Classical IOS is on the market for a long time, You will find this IOS on entry level routers like ISR or Enterprise switches like 6500 or 3750.
- IOS Cisco ISR Series Routers (800,1900,2900,3900 series), Cisco 7200 and 7600 Series and all legacy routers and Catalysts 6500
IOS XE is pretty similar to classic IOS, additional few debug commands and provide rapid feature delivery ..
- IOS XE – ASR 1000 Series and Catalyst 4500E Series Switches
IOS XR 's core is different micro-kernel architecture, memory-protected and preemptive multitasking etc....
In my opinion, the command syntax is pretty similar with Junos..
every detailed unnecessary commands like "commit", "address-family ipv4 unicast" for Static Routing !!! etc...
- IOS XR – Cisco CRS-1 and CRS-3, Cisco ASR9000 and Cisco XR12000
