Core Switch Upgrade - STP Questions

Unanswered Question
May 27th, 2008


My company is currently planning to upgrade our two existing core 6513 switches running CatOS 8.2(1) with two 6509-E switches running IOS (12.2(SXF10a) or later). We utilize a collapsed core design where the 6513's are the STP roots as well as serve as access-layer switches for server/BladeChassis connectivity. The existing CatOS 6513's are running PVST+ and we would like to run either MIST or RPVST+ on the new core switches. We have discussed performing this cutover by bringing the new 6509-E switches up in parallel with trunks to the 6513's, and making the 6509-E's the Root Bridges for our STP. This would give us the ability to migrate our servers/BladeChassis over to the new Core switches at our leisure, instead of over one weekend night.

I am going to pitch using MIST to my boss due to the inherent benefits regarding CPU resources. I understand that it requires a whole new level of knowledge than PVST+/RPVST+, but I'm willing to do the reading to make sure I am well versed before the upgrade. We also may end up going with RPVST+ for the convergence time benefits/ease of administration. My question is: Does the Cisco MIST implementation offer support for both PVST+ and RSTP+ regions within the same MIST Instance? Also, I've read about RPVST+ backward compatibilty with PVST+, but it seems that once a legacy bridge is detected, the port stays 'dumbed down' until manual intervention. I guess this wouldn't be an issue for us though, once all the servers have been migrated to the new core switches, we will be powering down the old 6513's and shutting the trunk links anyway.

Thanks for taking the time to read this.

Chris Williams

I have this problem too.
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chris.williamsEP Tue, 05/27/2008 - 08:21


Thanks for the reply. That is an excellent doc...I had stumbled on it before posting, but missed the Backward Compatibility section which answers my question.

What is the most typical place in the network to use as the border for an MST region? I.E. Would we run only MST on our core switches, but continue to run PVST+ or RPVST+ on our CBS3020 blade switches? Or would it be best to run MST on all supported L2 switches in the network?

Also, Is another benefit to MST the fact that it reduces the diameter of the spanning-tree? For instance, we have an L2 only VLAN that spans our entire campus...there is no LVI created for this L2 VLAN so it is unroutable, and exists just as one huge broadcast domain. if we add this VLAN to the MST region will it act to shrink the diameter of this side of the campus only seeing one switch (MST Region) of the other side?

Let me know if I am off-base with this assupmtion.



Amit Singh Tue, 05/27/2008 - 08:35

Hi Chris,

The MST region can exist anywhere in the network where a switch with MST talk to a switch running PVST+. In a proper STP design you should have MST running across all the switches on your network. If not then you might have STP convergence or STP loop issues on the network. I would suggest to run MST on all the switches across the whole network.

I really didnt get the second part of you post. As far as the Vlan a spanned across the whole network it will be the same broadcast domain. The same hold true with the PVST+ as well. The only difference here would be that with PVST+ each vlan will have it own root bridge and will send BPDU for each vlan. While in MST multiple vlans will be mapped in one instance and there will be only a single bpdu for all these vlans. A vlan mapped to one instance cannot be mapped to other MST instance.

-amit singh

chris.williamsEP Tue, 05/27/2008 - 08:40


Thanks for the reply. So basically, can it be said that MST reduces the amount of overall BPDUs that get transmitted on a segment? Let's say we have 1000 VLANs and we are sniffing each of the 1000 Broadcast Domains. We will see 1000 BPDUs (one for each VLAN) within a certain time period. Will we still see 1000 MST BPDUs (one for each instance, transmitted on 1000 VLANs) while using MST?

Mohamed Anees M... Tue, 05/27/2008 - 13:48

Yes. If you are going to create N instances, you will see N BPDUs. But if you are going to create one instance per one vlan, I would go with Rapid PVST+ instead of MST. And I agree with Amit that we need to have MST on the entire L2 domain. MST interoperability with others should be a temperory solution while migrating phase by phases.


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