Cisco IOS in a Nutshell, 2nd Edition

Chapter 16. Troubleshooting and Logging

This chapter includes a grab bag of techniques for troubleshooting and monitoring your router. The two tasks are closely related: it’s impossible to figure out what’s going wrong if you don’t have good logs that show what the router was trying (or failing) to do. We’ll start with ping and trace, two tools that are available on virtually any computer that can connect to a network.

ping

ping tests network connectivity by sending an ICMP echo-request message to the remote machine. The remote machine must respond with an ICMP reply message. The ping command takes a single argument: the address or hostname of the remote system. It prints various statistics about the responses it receives:

    Router>ping 10.10.1.2
    Type escape sequence to abort
    Sending 5, 100-byte ICMP Echos to 10.10.1.2, timeout is 2 seconds:
    !!!!!
    Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
    Router>

Each exclamation point indicates that the router has successfully received a response from the remote host. On a local network, you would expect a success rate very close to 100%. Across the Internet, you would tolerate some failures, but they should still be relatively rare. Many sites configure their routers to block ICMP traffic because it has been used in a number of denial-of-service attacks. Therefore, you can’t count on the ability to ping an arbitrary host on the Internet.

Tip

If ping locks up, you can abort by typing Ctrl+^ then x.

If you don’t give the hostname ...