In a next-hop static route, only the next-hop IP address is specified. The output interface is derived from the next hop. For example, in Figure 1, three next-hop static routes are configured on R1 using the IP address of the next hop, R2.
Before any packet is forwarded by a router, the routing table process must determine the exit interface to use to forward the packet. This is known as route resolvability. The route resolvability process will vary depending upon the type of forwarding mechanism being used by the router. CEF (Cisco Express Forwarding) is the default behavior on most platforms running IOS 12.0 or later.
Figure 2 details the basic packet forwarding process in the routing table for R1 without the use of CEF. When a packet is destined for the 192.168.2.0/24 network, R1:
1. Looks for a match in the routing table and finds that it has to forward the packets to the next-hop IPv4 address 172.16.2.2, as indicated by the label 1 in the figure. Every route that references only a next-hop IPv4 address and does not reference an exit interface must have the next-hop IPv4 address resolved using another route in the routing table with an exit interface.
2. R1 must now determine how to reach 172.16.2.2; therefore, it searches a second time for a 172.16.2.2 match. In this case, the IPv4 address matches the route for the directly connected network 172.16.2.0/24 with the exit interface Serial 0/0/0, as indicated by the label 2 in the figure. This lookup tells the routing table process that this packet is forwarded out of that interface.
It actually takes two routing table lookup processes to forward any packet to the 192.168.2.0/24 network. When the router performs multiple lookups in the routing table before forwarding a packet, it is performing a process known as a recursive lookup. Because recursive lookups consume router resources, they should be avoided when possible.
A recursive static route is valid (that is, it is a candidate for insertion in the routing table) only when the specified next hop resolves, either directly or indirectly, to a valid exit interface.
Note: CEF provides optimized lookup for efficient packet forwarding by using two main data structures stored in the data plane: a FIB (Forwarding Information Base), which is a copy of the routing table and an adjacency table that includes Layer 2 addressing information. The information combined in both of these tables work together so there is no recursive lookup needed for next-hop IP address lookups. In other words, a static route using a next-hop IP requires only a single lookup when CEF is enabled on the router.
Use the Syntax Checker in Figures 3 and 4 to configure and verify next-hop static routes on R2 and R3.