Statically Defined Routes

Statically Defined Routes
A statically defined route is one in which a route is manually entered into the router. A static
route can be entered into the router with the following command in global configuration mode:
ip route
prefix mask
{
address|interface
}
[
distance
]
The parts of this command are as follows:

prefix
is the IP route prefix for the destination.

mask
is the prefix mask for the destination.

address
represents the IP address of the next hop that can be used to reach the destination.

interface
is the network interface to use.

distance
is an optional parameter that represents the administrative distance.
As you can see, with the static route you can choose to either set the next-hop address
or use a connected interface on the router. You can also set the administrative distance of
the static route. (We will explain administrative distance a little later in this chapter.) When
a static route has the administrative distance set to a value other than the default value, it
is generally done to create what is known as a floating static route. Here is an example of
a configured static route:
2501A(config)#
ip route 192.168.20.0 255.255.255.0 172.16.50.1
If you want to configure a default route, all you need to do for the destination prefix
is set it to 0.0.0.0 and set the mask to 0.0.0.0. ANDing with a mask of all 0s turns any
intended destination address into all 0s. Comparing this to the configured destination prefix
of all 0s always gets a match. The mask length, however, is the shortest possible, with no
1s set, so any other match will always be chosen. When no other matches exist, this default
route will be used, hence its name.
You then need to decide what to set your next hop to. This default route will send any packets
that do not have a match in the routing table to the next hop defined.
The advantages to using static routes in an internetwork are that the administrator has total
control of what is in the router’s routing table and there is no network overhead for a routing
protocol. Using static routes for a small network is fine. It’s not going to be hard to implement,
and you have total control in the network.
The downfall of using only static routes is they do not scale well. What do we mean by that?
Let’s look at an example of how many routes you would need to enter for the number of routers
in these different internetworks, where the routers are daisy-chained with one link between each
pair of neighbors and the two end routers have stub Ethernets, resulting in each router being
connected to two network segments:

A network with two routers would require two static routes.

A network with three routers would require six static routes.

A network with 100 routers would require 9,900 static routes.
8
Chapter 1 
Routing Principles
The generic equation is the same one used to determine the number of full-mesh links in
WAN networking:
n
(
n
– 1) or
n
2
–
n
, where
n
represents the total number of routers in the internetwork.
As you can see, as an internetwork grows, the number of static routes the administrator
needs to control becomes unmanageable. Keep in mind that any static route you add, edit,
or delete will need to be propagated across all devices. What is the alternative? The alternative
is to use a routing protocol to dynamically learn routes.