Cisco Catalyst 2960 Series, Configuration Manual

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This error means that an internal EEM request for memory failed.
(_cerr_sub_err = 11) FH_ENOSUCHESID (unknown event specification ID)
This error means that the event specification ID could not be matched when the event was being registered
or that an event detector internal event structure is corrupt.
(_cerr_sub_err = 22) FH_ENULLPTR (event detector internal error - ptr is null)
This error means that an internal EEM event detector pointer was null when it should have contained a value.
(_cerr_sub_err = 26) FH_ESUBSIDXINV (invalid subscriber index)
This error means that the subscriber index was invalid.
(_cerr_sub_err = 54) FH_EFDUNAVAIL (connection to event detector unavailable)
This error means that the event detector was unavailable.
(_cerr_sub_err = 56) FH_EFDCONNERR (event detector connection error)
This error means that the EEM event detector that handles this request is not available.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
2021
unregister_counter