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Network Troubleshooting Tools


In order to correctly diagnose networking problems in Cisco routers running IOS, it is necessary to use  and become familiar with  the various commands available  via the IOS.

When in priviliged exec mode, there are many 'show' commands available. These allow you to see the status of many aspects of the router under consideration. Below is a list of the common commands available and their effect. This list is by no means exhaustive.
Command
 Effect
Router#show running-config Displays the current configuration that is held in RAM on the router. This is not necessarily the same as the startup configuration and will need to be saved with the command copy running-config startup-config or write.
Router#show ip route
Displays the routing table contents
Router#show ip  protocols
 Displays detailed information about the routing protocols in use, the networks currently being routed and the sources of routing information
Router#show protocols Displays summary information about the routed protocol and interface status and addressing information
Router#show ip interface Displays detailed information for all interfaces
Router#show ip interface brief
 Displays summary information for each interface and its address, validity of IP address, means by which IP address was applied, status of the interface and staus of the routing protocol on this interface
Router#show controllers [interface]
 Displays detailed information relating to an individual interfaces. When used with serial interfaces, thisw will display whether the interface is DCE or DTE
Router#show debugging
 Displays the state of each debugging option





Debugging Tools

The IOS debug tool allows dynamically generated information to be displayed. Commonly used with routing protocols to show events as they happen. These tools should be used sparingly when a router is in a production environment. Always turn debugging off when you have finished. This can be accomplished by putting no before the debug command that you originally used or alternatively turn off all debugging with the command undebug all.
Router#debug ip packet
 Displays sending and arrival of all packets. Useful for analysing the messages traveling between the local and remote hosts. IP packet debugging captures the packets that are process switched including received, generated and forwarded packets.
Router#debug ip ospf  events
 Displays sending and arrival of all packets involved in the ospf  routing. Useful for analysing routing errors.
Router#debug ip ospf  adj
 Debugs OSPF neighbor adjacency establishment process. The no form of this command disables debugging output.

Router#debug ip rip
 Displays sending and arrival of RIP packets and events related to building the RIP routing table.
Router#debug ip rip events
 Displays sending and arrival of RIP packets and events related to building the RIP routing table.
Router#debug eigrp fsm
 Displays debugging information about EIGRP feasible successormetrics (FSM).
Router#debug eigrp packets
 Displays general debugging information, including packets transmitted and received.