Is there a easy way to migrate the configuration from a ASA to a FWSM.
There is no tool to migrate from a ASA to FWSM but the migration path is pretty much straight forward.
Other than taking a slot in the Catalyst 6500 or 7600, power supply and a physical port on one of the modules to connect to the network, the FWSM has its own processor, code, etc. The only major difference is ASA is physical inteface and sub interfaces and FWSM is all about VLANS.
What version of code are you planning to run on the FWSM. FWSM 3.1 is based on Pix 7.0 code. If you already have an ASA Configuration, go the FWSM Configure the VLAN Interfaces and you should be able to pretty much copy/paste most of the stuff. Ofcourse, if you have any IPSEC, IPS configuration, they will not work on the FWSM.
All the best.
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