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We have a Dell stack switch (running RSTP) connected with a couple of Cisco switches (running PVST), and recently, something happened. I'm not sure where, but in our network, we started getting packet loss. Our network is big; it distributes to different, remote locations.

I have no idea who configured switches etc.

  1. Is it possible for RSTP and PVST to work together?
  2. Does it require any special configuration?
  3. In the core switch, I am sometimes seeing 'PortFast' Link UP and DOWN on the console. Is it because of spanning tree?
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide your own answer and accept it. – Ron Maupin Aug 6 '17 at 22:59
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RSTP is mapped to PVST+ VLAN 1 instance, so make sure you have VLAN 1 enabled on any trunks between the RSTP and PVST+ regions for the two to interoperate. Also, if you have PVST+ regions separated by an RSTP region, the PVST+ BPDUs are transparently forwarded across the RSTP region (so long as the VLANs are trunked), so the RSTP region acts like a shared network interconnecting any PVST+ regions.

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It is possible to mix PVST and MST (RSTP requires MST), but it needs some planning, and it can be fragile, as you have discovered. You would probably be better off using the same STP version across all your switches. You could change the STP on the Cisco switches to match the rest of the switches.

Cisco has several documents about how to interoperate with non-Cisco switches (e.g. Understanding Multiple Spanning Tree Protocol), or how to convert the Cisco switches to MST (e.g. Configuration example to migrate Spanning Tree from PVST+ to MST). There are also a lot of other documents about this subject, which you will find if you do a Google search.

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In this scenario, theory is a thing and real world is other. I have faced this same enviroment with Dell-Force10 (OS9) and Cisco switches several times. We have no problem with STP and PVST but we have strange behaviour when we mix RSTP and PVST+. My "solution" for this case is:

  • Do you need a single STP instance? Use slow and classical STP and PVST.
  • Can you keep two separate instances of STP? Use RSTP and PVST+ and deactive STP in interconnection links.

Good luck with troubleshoting if you still want to mix rapid versions because it's a headache.

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