31 Days Before Your CCNA Exam

Day 11Troubleshooting RoutingCCNA 640-802 Exam Topics■Troubleshoot routing issues.Key TopicsDuring the past three days, we have reviewed both static and dynamic routing, including static routes,default routes, RIPv1, RIPv2, EIGRP, and OSPF. These reviews by their very nature included somebrief discussion on troubleshooting routing issues related to each of the routing methods. Today wefinish our review of routing concepts and configuration with a focus on troubleshooting routing issues.The Basic CommandsTroubleshooting routing issues may begin with basic ping and traceroute commands to discoverwhere connectivity is lacking. In the case of a large network, however, these two commands areprobably not the most efficient way to find a problem. In addition, if these commands do trackdown a problem, you still have to discover the cause.A better method might be to start with your core devices. These devices should be a collectionpoint for all routes in the enterprise. To check for missing routes and track down the reason, thefollowing method can be used for issues related to dynamic routing:1. Check the routing tables for convergence with the show ip route command. All expectedroutes should be in the routing table. Barring a security policy that prevents some routes, thedevice should be able to route to any other location in the enterprise.2. If you find a missing route or routes, use the show ip protocols command to investigate therouting protocol operation on the local router. The show ip protocols command summarizesjust about every detail of a routing protocol’s operation. Helpful information for all protocolsincludes the following:— Enable routing protocol: If the expected routing protocol is not enabled, configure it.— Routing for networks: If a network that should be advertised is missing, it could be thatthe network command is missing for that route. However, it might also be that the interfaceor interfaces that belong to that network are not functioning. If so, use show ipinterface brief to isolate problems with interfaces.— Routing information sources: This is a list of neighbors from which the local router isreceiving updates. A missing neighbor could be a problem with the local router (missingnetwork command or down interface). Or the problem could be with the neighbor. ForEIGRP and OSPF, you can use the display of neighbor relationships as a first step in discoveringwhy a neighbor is not advertising routes to the local router (via the show ipeigrp neighbors and show ip ospf neighbor commands). If neighbor relationships areoperating as expected, log in to the neighbor router to discover why the neighbor is notadvertising routes.