AFAIK in order for Cisco PIX Failover functionality to work, you need to ensure that both units are the same model and have the same software version, encryption licenses, Flash memory, and the same RAM. Failover between the Cisco PIX 515E and 515 is not supported.
You need replace both of then with Cisco PIX 515E.
This link will tell you the difference between 515 and 515E and also have some FAQ about the 2 and also have an answer if you can use the 2 different model for to work together in a redundant hot standby pair.
PIX 515 has reached EOS status so i dont think you will be able to get that from Cisco instead you can think off going on for 515E which is the recommended replacement for the same.
But are you currently running any kinda loadbalancing/failover between them ?
if thats the case then you need to think off going on for similar models to achieve that.becoz thats the one of the main requirements before configuring failover or loadbalancing between them.
"Hardware Requirements for failover"
The two units in a failover configuration must have the same hardware configuration. They must be the same model, have the same number and types of interfaces, the same amount of Flash memory, and the same amount of RAM.
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