The firewall uses virtual routers to obtain Layer 3 routes to other subnets by you manually defining static routes or through participation in one or more Layer 3 routing protocols (dynamic routes). The routes that the firewall obtains through these methods populate the IP routing information base (RIB) on the firewall. When a packet is destined for a different subnet than the one it arrived on, the virtual router obtains the best route from the RIB, places it in the forwarding information base (FIB), and forwards the packet to the next hop router defined in the FIB. The firewall uses Ethernet switching to reach other devices on the same IP subnet. (An exception to one best route going in the FIB occurs if you are using ECMP, in which case all equal-cost routes go in the FIB.)

The Ethernet, VLAN, and tunnel interfaces defined on the firewall receive and forward Layer 3 packets. The destination zone is derived from the outgoing interface based on the forwarding criteria, and the firewall consults policy rules to identify the security policies that it applies to each packet. In addition to routing to other network devices, virtual routers can route to other virtual routers within the same firewall if a next hop is specified to point to another virtual router.

You can configure Layer 3 interfaces on a virtual router to participate with dynamic routing protocols (BGP, OSPF, OSPFv3, or RIP) as well as add static routes. You can also create multiple virtual routers, each maintaining a separate set of routes that aren’t shared between virtual routers, enabling you to configure different routing behaviors for different interfaces.

You can configure dynamic routing from one virtual router to another by configuring a loopback interface in each virtual router, creating a static route between the two loopback interfaces, and then configuring a dynamic routing protocol to peer between these two interfaces. The firewall supports only one hop between virtual routers. For example, with virtual routers A, B, and C, a route cannot go from A to B to C; it would have to go from A to C.

Each Layer 3 Ethernet, loopback, VLAN, and tunnel interface defined on the firewall must be associated with a virtual router. While each interface can belong to only one virtual router, you can configure multiple routing protocols and static routes for a virtual router. Regardless of the static routes and dynamic routing protocols you configure for a virtual router, one general configuration is required.