The set available is determined by the "mode" and the privilege level of the current user.
"Global configuration mode" provides commands to change the system's configuration, and "interface configuration mode" provides commands to change the configuration of a specific interface.
In all versions of Cisco IOS, packet routing and forwarding (switching) are distinct functions.
All commands are assigned a privilege level, from 0 to 15, and can only be accessed by users with the necessary privilege.Through the CLI, the commands available to each privilege level can be defined. Using the Embedded event manager feature, the interpreter can be scripted to react to events within the networking environment, such as interface failure or periodic timers.Trains more or less map onto distinct markets or groups of customers that Cisco is targeting.There were other trains from time to time, designed for specific needs — for example, the 12.0AA train contained new code required for Cisco's AS5800 product.Cisco recommend upgrading to Maintenance releases where possible, over Interim and Rebuild releases.
Cisco says, "A train is a vehicle for delivering Cisco software to a specific set of platforms and features." Before Cisco IOS release 15, releases are split into several trains, each containing a different set of features.Routers come with IP Base installed, and additional feature pack licenses can be installed as bolt-on additions to expand the feature set of the device.The available feature packs are: An Interface Descriptor Block, or simply IDB, is a portion of memory or Cisco IOS internal data structure that contains information such as the IP address, interface state, and packet statistics for networking data.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 d CEF (Distributed CEF).Rebuilds are produced to either quickly repair a defect, or to satisfy customers who do not want to upgrade to a later major revision because they may be running critical infrastructure on their devices, and hence prefer to minimise change and risk.