Questions tagged [cisco-ios]

Cisco IOS is an embedded operating system for Cisco networking products. It is found in nearly all of Cisco's enterprise-class switches and routers.

Cisco IOS is a monolithic-kernel OS with no GUI. It is designed to perform some of its functions in software, and to delegate others to specialized hardware. For example, routing and other protocols run as IOS processes and contribute to the Routing Information Base (RIB). The RIB is processed to generate the final IP forwarding table (FIB, Forwarding Information Base), which is used by the forwarding function of the router. However, 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 releases and supports several versions of the IOS software, roughly parallel to the stable/unstable/nightly versions of open source projects. Cisco calls these versions "trains," and it releases several of them. The mainline and T trains are the most common, and the rest are specialized for specific groups of customers.

• The mainline train is designed to be the most stable release the company can offer, and its feature set never expands during its lifetime. Updates are released only to address bugs in the product.
• The T is the Technology train, gets new features and bug fixes throughout its life, and is therefore potentially less stable than the mainline. The T train, once mature, becomes the basis for the next version of the mainline train.
• The S is the Service Provider train, runs only on Cisco's core router products and is heavily customized for Service Provider customers.
• The E is the Enterprise train, customized for implementation in enterprise environments.
• The B is the broadband train, meant to support broadband features.
• The X* trains (e.g. XA, XB) are a series of speciality trains and are not widely available.