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184 Chapter 6: IP Routing
Foundation Topics
The simplest form of routing is static routing. The chore of finding routes and propagating them
throughout a network is beyond the scope of the static route. A router programmed for static
routing forwards packets out of designated ports. After the link between a destination address
and a router port is configured, there is no longer any need for a router to communicate its
information about routes. Default static routes are entered as 0.0.0.0 and the next hop. A default
router is one that all others look to for networks that are not in their tables. Any IP network
address can be manually entered into the routing table. There are many benefits to using static
routes. For instance, statically programmed routes can make for a more secure network. In
addition, there is no overhead on the network for routing updates.
Dynamic routing happens when routers talk to neighboring routers, informing each other about
what networks each router is currently connected to. Routing decisions are based on metrics.
When a route goes down or changes, a dynamic route is automatically updated in a routing
table. Routers that perform dynamic routing communication use a routing protocol, of which
there are many to choose. Some of the dynamic routing protocols available for use include RIP,
IGRP, EIGRP, and OSPF.
Distance-vector, link-state, and hybrid are three types of dynamic routing protocols. This
chapter discusses the issues that relate to routing protocols in greater detail.
Static Versus Dynamic Routing
A stub is a section of the internetwork that can be accessed by one path. Because static routes
do not change when a link goes down, they can effectively be used to connect to a stub. Static
routes can save bandwidth if there is only one path to the network. A stub is a part of the
internetwork that can be reached by a single route. Figure 6-2 shows an example of a static route
to a stub network.
In addition, static routing is much more resource-efficient than dynamic routing. Static routing
uses no bandwidth across the links and doesn't waste router resources. On the other hand,
statically programmed routers cannot discover routes. Static routers lack the capacity to
communicate routing information with other routers. Statically programmed routers can only
forward packets using routes defined by a network administrator. As networks grow larger and
add redundant paths to destinations, static routing can become an administrative nightmare.
Any changes in the availability of routers or transmission facilities in the WAN must be
manually programmed. Networks that have complex topologies that offer multiple potential
paths require dynamic routing.
87200333.book Page 184 Wednesday, August 22, 2001 2:37 PM