RSTP and RPVST+ interoperability

[ifvelascosz]

Hello,

I need to partially migrate some switches, and during this process, I'm forced to use RSTP to switches that only support that standard. In this technical guide I'm sharing, I see that Cisco recommends some interoperability options, but it's confusing to me whether combining RPVST+ and MSTP downgrades to PVST+ or not.

Please help me understand if it's possible to combine RPVST+ and MSTP with restrictions but without downgrading, or if this combination downgrades.

Thank you,

reference: https://www.cisco.com/c/en/us/td/docs/routers/ncs5xx/ncs520/configuration/guide/LAN-switch/17-1-1/b-lanswitch-17-1-1-ncs520/b-lanswitch-17-1-1-ncs520_chapter_0110.pdf

[Jens Albrecht · Accepted solution]

The combination of MSTP with RPVST+ works just fine and I have never seen any "downgrading" and really have no idea why such a thing should ever happen.

If you configure a switch to run RPVST+ it will use this protocol to communicate with all RPVST+ neighbors as well as all MSTP neighbors as MSTP uses the same timers under the hood. Only if you add a PVST+ switch to your network, then the RPVST+ switch needs to fallback to PVST+ on those links connected to PVST+ neighbors. Even then the communication with connected RPVST+ and MST neighbors will continue to use RPVST+.

This is valid as long as you only use Cisco switches. Once you add 3rd-party switches you are forced to run MSTP because it is the only STP protocol that Cisco implemented according to the RFC standards.

[MHM Cisco World]

Can you more clear which mode you have 

RSTP or Rapid-PVST ?

Can you use MST?

MHM

[ifvelascosz]

Thank you for your responses.

I have a Phoenix Contact Switch 2000 that only supports RSTP, and I'm migrating in stages to a Rockwell Automation Stratix 5800 (manufactured by Cisco). The Stratix 5800 supports PVST+, RPVST+, and MSTP, like all Cisco switches. Reading the official Cisco documentation, I understand that the best thing to do is configure MSTP on the new Stratix switches and leave all VLANs on instance 0. However, I came across the document I'm sharing (official Cisco document) where it indicates that combining RPVST+ and MSTP will downgrade to PVST+, but the table with the highlighted points seems contradictory to me. I've never done this, so I'd like to confirm with you if it's an error in the documentation or if it actually downgrades to PVST+.

[MHM Cisco World]

Unfortunately' RSTP not interoperate with PVST nor PVST+.

This RST is IEEE standard so you need to use only MST (which also IEEE standard) and it interoperate with RSTP.

MHM