Page 307 of Mac OS X Deployment v10.5 says to change:
in /etc/swupd/swupd.plist to cascade updates from one SUS to another. My experience has been that it's incorrect. Here's the line that works:
index.sucatalog lists the actual update packages themselves whereas we want a list of SU servers. catalogs.sucatalog looks exactly like mirror-config-1.plist, except that it lists your master SUS's address.