Fundamentals of Routing
Dynamic routing is the process of using protocols to find and update routing
tables on routers and to maintain a loop-free, single path to each network.
This is easier than static or default routing, but you use it at the expense of
router CPU processes and bandwidth usage on the network links. A routing
protocol defines the set of rules used by a router when it communicates
between neighbor routers.
Once the router process knows the metric values of each path, then rout-
ing decisions are made. When a route is learned from different sources, the
router will first choose the route with the lowest administrative distance. If
two routes have the same AD, then the router will use the routing metrics to
determine the best path to the remote network. If the AD is the same in both
routes, as well as the metrics, then the routing protocol will load balance.
There are two types of dynamic routing protocols used in internetworks:
Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP).
IGP routing protocols are used to exchange routing information with routers
in the same autonomous system (AS). An AS is a collection of networks
under a common administrative domain. EGPs are used to communicate
between ASes. An example of an EGP is the Border Gateway Protocol (BGP),
which is discussed in Chapters 8 through 9.
There are two classes of dynamic routing protocols:
Distance-vector The distance-vector protocol uses the distance to a
remote network as a determination of the best path to a remote network.
Each time a packet goes through a router, it's called a hop. The route with
the least number of hops to the remote network is determined to be the
best route. The vector is the determination of direction to the remote
Examples of a distance-vector protocol are RIP and IGRP.
However, not all distance-vector protocols use hop count in their metric.
IGRP uses bandwidth and delay of the line to determine the best path to a
remote network. It is considered a distance-vector protocol because it sends
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