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Learning Cisco's Hidden Commands

written by: George Garza•edited by: Michele McDonough•updated: 4/24/2010

Cisco makes routers and switches, two main networking devices that connect users across many different network domains. The commands to configure the devices are well known to any CCNA, CCNP, or CCIE certificate holder. But there are a series of hidden commands that can be explored as well.

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    Cisco Commands

    The configuration of Cisco hardware is performed from the command line. Unlike other hardware devices that have a GUI (Graphical User Interface) to use for configuration, Cisco makes no attempt at making it easy. Indeed, there are hundreds of commands that a Cisco Engineer must learn in order to configure the device. These commands, moreover, are frequently not intuitive – at least not intuitive enough to make one say that if you learn one, you can learn the others because they are similar.

    Most common commands can be discovered with the "?". But hidden commands cannot be uncovered with that marker, "?". The Tab key used for autocomplete cannot be used either. Hidden commands are not documented in Cisco PDFs or on websites. Some of the output is used strictly for engineering purposes. Finally, they are not supported by Cisco, which means that the results cannot be guaranteed.

    Cisco hidden commands were put in place by engineers who were designing the Internetwork Operating System (IOS). They were interested in testing the IOS to see if it was performing as expected.

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    Cisco Hidden Commands: Exec Commands

    The following commands are hidden from users at any privilege level in a Cisco router or switch device. Some commands are informative, but others offer nothing of substance. If used incorrectly, some will even lock the router.

    Exec commands

    These commands are general and are used to provide information about the configuration.

    • @clear profile – This clears cpu profiling.
    • @debug par mo – This debugs parser modes.
    • @sh chunk – This shows the chunks of memory that are allocated to processes.
    • @sh idb – This shows the interface database.
    • @sh in stats – This will provide the switching path on the interface.
    • @sh parser modes – This identifies the access-tree associated with the current process.
    • @sh region – This will display the image layout.
    • @sh proc all-events – This will display all process events.

    This short series can be beneficial to the Cisco engineer only if the current command set is lacking. But, that is unlikely given that the normal commands are well documented.

    Engineers, use these commands to test the Exec mode and display information about how the feature set works in this environment. Once a command is run, it will display certain results. The hidden commands are used in addition to the normal commands to see how the regular commands affect other operating system elements.

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    Cisco Hidden Commands: Configuration Mode Commands

    In the configuration mode, the essentials of router or switch operations are defined. This is the most important mode necessary to prepare the device to run programs or protocols.

    • @scheduler max-task-time 200 – Get the last value in milliseconds.
    • @scheduler heapcheck process – To validatee memory after processing.
    • @scheduler heapcheck poll – To check if memory is valid after polling.
    • @scheduler run-degraded – To test the scheduler in a failure mode.
    • @service log backtrace – This provides a traceback mechanism for every logging instance.

    This set of hidden commands can be used for additional configuration information. They will provide additional information that is not normally found after running the normal set of commands.

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    Summary and Sources

    Cisco routers and switches are the two most common appliances used by network administrators. To configure both appliances, administrators must learn a bevy of commands that are well documented and are supported by Cisco. The hidden commands are usually commands used by the Cisco Engineers that designed the IOS. They work primarily to test and verify the performance of the operating system that will be released to the public. Once the IOS is released, the hidden commands are not necessary, so they remain undocumented. Nevertheless, they are there.

    Sources:

    Elemental Network Undocumented Cisco Commands

    CISCO Undocumented Commands

    MyUC500