Cisco Systems Inc. gave an upbeat forecast for fiscal fourth-quarter sales, a signal of healthy demand for equipment and software that runs the internet and corporate networks. Chief Executive Officer Chuck Robbins has been working to make Cisco less dependent on the market for expensive equipment by making the company’s hardware more flexible and selling more networking services and software. For now, increased spending on networking gear in general may be buoying Cisco’s main business, buying more time for its increasing backlog of deferred software and services revenue to translate into more sustained sales growth.