MST to PVST

[Psmurali89]

Hi All

I got some issues with STP. 

I got MST running on switch1 and PVST on switch2. 

Switch1 -> ISP -> Switch2 -> Router

I configured VLAN922 on switch1 as SVI and added to trunk port that connects to ISP.  In switch2 vlan922 is Layer2, as Layer3 is in router. As soon as i add vlan 922 in trunk port in switch2 that connects to ISP, I get the following in Switch1. 

: %SPANTREE-2-PVSTSIM_FAIL: Blocking designated port Twe2/0/22: Inconsitent superior PVST BPDU received on VLAN 922, claiming root 8192:34bd.c8a3.ab00

I understand that I need to alter the priority for vlan922 to fix this issue but not sure how and which switch that i need to do this and what should i change it to? Switch2 which runs PVST will be decommissioned soon so i dont want to mess around by changing to MST at this stage. 

Any suggestions is much appreciated. 

[Elliot Dierksen]

It isn't the priority, it is the fact that it is MST versus PVST. Does the PVST switch have any downstream connections? If not, then it is as simple as changing one global command in that switch.

spanning-tree mode mst

 It has been a long time since I had to deal with MST, but I have a recollection that it is dependent on VLAN 1, but I am not positive of that. If VLAN 922 only lives on that single PVST switch with no other connections, you could try turning off spanning tree for that vlan this way.

no spanning-tree vlan 922

 Be REALLY CAREFUL with that! That would not be my first choice since then you could end up with loops in that VLAN.

[Psmurali89]

Thank you. I was thinking of changing that PVST switch to MST. The PVST switch has also got other VLAN's in that trunk port which connects to ISP, this switch is just L2. All L3 for those VLAN's are configured in ASR router. 

If i change this switch to MST I take it there will be atleast 30 sec downtime? And i hope below is the right way to do it? 

no spanning-tree mode pvst

no spanning-tree extend system-id

spanning-tree mode mst
spanning-tree extend system-id
spanning-tree mst configuration
name xxx
revision 1
instance 1 vlan 1-4094

this switch connects to another switch which runs on MST. By mistake only this switch was configured as PVST. 

[Elliot Dierksen]

I don't think there is any reason to "no" out the pvst stuff. I don't think that will work anyway since it MUST have a mode for spanning tree. I would think it would only be "spanning-tree mode mst", BUT if any of those other commands are used in your other MST switches then I would config this one to match. Disclaimer: I don't run MST anywhere. I used to when my home switches were a mix of HP and Cisco. They are all Cisco now and so I am using rapid-pvst.

Yes, I would expect a 30-120 disruption while spanning tree sorts itself out when you change to MST.

[MHM Cisco World]

NOTE:- ANY RECOMMEND for L2 protocol must be double check before apply it can cause LOOP 

the issue with STP multi protocol compatible is the accept the BPDU 
please check below command 
and again friend please see above notice
http://www.bscottrandall.com/1.4.4.4.html

[Psmurali89]

Hi 

I think i have found a solution for this. I tried in my lab and it seems to fix the issue. 

* I changed the priority for vlan 922 in PVST switch to lower value (most preferred)

spanning-tree vlan 922 priority 12288

* As soon as i done the above in PVST switch, i see the below error in MST switch and that port is blocked. 

%SPANTREE-2-PVSTSIM_FAIL: Blocking designated port Gi0/0: Inconsitent superior PVST BPDU received on VLAN 922, claiming root 13210:5000.0020.0000

* I changed the priority for VLAN922 in PVST switch to higher value (least preferred) 

spanning-tree vlan 922 priority 40960

* As soon as i done the above in PVST switch, i see the below in MST switch and the port is unblocked. 

%SPANTREE-2-PVSTSIM_OK: PVST Simulation inconsistency cleared on port GigabitEthernet0/0.

So if you set the priority value of that vlan in PVST switch to higher value (least preferred) then this issue seems to be fixed. 

[MHM Cisco World]

thanks thanks thanks for sharing this, 
I will also lab this and update you. 
let see why priority effect MST-PSTP compatible.