CCNP Switch - VLAN to STP relationship?

Node Man

Hi Everyone,
I hope you are having a nice day. I have a question: Is there, or should there be a relationship between VLANs and STP? Also should STP have a relationship to a Switch Block? For example, should every Switch Block have its own instance of STP?

Thanks!

fredrikjj

It depends on what STP type you are using. Read about classic STP, PVST+ and MST.

Danielh22185

Conceptually and traditionally speaking every vlan will have its own separate instance of STP running. This follows the default behavior of Per Vlan Spanning Tree Protocol. So if you have 100 vlans on the switch essentially you will also have 100 instances of STP running. However that could severly impact the CPU on the switch and is not a real life scenario, yet explains the point. The only way to minimize this would be to run MST (Multiple Spanning Tree) where you group vlan spanning tree instances into groups or essentially map those spanning tree instances to groups. The idea is not welcomed much in real world practice but is an NP topic.

theodoxa

Common Spanning-Tree: 1 STP Instance for All VLANs
Per-VLAN Spanning-Tree: 1 STP Instance for Each VLAN
Multiple Spanning-Tree: 1 STP Instance for 1 or More VLANs. You Map 1 or More VLANs to an MST Instance. Effectively a middle ground between CST and PVST.

As for a Switch Block, STP Instances and Individual VLANs should be confined to a single switch block. You don't want them crossing the Core/Backbone.

OfWolfAndMan

However that could severly impact the CPU on the switch and is not a real life scenario, yet explains the point.

That's why you'd run the root bridge on a Catalyst Chassis switch (i.e. 4506-E or 6509-E). Even then, some of the smaller catalyst switches could handle that. I'd be worried about spanning tree loops. Another big thing to worry about is the virtual MAC address per instance of spanning tree. That's why they implemented the system extend id in rapid PVST to reduce it to one MAC and use the extend id to uniquely identify.

Common spanning tree is not used in Cisco switches FYI (Although there are other vendors that have). Focus on PVST and MST. Also focus on Rapid PVST (MST uses Rapid Spanning Tree by default). PVST is default on Cisco switches.