| In this section,
several scenarios are presented using static routes and gateways of
last resort. These concepts are some of the most difficult for those
new to the nuances of IP routing to understand. To truly understand
routing, you must have a solid grasp on the fundamentals of local
domains versus nonlocal domains, gateways of last resort, summarized
routes, and redistribution of static routes and default metrics.
People who are fully
conversant in IP routing can analyze any routing table and determine
what will happen when a packet to any possible destination is
encountered.
Using Static Routes
In the scenario shown
in Figure , you can see what happens when an improper
static route has been entered to fix a routing problem.
In Figure ,
the router A routing table has been edited to show what happens when
an improper static route is added to a router. Router A has a static
route to the major network of 168.71.0.0 via 168.71.6.3. Although
this will enable router A to forward packets to 168.71.8.0, router A
cannot reach 168.71.9.0. In fact, if router A received a packet for
168.71.9.1 on its Ethernet interface, it would forward it to router
B. The proper way of doing this is shown in Figure .
In Figure ,
the routing table has been fixed to allow full connectivity to the
subnets shown. Router A now has access to all local subnets of
168.71.0.0. It also has access to a gateway of last resort and a
pointer via the summarized route that extends from 168.71.0.0 via
179.12.9.2 to other unknown subnets of 168.71.0.0 that can be found
somewhere in the rest of the world.
In Figure ,
the routing table has been changed to show what would happen if IGRP
were used in the local domain to allow full connectivity to the
subnets shown and to allow static routes to all other subnets of
168.71.0.0 and other nonlocal domains. It is easier to use a dynamic
protocol than static routes because they are easier to maintain.
Router A now has
access to all local subnets of 168.71.0.0. Router A also has a
gateway of last resort and a pointer via the summarized route that
extends from 168.71.0.0 via 179.12.9.2 to other unknown subnets of
168.71.0.0.
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