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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 “bump in the wire,” or a “stealth firewall,” and is not seen as a router hop to connected devices. The security appliance connects the same network on its inside and outside ports. Because the firewall is not a routed hop, you can easily introduce a transparent firewall into an existing network; it is unnecessary to readdress IP.

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Maintenance is facilitated because there are no complicated routing patterns to troubleshoot and no NAT configuration.

Even though the transparent mode acts as a bridge, Layer 3 traffic, such as IP traffic, cannot pass through the security appliance unless you explicitly permit it with an extended access list. The only traffic allowed through the transparent firewall without an access list is ARP traffic. ARP traffic can be controlled by ARP inspection.

In routed mode, some types of traffic cannot pass through the security appliance even if you allow it in an access list. Alternatively, the transparent firewall can allow any traffic through with either an extended access list (for IP traffic) or an EtherType access list (for non−IP traffic).

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For example, you can establish routing protocol adjacencies through a transparent firewall; you can allow VPN (IPSec), OSPF, RIP, EIGRP, or BGP traffic through based on an extended access list. Likewise,protocols such as HSRP or VRRP can pass through the security appliance.

Non−IP traffic (for example, AppleTalk, IPX, BPDUs, and MPLS) can be configured to go through with an EtherType access list.

For features that are not directly supported on the transparent firewall, you can allow traffic to pass through so that upstream and downstream routers can support the functionality. For example, with an extended access list, you can allow DHCP traffic (instead of the unsupported DHCP relay feature) or multicast traffic, such as that created by IP/TV.

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. For example, if your syslog server is located on a remote network, you must use a static route, so the security appliance can reach that subnet.

You can set the adaptive security appliance to run in the default routed firewall mode or transparent firewall mode. When you change modes, the adaptive security appliance clears the configuration because many commands are not supported in both modes. If you already have a populated configuration, be sure to back up this configuration before you change the mode; you can use this backup configuration for reference when you create a new configuration.

For multiple context mode, you can use only one firewall mode for all contexts. You must set the mode in the system execution space. For multiple context mode, the system configuration is erased, which removes any contexts. If you again add a context that has an existing configuration that was created for the wrong mode, the context configuration does not work correctly.

Note: Be sure to create your context configurations for the correct mode before you add them again, or add new contexts with new paths for new configurations.

Note: If you download a text configuration to the security appliance that changes the mode with the firewall transparent command, be sure to put the command at the top of the configuration; the adaptive security appliance changes the mode as soon as the command is executed and then continues to read the configuration that you downloaded. If the command occurs later in the configuration, the adaptive security appliance clears all previous lines in the configuration.
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