The routing protocols use different metrics
to measure the distance and desirability of a path to a destination network. Because it is not
possible to directly select from information provided by different routing protocols, the
Cisco routing process assigns a weight, known as the administrative distance, to each
source of information. The best, most trusted source has the lowest number.
Routing Tables
As part of the path determination process, the routing process builds a routing table that
identi?¬?es known networks and knows how to reach them. Figure 4-3 shows how a routing
table maintains network locations.
Which Path?
Exploring the Functions of Routing 241
Figure 4-3 Routing Tables
Routing metrics vary depending on the routing protocol used. Figure 4-3 shows how routers
keep a table of information used to decide how to forward packets.
Routing Table Information
The routing table consists of an ordered list of ???known??? network addresses??”that is, those
addresses that have been learned dynamically by the routing process or the statically
con?¬?gured, directly connected networks. Routing tables also include information on
destinations and next-hop associations. These associations tell a router that a particular
destination is either directly connected to the router or that it can be reached through
another router, called the next-hop router, on the way to the ?¬?nal destination.
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