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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def router_id(self, **kwargs):
 """Configures device's Router ID. Args: router_id (str):
 Router ID for the device. rbridge_id (str):
 The rbridge ID of the device on which BGP will be configured in a VCS fabric. callback (function):
 A function executed upon completion of the method. The only parameter passed to `callback` will be the ``ElementTree`` `config`. Returns: Return value of `callback`. Raises: KeyError: if `router_id` is not specified. Examples: Traceback (most recent call last):
 KeyError """

        router_id = kwargs.pop('router_id')
        rbridge_id = kwargs.pop('rbridge_id', '1')
        callback = kwargs.pop('callback', self._callback)
        rid_args = dict(rbridge_id=rbridge_id, router_id=router_id)
        config = self._rbridge.rbridge_id_ip_rtm_config_router_id(**rid_args)
        return callback(config)