In our company we can use two internet connection, one is satellite and other is DSL connection ? DSL connection connect to juiper SSG 20 firewall then connect to 3 com switch. 2nd Satellite connection is connected to ISA 2006 server then connected to 3com switch? Both connection connected to 3Com switch all lan connection is connected to 3 com switch. But sometimes internet is not working why ?

You need to decide which one to use for your computers. Your computers will not perform any type of "load balancing". ISA Server can only get you there part of the way by performing an active/passive failover between two ISP connections. There is an option to automatically turn on a backup Internet connection when the first one fails.

I do not think that an Upstream server configuration will work either. You may want to query the folks at ISAServer.org - Microsoft ISA Server Firewall Resource Site: Articles & Tutorials.