/Software\ Update/Config/swupd/swupd.plist