Cisco Catalyst switches originally did not run IOS—the early chassis-based switches were CatOS-based. This was because the technology for these switches came from other companies that Cisco acquired, such as Crescendo, Kalpana, and Grand Junction.
CatOS may appear clunky to those who have used only IOS, but there are some distinct advantages to using CatOS in a switching environment. One of these advantages can also be considered a disadvantage: when a Catalyst 6500 runs CatOS and also has an MSFC for Layer-3 functionality, the MSFC is treated like a separate device. The switch runs CatOS for Layer-2 functionality, and the MSFC runs IOS for Layer-3 and above functionality. This separation can be easier to understand for people who do not have experience with IOS Layer-3 switches, but for those who are used to IOS-based switches like Catalyst 3550s and 3750s, the need to switch between operating systems can be burdensome and confusing.
Because all of the new Cisco Layer-3 switches (such as the 3550 and the 3750) run only IOS, learning the native IOS way of thinking is a smart move, as that’s clearly the direction Cisco has taken. At one point, Cisco actually announced plans to discontinue CatOS, but there was such an uproar from die-hard CatOS users that the plans were scrubbed. I still see CatOS out in the field, but they are disappearing.
Another advantage of CatOS over IOS is the concise way in which it
organizes information. An excellent example is the