The linked video provides guidance for optimal IOS version selection.The large number of IOS versions makes choosing the best version for your router or switch difficult. You must pick the most reliable version which includes the features you need. Different IOS "packages" have different features. For example, the "LAN base" package includes basic switching code. "IP base" adds access-layer routing features (RIP and EIGRP-stub). "IP services" adds most layer-3 routing protocols (OSPF, EIGRP, BGP). "Advanced IP services" adds IS-IS and MPLS.Picking a version also means picking one with recently introduced features you need. For example, 16-port 10-gigabit ethernet card support was added to the 6500 line in 12.2(33)SXH code. If you require that card, you cannot pick an older version, such as 12.2(18)SXF. The release notes include details on recently added features.Finally, of all the versions that have the features you require, you want to pick the most stable version. That means picking a version that has been "rebuilt" with many bugfix-only releases. Picking 12.4(2)T, where 60 new features were just introduced, would be a bad idea. On the other hand, 12.4(23) (the lack of a letter means it is a mainline release) would be a good choice because that release has undergone dozens of releases since significant numbers of features were introduced.