Because the ASA 5505 license is permanently burned into the hardware chassis, the license stays with the box forever. The most cost-effective way to get an upgraded ASA 5505 is to buy one secondhand on platforms like eBay.
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If you have a legitimate PAK (Product Activation Key) from a previous purchase, you can use the Cisco Licensing Portal to generate the necessary activation key. How to Activate a Valid License Key cisco asa 5505 license key generator best
ciscoasa(config)# exit ciscoasa# write memory ciscoasa# reload Because the ASA 5505 license is permanently burned