Cisco IOS - Architecture

Architecture

In all versions of Cisco IOS, packet routing and forwarding (switching) are distinct functions. Routing and other protocols run as Cisco IOS processes and contribute to the Routing Information Base (RIB). This is processed to generate the final IP forwarding table (FIB, Forwarding Information Base), which is used by the forwarding function of the router. On router platforms with software-only forwarding (e.g., Cisco 7200) most traffic handling, including access control list filtering and forwarding, is done at interrupt level using Cisco Express Forwarding (CEF) or dCEF (Distributed CEF). This means IOS does not have to do a process context switch to forward a packet. Routing functions such as OSPF or BGP run at the process level. In routers with hardware-based forwarding, such as the Cisco 12000 series, IOS computes the FIB in software and loads it into the forwarding hardware (such as an ASIC or network processor), which performs the actual packet forwarding function.

Cisco IOS has a "monolithic" architecture, which means that it runs as a single image and all processes share the same memory space. There is no memory protection between processes, which means that bugs in IOS code can potentially corrupt data used by other processes. It also has a run to completion scheduler, which means that the kernel does not pre-empt a running process — the process must make a kernel call before other processes get a chance to run. For Cisco products that required very high availability, such as the Cisco CRS-1, these limitations were not acceptable. In addition, competitive router operating systems that emerged 10–20 years after IOS, such as Juniper's JUNOS, were designed not to have these limitations. Cisco's response was to develop a new version of Cisco IOS called IOS XR that offered modularity and memory protection between processes, lightweight threads, pre-emptive scheduling and the ability to independently restart failed processes. IOS XR uses a 3rd party real-time operating system microkernel (QNX), and a large part of the current IOS code was re-written to take advantage of the features offered by the new kernel. But the microkernel architecture removes from the kernel all processes that are not absolutely required to run in the kernel, and executes them as processes similar to the application processes. Through this method, IOS XR is able to achieve the high availability desired for the new router platform. Thus IOS and IOS XR are very different codebases, though related in functionality and design. In 2005, Cisco introduced IOS XR on the Cisco 12000 series platform, extending the microkernel architecture from the CRS-1 to Cisco's widely deployed core router.

In 2006, Cisco has made available IOS Software Modularity which extends the QNX microkernel into a more traditional IOS environment, but still providing the software upgrade capabilities that customers are demanding. It is currently available on the Catalyst 6500 enterprise switch.