Fully Specified Static Route
In a fully specified static route, both the output interface and the next-hop IP address are specified. This is another type of static route that is used in older IOSs, prior to CEF. This form of static route is used when the output interface is a multi-access interface and it is necessary to explicitly identify the next hop. The next hop must be directly connected to the specified exit interface.
Suppose that the network link between R1 and R2 is an Ethernet link and that the GigabitEthernet 0/1 interface of R1 is connected to that network, as shown in Figure 1. CEF is not enabled. To eliminate the recursive lookup, a directly connected static route can be implementing using the following command:
R1(config)# ip route 192.168.2.0 255.255.255.0 GigabitEthernet 0/1
However, this may cause unexpected or inconsistent results. The difference between an Ethernet multi-access network and a point-to-point serial network is that a point-to-point network has only one other device on that network, the router at the other end of the link. With Ethernet networks, there may be many different devices sharing the same multi-access network, including hosts and even multiple routers. By only designating the Ethernet exit interface in the static route, the router will not have sufficient information to determine which device is the next-hop device.
R1 knows that the packet needs to be encapsulated in an Ethernet frame and sent out the GigabitEthernet 0/1 interface. However, R1 does not know the next-hop IPv4 address and therefore it cannot determine the destination MAC address for the Ethernet frame.
Depending upon the topology and the configurations on other routers, this static route may or may not work. It is recommended that when the exit interface is an Ethernet network, that a fully specified static route is used including both the exit interface and the next-hop address.
As shown in Figure 2, when forwarding packets to R2, the exit interface is GigabitEthernet 0/1 and the next-hop IPv4 address is 172.16.2.2.
Note: With the use of CEF, a fully specified static route is no longer necessary. A static route using a next-hop address should be used.
Use the Syntax Checker in Figure 3 and 4 to configure and verify fully specified static routes on R2 and R3.