Traditionally a firewall is a routed hop and acts as a default gateway for hosts that connect to one of its screened subnets. A transparent firewall on the other hand is a Layer 2 firewall that acts like a “stealth firewall” and is not seen as a router hop to connected devices.
When the security appliance runs in transparent mode, the outbound interface of a packet is determined by a MAC address lookup instead of a route lookup. Route statements can still be configured, but they only apply to security appliance-originated traffic.
Features that are not supported on transparent mode are:
-Cannot be a DHCP server but can pass relay commands with extended ACL
-only site-to-site VPN for management is supported.
-Extended ACLs can pass connections.
-SSL VPN is not supported.
-Need ACL to pass traffic
To convert the firewall from the router mode to the transparent mode you need to enter the firewall transparent