Firewall Mode and Security Context Mode
The ASA runs in a combination of the following modes:
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Transparent firewall or routed firewall mode
The firewall mode determines if the ASA runs as a Layer 2 or Layer 3 firewall.
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Multiple context or single context mode
The security context mode determines if the ASA runs as a single device or as multiple security contexts, which act like virtual devices.
Some commands are only available in certain modes.