The M releases are extended maintenance releases, for which Cisco will supply bug fixes for a period of 44 months. The T releases are standard maintenance releases, for which Cisco will only supply bug fixes for a period of 18 months.

Cisco IOS Packaging and Feature Sets

Nearly all of the Cisco equipment which is running IOS also has one or more “feature sets” or “packages”, normally eight packages for Cisco routers and five packages for Cisco network switches. As an example, Cisco IOS releases intended for deployment on Catalyst switches are obtainable as “standard” versions (supplying just basic IP routing), “enhanced” versions, which supply you with complete IPv4 routing support, and “advanced IP services” versions, which supply the improved features along with IPv6 support.

Each distinct package relates to one service category, including:

  • IP Data
  • Converged Voice & Data
  • Security & VPN

The precise feature set needed for a certain function can be found using the Cisco Feature Set Browser.

Starting with the 1900, 2900 and 3900 series of Cisco ISR Routers, Cisco revised the licensing model of IOS. Routers come out of the box with IP Base preinstalled, and supplementary feature pack licenses can be deployed as bolt-on additions to enlarge the feature set of the routing device. The obtainable feature packs are:

  • Data adds features including BFD, IP SLAs, IPX, L2TPv3, Mobile IP, MPLS, SCTP.
  • Security adds features including VPN, Firewall, IP SLAs, NAC.
  • Unified Communications (UC) adds features including CallManager Express, Gatekeeper, H.323, IP SLAs, MGCP, SIP, VoIP, CUBE(SBC).

An Interface Descriptor Block, long for IDB, is a part of memory or Cisco IOS internal data structure which comprises of information like the IP address, interface state, and packet statistics for networking data. Cisco’s IOS software keeps one IDB for each hardware interface in a specific Cisco switch or router and one IDB for each subinterface. The number of IDBs existing in a system differs with the Cisco hardware platform types.

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