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Chapter 2
Routing Principles
out the complete routing table at periodic intervals. The periodic routing
updates from a distance-vector router are sent only to directly connected
routers and sent as a broadcast of Since the updates
include all routes that the sending router knows about, this is sometimes
referred to as "routing by rumor" because a router will accept information
from a neighbor as correct. The disadvantage to distance-vector protocols is
that the periodic updates consume bandwidth even if there are no topology
changes to report.
Link-state Typically called shortest path first, link-state routers create
three separate tables. One of these tables keeps track of directly attached
neighbors, one determines the topology of the entire internetwork, and
one is used for the routing table. Link-state routers know more about the
internetwork than any distance-vector protocol. An example of an IP
routing protocol that is completely link-state is OSPF.
To send routing updates, the link-state router uses a triggered-update type
of announcement. These announcements are sent from a router only
when a topology change has occurred within the network. The advantage
of link-state routing over distance-vector is that when an update occurs,
only the information about the link that changed is contained in the
There is no set way of configuring routing protocols for use with every
business. This task is performed on a case-by-case basis. However, if you
understand how the different routing protocols work, you can make good
business decisions.
Both distance-vector and link-state routing protocols are discussed in
more detail later in this chapter.
Classful Routing
he basic definition of classful routing is that subnet mask informa-
tion is not carried within the routine, periodic routing updates. This means
that every interface and host on the network must use the same subnet mask.
Examples of classful routing protocols are the Routing Information Protocol
version 1 (RIPv1) and the Interior Gateway Routing Protocol (IGRP).
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