Rapid-PVST+ Interacting with MSTP

[mogli]

Hi,

we want to implement MSTP at customer side. unfortunately there are some legacy switches who don't understand MSTP.

so we'll have to implement a "hybrid scenario" where the "core" speaks MSTP using instances and the access-switches are

using their "old" spanning tree protocol (talking with the Instance 0 of the MSTP).

now my question:

when the core is migrated to MSTP, is the IST Spanning Tree able to talk with legacy switches in a "rapid way"

(if the support it)?

i found a cisco document which deals with this topic, but i'm not completely sure that it answers my question....

Rapid-PVST+ Interacting with MSTP

An MSTP switch interacts with a Rapid-PVST+ switch in the same way that an MSTP switch interacts

with PVST+ switch. (See IST Interacting with PVST+.) The MSTP switch will send IST BPDUs in

802.1D format on all VLANs to the Rapid-PVST+ switch and IST will consider the port connected to

the Rapid-PVST+ switch to be at the boundary of the MST region.

Has anyone of you experience with such "hybird" MST/Rapid-PVST+ configurations?

thanks a lot for your answers &

kind regards,

Bernhard

[Francois Tallet · Accepted solution]

Hi Bernard,

In that scenario, as mentioned in the document, the MST region is going to send plain STP BPDUs to the rapid-PVST region. So basically, only STP will be run at the boundary between MST and rapid-PVST. In particular, there will be no proposal agreement possible on the vlans because MST only runs one instance (the CIST, instance 0), while rapid-pvst runs several instances (one per vlan). So the MST side does not have a per-vlan context that would allow it to answer the proposal from each individual PVST+ instance. To be honest with you, that could probably have been enhanced, but the interaction rapid-PVST/MST has never been a priority, as we hope that customers will be able to migrate the whole network to MST and only use this interaction as a temporary solution.

Regards,

Francois

[Francois Tallet]

PS: sorry for the spelling error in your first name. I wrote it as a French name, personal bias;-)

[mogli]

hi francois,

thanks for your answer, for what reason ever i didn't receive a e-mail notification to your reply (maybe i forgot to check the checkbox).

i agree with you that the interaction between Rapid-PVST+ and MST is not handled with priority - our case is only a temporay workaround.

anyhow, in the meanwhile i've already answered the qustion:

yes, the interaction between the CIST and the "outside" RPVST+ domain works in rapid-fashion.

we implemented the hybrid solution at customer side and it works fine.

if you don't forget to enable "portfast" on the user ports of the RPVST+ Cloud (and therefore run into the holdtime of 15sec), the switchover is as rapid assumed.

kind regards,

bernhard

[antilope1]

Hello Francois, 

To change rapid-pvst+ topology in a Cisco infrastructure for a MSTP topology, I suppose I can change the MSTP on the L3 equipment first and then on the others. But I don't know if this is the right procedure or if all the equipments should be changed simultaneously. 

Could you help me with this? 

Thank you very much.