About IPS Interfaces
This section describes IPS interfaces.
IPS Interface Types
IPS-only mode interfaces bypass many firewall checks and only support IPS security policy. You might want to implement IPS-only interfaces if you have a separate firewall protecting these interfaces and do not want the overhead of firewall functions.
The firewall mode only affects regular firewall interfaces, and not IPS-only interfaces such as inline sets or passive interfaces. IPS-only interfaces can be used in both firewall modes.
IPS-only interfaces can be deployed as the following types:
Inline Set, with optional Tap mode—An inline set acts like a bump on the wire, and binds two interfaces together to slot into an existing network. This function allows the FTD to be installed in any network environment without the configuration of adjacent network devices. Inline interfaces receive all traffic unconditionally, but all traffic received on these interfaces is retransmitted out of an inline set unless explicitly dropped.
With tap mode, the FTD is deployed inline, but the network traffic flow is undisturbed. Instead, the FTD makes a copy of each packet so that it can analyze the packets. Note that rules of these types do generate intrusion events when they are triggered, and the table view of intrusion events indicates that the triggering packets would have dropped in an inline deployment. There are benefits to using tap mode with FTDs that are deployed inline. For example, you can set up the cabling between the FTD and the network as if the FTD were inline and analyze the kinds of intrusion events the FTD generates. Based on the results, you can modify your intrusion policy and add the drop rules that best protect your network without impacting its efficiency. When you are ready to deploy the FTD inline, you can disable tap mode and begin dropping suspicious traffic without having to reconfigure the cabling between the FTD and the network.
Tap mode significantly impacts FTD performance, depending on the traffic.
Inline sets might be familiar to you as "transparent inline sets," but the inline interface type is unrelated to the transparent firewall mode or the firewall-type interfaces.
Passive or ERSPAN Passive—Passive interfaces monitor traffic flowing across a network using a switch SPAN or mirror port. The SPAN or mirror port allows for traffic to be copied from other ports on the switch. This function provides the system visibility within the network without being in the flow of network traffic. When you configure the FTD in a passive deployment, the FTD cannot take certain actions such as blocking or shaping traffic. Passive interfaces receive all traffic unconditionally. and no traffic received on these interfaces is retransmitted. Encapsulated remote switched port analyzer (ERSPAN) interfaces allow you to monitor traffic from source ports distributed over multiple switches, and uses GRE to encapsulate the traffic. ERSPAN interfaces are only allowed when the FTD is in routed firewall mode.
About Hardware Bypass for Inline Sets
For certain interface modules on the Firepower 9300 and 4100 series (see Requirements and Prerequisites for Inline Sets), you can enable the Hardware Bypass feature. Hardware Bypass ensures that traffic continues to flow between an inline interface pair during a power outage. This feature can be used to maintain network connectivity in the case of software or hardware failures.
Hardware Bypass Triggers
Hardware Bypass can be triggered in the following scenarios:
FTD application crash
FTD application reboot
Security Module reboot
Firepower chassis crash
Firepower chassis reboot or upgrade
Firepower chassis power loss
Security Module power loss
Hardware bypass is intended for unplanned/unexpected failure scenarios, and is not automatically triggered during planned software upgrades. Hardware bypass only engages at the end of a planned upgrade process, when the FTD application reboots.
Hardware Bypass Switchover
When switching from normal operation to hardware bypass or from hardware bypass back to normal operation, traffic may be interrupted for several seconds. A number of factors can affect the length of the interruption; for example, copper port auto-negotiation; behavior of the optical link partner such as how it handles link faults and de-bounce timing; spanning tree protocol convergence; dynamic routing protocol convergence; and so on. During this time, you may experience dropped connections.
You may also experience dropped connections due to application identification errors when analyzing connections midstream after the return to normal operations.
Snort Fail Open vs. Hardware Bypass
For inline sets other than those in tap mode, you can use the Snort Fail Open option to either drop traffic or allow traffic to pass without inspection when the Snort process is busy or down. Snort Fail Open is supported on all inline sets except those in tap mode, not just on interfaces that support Hardware Bypass.
The Hardware Bypass functionality allows traffic to flow during a hardware failure, including a complete power outage, and certain limited software failures. A software failure that triggers Snort Fail Open does not trigger a Hardware Bypass.
Hardware Bypass Status
If the system has power, then the Bypass LED indicates the Hardware Bypass status. See the Firepower chassis hardware installation guide for LED descriptions.