3.1 Routing
3.1.2 Static routing

The simplest form of routing is preprogrammed and, consequently, static routes. The tasks of discovering routes and propagating them throughout a network are left to the network's administrator(s). A router programmed for static routing forwards packets out of predetermined ports. After the relationship between a destination address and a router port is configured, routers do not attempt route discovery or even communicate information about routes because it is the characteristic of static routing that route discovery is not performed.

There are many benefits to using static routes. For instance, statically programmed routes can make for a more secure network. There can be only a single path into, and out of, a network connected with a statically defined route. That is, of course, unless multiple static routes are defined.

Another benefit is that static routing is much more resource efficient. Static routing uses far less bandwidth across the transmission facilities, doesn't waste router CPU cycles trying to calculate routes, and requires far less memory. In some networks, you might even be able to use smaller, less expensive routers by using static routes. Despite these benefits, you must be aware of some inherent limitations to static routing.

Simply put, static routes are configured by the network administrator, they are not learned and thus are not dynamic. It's a relatively simple table look up rather than a best-path calculation, and requires fewer resources than a dynamic protocol since the paths never change and the routers don't exchange routing information with each other.

Drawbacks to Static Routing

In the event of a network failure, or other source of topology change, the onus is on the network administrator to manually accommodate the change. Figure illustrates this point.

In this simple example, the networks' administrators have collaborated on a route redistribution scheme that they believe will minimize their workload as well as network traffic loads. The internetwork is relatively small, consisting of three different networks, one of which supports a stub network. Each network uses its own address space and a different dynamic routing protocol. Given the innate incompatibility of the three different routing protocols, the administrators chose not to redistribute routing information among their networks. Rather, they aggregated the routes into network numbers, and statically defined paths for them. Figure summarizes the routing tables of the three gateway routers. Router D connects a small, stub network to the other networks. As such, this router uses its serial port as a default gateway for all packets addressed to any IP address that does not belong to 192.168.126.

In this scenario, Router A forwards all packets addressed to any hosts within the 172.16 network address space to Router B. Router A also forwards all packets addressed to hosts within network 192.168 to Router C. Router B forwards all packets addressed to any hosts within the 192.168 address space to Router C. Router B forwards packets addressed to hosts within Network 10 to Router A. Router C forwards all packets destined for Network 10 to Router A, those packets destined for 172.16 to Router B. Additionally, Router C forwards packets addressed to 192.168.126 to Router D, its stub network. This network is a stub because it is literally a dead-end in the network. There is only one way in---and one way out. This small network depends completely on its link to Router C for connectivity to all the internetworked hosts.

In Figure , the transmission facility between Gateway Routers A and C has failed. In this example, a failure will result in unreachable destinations despite the fact that an alternative path is available for use.

The effect of this failure is that end systems in networks 10 and 192.168 cannot communicate with each other, even though a valid route exists through Router B. Figure summarizes the effects of this type of failure on the routing tables.

The lack of any dynamic mechanism prevents Routers A and C from recognizing the link failure. They are not using a routing protocol that would otherwise discover and test the qualities of the links to known destinations. Consequently, they cannot discover the alternative path through Router B. Although this is a valid and usable route, their programming prevents them from discovering or using it. This situation will remain constant until the network administrator takes corrective action manually.

What's Static Routing Good For?

At this point, you might be wondering what possible benefit there might be in statically defined routes. Static routing is good for very small networks that only have a single path to any given destination. In such cases, static routing can be the most efficient routing mechanism because it doesn't consume bandwidth trying to discover routes or communicate with other routers.

As networks grow larger, and redundant paths to destinations are added by the network administrator, static routing becomes a labor-intensive liability. Any changes in the availability of routers or transmission facilities in the WAN must be manually discovered and programmed in. WANs that feature more complex topologies that offer multiple potential paths absolutely require dynamic routing. Attempts to use static routing in complex, multipath WANs will defeat the purpose of having that route redundancy.

At times, statically defined routes are desirable, even in large or complex networks. Static routes can be configured to enhance security. Your company's connection to the Internet could have a statically defined route to a security server. No ingress would be possible without having first passed whatever authentication mechanisms the security server provides.

Alternatively, statically defined routes might be extremely useful in building extranet connections using IP to other companies that your employer does a lot of business with. Finally, static routes might be the best way to connect small locations with stub networks to your WAN. The point is that static routes can be quite useful. You just need to understand what they can and can't do.