In the last part of our series on Cisco's drivers and challenges in the service provider market, we focus on the company's venerable routing software, IOS, which may present Cisco with one of its most daunting technical challenges. IOS routing code is often considered a blessing and a curse for the company: It's widely installed, yet it’s 15 years old and its roots are in enterprise networking.

Analysts and competitors bash it for being monolithic, unstable and a CPU resource hog - Juniper continually states that IOS saps wire-speed performance from the 12000 series routers when additional software-based services are turned on.

“Yes, it's a monolithic piece of code that is sensitive to changes and new additions,” says Joe McGarvey, an analyst at Current Analysis. “At the same time, it runs throughout its product line and is ingrained in thousands of environments. It's sort of like the weather: There's no point in complaining about it, because you can't do anything about it.”

Cisco is, however, making incremental changes to the current IOS specifically for service provider duty, says Roland Acra, Cisco senior vice president and service provider CTO.

“We’ve done a lot with IOS specifically for the service provider space,” he says, citing recent scalability, reliability and high-availability enhancements such as non-stop forwarding, stateful switchover and Globally Resilient Internet Protocol capabilities. “The amount of investment in IOS is going predominantly for service provider purposes. Many of our large carrier customers have installed these capabilities and found tremendous uptime improvements. The adoption is high because these are the guys who asked us to do these things.”

There's been years of speculation that Cisco is rewriting IOS or developing a new, unique operating system from the ground up specifically for carriers to meet their requirements for modularity, protected memory operation and availability.

This new software is referred to as "IOS NG," where NG stands for "next generation." But Acra says talk of a new operating system is overstated, and that Cisco will not take a "flash cut" approach to optimizing its routing code for the service provider environment.

Cisco is doing a lot of "piecemeal" work, Acra has said, to modify IOS platform software, operating system and applications exclusively for the service provider space. IOS images currently shipping to service providers have had little to do with the same code that runs in enterprise routers.