clarification on cst, pvst, pvst+ and rst

aocferreira

hi,

i'm a little bit confused with all these terms.. So, I think it's like this:

- CST (802.1d) is the original STP. It runs one instance for all VLANs and the algorithm is pretty bad

- PVST uses same algorithm but runs one instance of spanning tree per VLAN (Cisco proprietary)

- PVST+ introduced some features like uplinkfast, backbone fast and port fast (also Cisco proprietary). Basically, a better algorithm

- RST (802.1w) is an industry standard as a response to PVST+, i.e, a better algorithm that allows networks to converge faster

Can you please confirm? Missing something?

tomtom1

PVST+ is an improvement to PVST, but the features such as portfast, uplinkfast and backbonefast were already available in 802.1d (STP / CST). PVST+ is an enhancement that brought 802.1Q support to the table instead of ISL.

What you're still missing is MST, basically a middle ground between PVST and CST. MST allows a single STP instance to be mapped to different VLANs that share the same logical topology and / or bandwidth requirements. This lowers the CPU usage on the switches, seeing as it does not have to have a STP instance for every single (active) VLAN on the switch.

MST uses the concept of regions for the mappings. For switches to belong to the same region, they need to agree on:

-> The instance to VLAN mapping
-> Region name
-> Configuration revision number