Defaulting a Cisco interface…

One pain with Cisco IOS is trying to get a configured interface back to defaults. Half the time you don’t even remember what those were.

If it’s a sub-interface you can “no” it, but you will still have configuration left behind:

cisco(config)#no interface ATM1/0.1
Not all config may be removed and may reappear after reactivating the sub-interface

with physical top level interfaces you can’t “no” them at all anyway.

The answer is to use the “default” command:

cisco(config)#default interface ATM1/0.1
Building configuration…

Interface ATM1/0.1 set to default configuration

Curiously this doesn’t seem to clear PVC definitions!

NOTE: If you do this on the primary physical interface, all sub-interfaces will be defaulted and deleted (which may or may not be what you want). So use carefully!

UPDATE: Well the docs say the sub-interfaces will be deleted, but they’re not in my experience. Also it’s not even clear if this works on sub-interfaces. The combination of a “no” on the sub-interface first and then a “default” after the fact seemed to maybe work, but no promises.

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