Cisco switches - they differ immensely
The Catalyst 8500 runs Classical IOS as we all know it from the routers. (This isn't very strange since the 8500CSR *is* a router. The MSR also has Classical IOS CLI, but that's really just a souped- up Lightstream 1010 ATM switch).
The Catalyst switches that came from the Crescendo acquisition (Catalyst 1200, 1400, 2900, 4000, 5000, 6000 switches) use what folks commonly refer to as Crescendo IOS - Cisco calls it CatOS, since that's were the CLI came from.
The Catalyst 6000's with MSFC can also run Classical IOS where you don't get the Crescendo CLI for the Layer-2 part of the box. The MSFC is similar to the Cat 5000 RSM or RSFC 'router module for switches'.
The 2900 G switches run 'Crescendo IOS', the same as the Catalyst 4000, 5000 and 6000 switches. Actually, 2900 Gs are fixed-config Catalyst 4000 switches and should have been named accordingly. One big difference between the 2948G/2980G and the Cat 5000/5500 is the backplane speed. Unlike the 5000, the 2948G/2980G are fast! Gigabit traffic can run a Cat5500 into the ground, while it is very difficult to saturate either the 2948G or 2980G in the same scenario.
The 2900 G L3 switches run 'Classical IOS' (router-IOS) because it *is* a router. Yes, it's a very fast router, but it's not 'a switch with a router module' like the 5000s with RSM og RSFC or the 6000s with MSFC. Hardware-wise, it's a fixed-config 8510 CSR router in a small box. It's actually a 2948G and a 8510 router in the same box. Although this is a different module than the Cat 5000/5500 use, the command syntax is almost identical. If you learn the 2948G/2980G command language, you'll know the Cat 5000 language, and vice versa. A few features are supported on one and not on the other. For example, show crashdump exists only on the 29xx/4000, while the RSM and related features like MLS exist only on the 5000/5500.
The XLs (2900, 3500) run 'Workgroup IOS'. It looks a lot like the routers but have a number of very irritating differences and is nothing like the 5000 series switches.
The 1900/2800's also have an 'enterprise' version which also has a CLI somewhat like the XL's, but it's not the same. From a configuration management standpoint they are clumsy. (They also have the menus as the Classical Grand Junction IOS).
The 1900/2800's use Grand Junction IOS, while the 3000's use Kalpana IOS. (Again, those are the companies Cisco acquired where the technology was originally developed).
Note on CDP:
CDP (HDLC protocol type value 0x2000) is a media independent layer-2 device discovery protocol. It uses a small multicast packet to the common destination address 01-00-0c-cc-cc to send and receive periodic messages. CDP packets are sent with a TTL value greater than zero after an interface is enabled and with a TTL of zero immediately before and interface is idled down (for quick state discovery). Cisco devices never forward CDP packets.