Stateful Failover

Hi,

Just learned from Cisco Document, that if I want to set up a stateful failover configuration between two PIX515E.

A normal failover architect :

1. a failover cable
2. check if the IOS supports the failover structure by the command "sh ver"

I would like to know when the command "sh ver" is valid :

1. issue after two PIX515 are connected
2. issue this command no matter they are connected together or not

I found the "failover" function is disabled when I issued the command before two PIX515 are connected....does it mean I have to purchase a license upgrade ?

Which IOS version or above will support the failover function ?

For a stateful failover, I have to purchase:

1. A LAN interface for each PIX
2. the Logical Update ( LU ) software module for each PIX

What else I have to purchase for the stateful failover ?

Thank you very much for your kindly help.

Cheers