An ASA in transparent mode is often referred to as a “bump in the wire,” or a “stealth firewall” because the ASA functions like a Layer 2 device and is not considered a router hop. In the figure below, the ASA is only assigned an IP address on the local network for management purposes. This mode is useful to simplify a network configuration, or when the existing IP addressing cannot be altered. However, the drawbacks include no support for dynamic routing protocols, VPNs, QoS, or DHCP Relay.
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