3.1 Routing
3.1.2 Static routing
Static routing is useful in very simple networks that do not have multiple paths to any destination network. Static routing reduces the memory and processing burdens on a router. Even on large internetworks, administrators often configure static routes on access routers that connect stub networks, or networks that have only one way in and one way out. In Figure , RTZ is configured with a static route to 172.24.4.0 /24.

To configure static routing on a Cisco router, you must use the ip route command. This command uses the following syntax:

Router(config)#ip route destination-prefix destination-prefix-mask {address | interface} [distance] [tag tag] [permanent]

Figure describes the parameters that are used with this command.

You can manually add an entry to a routing table using one of two variations on the ip route command:

RTA(config)#ip route 10.6.0.0 255.255.0.0 s1

or

RTA(config)#ip route 10.7.0.0 255.255.0.0 10.4.0.2

Both of these global configuration commands will add a static route to the routing table. The first example maps a network prefix (10.6.0.0/16) to a local physical interface (S1) on the router the same way that a directly connected network is mapped to an interface. The second example maps the network prefix (10.7.0.0/16) to the next-hop address (10.4.0.2). Although these commands will both add valid static routes to the router's table, notice that the format of these two static routing table entries is different.

In Figure , the static route to 10.6.0.0 shows as a static route (notice the S at the beginning of the line), but is formatted similar to a directly connected route (even though it is not) because the router has just been configured to forward packets for 10.6.0.0 out S1. The static route to 10.7.0.0, which is configured with a next-hop address, is formatted the same way that dynamic routes are; the next-hop address is included in the routing table because the packets destined for 10.7.0.0 should be forwarded to another router's interface at 10.4.0.2. What is the difference between these two kinds of static routes?

When using a routing protocol such as RIP or IGRP, static routes that show as directly connected will automatically be advertised to other routers as long as the appropriate network command has been issued. The next-hop static route will not be advertised without additional configuration. Static routes can be included in updates if they are injected, or redistributed into the dynamic routing protocol.

When an interface goes down, all static routes mapped to that interface are removed from the IP routing table. In addition, when the router can no longer find a valid next hop for the address specified in a static route, the static route is likewise removed from the table. An alternative method is to map a static IP address to a loopback interface.

Note: As a rule, you should always use the next-hop address when defining a static route on a multi-access network such as Ethernet. A router interface on a multi-access network could have several link partners, so you must use the next-hop address to specify which neighbor should receive traffic for a given network.

Static routes are also good to use when having trouble with a routing protocol. At a multi-campus college, the ISP was upgrading its infrastructure. As each campus was converted to the new ISP infrastructure, problems occurred with the RIP routing protocol. As a quick solution, the WAN staff created static routes until the ISP issue was resolved.

Static routing does not suit large, complex networks that include redundant links, multiple protocols, and meshed topologies. Routers in complex networks must adapt to topology changes quickly and select the best route from multiple candidates. Dynamic routing is the best choice for complex networks or in any other network in which automation is preferred over human intervention.