Page 307 of Mac OS X Deployment v10.5 says to change:

<key>metaIndexURL</key>
<string>http://swscan.apple.com/content/meta/mirror-config-1.plist</string>

to

<key>metaIndexURL</key>
<string>http://sus.pretendco.com:8088/index.sucatalog</string>

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:

<key>metaIndexURL</key>
<string>http://sus.pretendco.com:8088/catalogs.sucatalog</string>

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.