102 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 9 Copyright © 2004, Cisco Systems, Inc.
9.1 Examining the Routing Table Essential Labs: 9.1.1, 9.1.2, and 9.1.8
Optional Labs: None
Core TIs: All
Optional TIs: none
Course-Level Claim: Students can configure, verify, analyze, and troubleshoot simple
distance vector routing protocols.
Certification-Level Claim: Students can troubleshoot and configure routing protocols based
on user requirements.
Hands-on skills: none
9.1.1 The show ip route command
One of the primary functions of a router is to determine the best path to a given destination. A
router learns paths from the configuration or from other routers through routing protocols. They
use RAM to store this routing information in routing tables. A routing table contains the best
available routes to destinations. The show ip route command displays the contents of the
IP routing table. The routing table contains entries for all known networks and subnetworks
and a code that indicates how that information was learned. Discuss how valuable the show
ip route command is to network troubleshooting.
Routes can be added to a router through two methods:
• Static routing – An administrator manually defines routes. These routes do not
change until a network administrator manually programs the changes.
• Dynamic routing – Routers follows rules defined by a routing protocol to
exchange routing information. These routes change automatically as neighboring
routers update each other with new information.
Discuss with the students the differences between static routing and dynamic routing. This is
an important concept for the students to understand. The instructor should also emphasize
that the router would not know what to do with a packet if there was no route to forward it
toward the destination.
9.1.2 Determining the gateway of last resort
Routers do not maintain routes to every possible destination. Instead, routers can keep a
default route, or a gateway of last resort. The router will use this default route to forward a
packet to a different router. Default routes can be statically entered by an administrator or
dynamically learned through a routing protocol. Before routers can dynamically exchange
information, an administrator must configure at least one router with a default route.