Cisco Systems CCNA 2 manual Examining the Routing Table, Show ip route command

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.

102 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 9 Copyright ♥ 2004, Cisco Systems, Inc.