As part of a new implementation, I have the requirement to interconnect a vPC Cluster of Cisco Nexus N9Ks (peer-switch/peer-gw enabled), with another pair of third-party multi-services platform running [for the purpose of this analysis] as a pair of independent switches.
The Nexus pair is the Core (Root Bridge and Gateway) of several VLANs for the IT Network and they are running Rapid Per-VLAN STP. Whereas the other 2 switches are the core for a transport (mpls-tp) network, and they only support RSTP (NO per-VLAN nor support for RPVSTP or MST).
As each of the vPC cluster members is supossed to have a connection to one of the two other switches for redundancy, we have first connected the vpc member 1 to the Switch 1 (third party switch), and expectedly noticed that the Nexus sees itself as RB for all the VLANs as it has a configured priority of 4096.
While the Switch1 also sees itself as RB for its RSTP instance, even when it has a priority of 24576. Since the Nexus is running per-VLAN STP and the Switch1 is running just RSTP, evidently both devices cannot properly understand each others BPDUs and agree upon the Nexus being the Root Bridge for all the VLANs.
As of now the network is working like that with no issues.
However, the time will come to bring-up the second/redundant link, between the vpc Member 2 (nexus) and the other Switch2. And there we may have a problem or even a loop if we don't get the STP domains to properly work together.
Essentially the purpose is to integrate both STP domains while keeping the Root Bridge role in the vPC Cluster (Nexus switches) and getting STP to block a port in the Switch2 (which has a priority of 28672) to prevent loops from ocurring.
Since the Switches 1 and 2, do not support RPVSTP or MST. I understand that the way to go is to configure the Nexus to run MST (correct me if I am wrong or there are other feasible options around).
If that is the case, I have some doubts that I would appreaciate assessing with the community:
0. Is there any other option to integrate the domains without resorting to MST?
1. In order to implement MST on the Nexus, and reach the objective, is it required to configure MST globally or can I keep RPVSTP as global and run MST just in the interfaces that connect to Switch 1 and Switch 2?
2. If I need to move to MST globally, I understand that such migration will have impact on the whole IT network and such should be properly analyzed to define new boundaries and keep rpvstp running on the distribution and access layer switches..
Thoughts, insights and recommendations are welcome.
Attached is a diagram for reference.
Thank you
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@dm2020 @paul driver - I am tagging you since I saw a similar post: https://community.cisco.com/t5/switching/rstp-and-rpvst-integration/td-p/3739140
