For your information, here is a set of rules to be followed for assigning security level to an interface.
To define an interface for a FWSM:
a. Enter the identification number of the Virtual Local Area Network (VLAN) associated with the FWSM
interface in the VLAN ID field. Values are 1-4095.
b. Select the Interface Enable check box.
c. Enter the interface name.
--------------------------------------------------------------------------------
Note The inside and outside interfaces are partially defined for the Global group by default;
however, you must edit the interfaces to include the additional information.
--------------------------------------------------------------------------------
d. Enter the security level that the interface will enforce. Valid values are 0-100 (100 = greatest
security level).
Outside interface is always 0.
Inside interface is always 100.
DMZ is 1-99. (Interfaces set to the same security level cannot communicate with each other.)