* For layer 2 troubleshooting, use the show interfaces command and CDP. For layer 3 troubleshooting, use ping and traceroute. For layer 7 troubleshooting, use telnet. For detailed troubleshooting, use debug.
* CDP is used to learn basic information about directly connected Cisco devices. It uses a SNAP frame format and generates a multicast every 60 seconds. It is enabled, by default, on a Cisco device.
* To execute an extended ping or traceroute, you must be at Privilege EXEC mode. Ping tests only if the destination is reachable, while Traceroute lists each layer 3 device along the way to the destination.
* To suspend a telnet session, use the CTRL-SHIFT-6, X, or CTRL-^ control sequence.
* The debug commands require Privilege EXEC access. To disable all debug commands, use no debug all or undebug all.
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