A configuration space bus includes a configuration space on a primary interface and an extension or secondary interface in communication with a configuration space of the primary interface. When the primary interface receives a transaction request which it does not recognize, the transaction request is passed to the secondary interface for processing. The primary bus then waits for a response from the secondary bus. If the primary interface receives a transaction request which it does recognize, that transaction request is processed by the primary bus. The extension interface allows the primary bus to receive and process industry standard specification defined commands as well as forward commands defined by a user to the extension bus for processing. Multiple buses may be cascaded to form a primary extension interface, a secondary interface, a third interface, etc. A transaction request is passed down through such a chain of interfaces until an interface recognizes and processes it.