“Non-Cisco 802.1Q switches maintain only a single instance of spanning tree (the Mono Spanning Tree, or MST) that defines the spanning tree topology for all VLANs. One final note on Cisco’s cover up of the acronym MST – Mono Spanning-Tree:Ĭisco acknowledges MST both as Mono-Spanning Tree noted here: This seems amazingly not agreed upon by the Cisco community, but for exam day, I would personally go with Cisco’s documentation that it is defined in IEEE 802.1Q. Per Cisco’s official documentation, CST was introduced and defined by IEEE 802.1Q (Dot1q Trunking), so this is the standard I would tend to go with if asked on exam day, however if you research this on forums many highly respected members of the community call CST “Legacy STP” and advise it is defined under IEEE 802.1d STP. This confused the daylights out of me, if it was not configurable than WHERE IS IT?ĬST is not a configurable STP mode in general, however its common STP instance concept is used with MST – More on this in the MST segment.Ī quick word on the oddity of the IEEE definition of CST Rapid-pvst Per-Vlan rapid spanning tree mode
#CST TO MST FULL#
MST = “Multiple” Spanning-Tree instances (802.1s).PVST = One instance “Per VLAN” (802.1d).CST = One “common” instance for all VLANs (802.1q).With “Common” Spanning-Tree or CST the name is really the recipe, as one “Common” Spanning-Tree instance exists for all VLANs on the switch, which looking at different STP names they also come more full circle To review MST you must start at the beginning of STP’s existence, which is an industry standard across all switch vendors called “Common Spanning-Tree”, which uses IEEE 802.1q Trunks to run a single STP instance for all VLANs over Trunk ports. I assume this is enough knowledge to get by the CCNP SWITCH exam, but this is far from an exhaustive review on all topics relating to Multiple Spanning-Tree, so I definitely encourage using multiple sources for in depth study of MST!Ĭommon Spanning-Tree (CST) – The beginning of STP! I have spent days researching Cisco documentation / Blogs / Discussions / Training material to cross reference MST information, as this is a very complex topic with quite a bit of conflicting information even from Cisco. The above Topology is configured waaaay toward the bottom of the post, there is a lot of ground to cover in terms of the evolution of STP, and clarifying some differences in flavors.