In a virtual wire deployment, you install a firewall transparently on a network segment by binding two firewall ports (interfaces) together. The virtual wire logically connects the two interfaces; hence, the virtual wire is internal to the firewall.

Use a virtual wire deployment only when you want to seamlessly integrate a firewall into a topology and the two connected interfaces on the firewall don't need to do any switching or routing. For these two interfaces, the firewall is considered a bump in the wire.

A virtual wire deployment simplifies firewall installation and configuration because you can insert the firewall into an existing topology without assigning MAC or IP addresses to the interfaces, redesigning the network, or reconfiguring surrounding network devices. The virtual wire supports blocking or allowing traffic based on virtual LAN (VLAN) tags, in addition to supporting security policy rules, App-ID, Content-ID, User-ID, decryption, LLDP, active/passive and active/active HA, QoS, zone protection (with some exceptions), non-IP protocol protection, DoS protection, packet buffer protection, tunnel content inspection, and NAT.

Each virtual wire interface is directly connected to a Layer 2 or Layer 3 networking device or host. The virtual wire interfaces have no Layer 2 or Layer 3 addresses. When one of the virtual wire interfaces receives a frame or packet, it ignores any Layer 2 or Layer 3 addresses for switching or routing purposes, but applies your security or NAT policy rules before passing an allowed frame or packet over the virtual wire to the second interface and on to the network device connected to it.

You wouldn’t use a virtual wire deployment for interfaces that need to support switching, VPN tunnels, or routing because they require a Layer 2 or Layer 3 address. A virtual wire interface doesn’t use an interface management profile, which controls services such as HTTP and ping and therefore requires the interface have an IP address.

All firewalls shipped from the factory have two Ethernet ports (ports 1 and 2) preconfigured as virtual wire interfaces, and these interfaces allow all untagged traffic.