RSTP root with MSTP

Routing & Switching Spanning Tree
1

Hello,

I am studying for CCNP ENCOR and I could not get clear understanding how RSTP and MST can speak with each other. Specifically how it’s possible to make RSTP switch a root bridge.

So this is the scenario:

I have MST region with 3 switches and 1 Switch outside the region running RSTP.
My understanding how it works is this. (most likely wrong understanding)
To make RSTP switch a root bridge for entire spanning-tree topology I should make RSTP priority on all vlans less then any MST switch.

This is the lab I setup in EVE-NG
I have configured all switches with three vlans from 1 to 3.
I have only default IST region 0
I was unable to upload more than 1 screenshot so I cramped everything in 1 file. hope you can see everything

topology - Copy
topology - Copy1510×1614 210 KB

On SW1 I receive this log message:

%SPANTREE-2-PVSTSIM_FAIL: Blocking root port Gi0/1: Inconsitent inferior PVST BPDU received on VLAN 2, claiming root 2:5000.0006.0000

and port role that’s going to outside_sw is Root but BKN(broken?)
What I am missing, why this is happening? How BPDU can be inferior when it shows that root is outside_sw and priority is 1 and in the region all switches has higher (default) priority?Preformatted text

2

Hello Netdev

MST does a root port check at the “border” between the MST and RSTP regions. In order for one of these boundary interfaces to become a root port, the RSTP BPDUs from all VLANs except from VLAN1 must be identical or superior compared to the RSTP VLAN1 BPDU. The VLAN1 priority must be equal to or greater than the rest.

This same problem arises when interoperating MST with PVST+. This interoperation is described in this lesson, in the section where the PVST+ switch is being made root bridge. You will notice the exact same error message that you are receiving as well:
https://networklessons.com/spanning-tree/mst-pvst-interoperability#PVST_Root_Bridge
In this lesson, you will also see the resolution to this problem.

I hope this has been helpful!

Laz