The best replacement for isa server is squid.

The best alternatives to isa server are rated as following:
★★★★★  squid
★★  cisco asa
★★  hardware firewall
★  bluecoat
★  tmg
  proxy server

Your alternative for isa server is not listed? Suggest!
Speaking about isa server, one probably shouldn't forget about dhcp.

Other search terms related to isa server and their ratings are:
★★★★★ dhcp
You have in mind a companion for isa server and it is not yet here? Propose!



Google Trends

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isa server
squid
cisco asa