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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. 
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