PVST to MST

[Psmurali89]

Hi All,

I got 3 existing stack switches in MST with the below config. 

!

spanning-tree mode mst

spanning-tree extend system-id

!

spanning-tree mst configuration

name xxxx

revision 1

instance 1 vlan 1-4094

!

spanning-tree mst 0-1 priority 8192

!

We connected another standalone switch to stack1 and by mistake we configured as PVST. All switches are L2 only. I want to change this one switch from PVST to MST using the same config above. Will this have impact on all other 3 stack switches? I will be making this change in maintenance window anyway. 

[marce1000]

 - You may find this article useful : https://community.cisco.com/t5/switching/migrating-from-rapid-pvst-to-mst/m-p/1792072#M192453

 M.

[Psmurali89]

Thank you. 

[MHM Cisco World]

let me lab and I will share with you point I see may be it can help you. 

[Psmurali89]

Thank you. Let me know - Am planning to make this change tomorrow. 

[paul driver]

Hello
No it should not cause any issue to you existing stack , as long as you configure the switch correctly, the MST stack will presently see that switch in its own PVST domain with each trunk interconnect set a boundary port toward that PVST domain switch.

If you can disconnect the switch from the stack , then make the change, otherwise perform the following

On single switch
spanning-tree mst configuration
name xxxx
revision 1
instance 1 vlan 1-4094
spanning-tree mst 0-1 priority 65356
spanning-tree mode mst

.

[Psmurali89]

Thank you. 

This standalone switch connects to Stack1 via single trunk port. I will try and disconnect this trunk port, if not I will just change the STP to MST. 

Do i need to configure the priority as 65356 in this standalone switch? All other stack switches has same MST config as below so I was wondering to copy the same config to this standalone switch? 

!

spanning-tree mode mst

spanning-tree extend system-id

!

spanning-tree mst configuration

name xxxx

revision 1

instance 1 vlan 1-4094

!

spanning-tree mst 0-1 priority 8192

!

[paul driver]

Hello

---

@Psmurali89 wrote:

Do i need to configure the priority as 65356 in this standalone switch? All other stack switches has same MST config as below so I was wondering to copy the same config to this standalone switch? 

---

 I assume this switch is an edge switch meaning it is not connecting to any other downstream switches and its only upstream interconnects are via this existing stack, if so YES set the mst instance priority to an higher value than the existing switch stack it’s going to connect to.

Lastly IF you are making the changes whilst the switch is STILL connected and active to the stack then apply the mst configuration as i stated above and shown again for reference .

spanning-tree mst configuration
name xxxx
revision 1
instance 1 vlan 1-4094
spanning-tree mst 0-1 priority 65356
spanning-tree mode mst  <--- append this last 

[Psmurali89]

Thank you. It makes more sense. This switch is connecting only to existing stack1 and we have one Fibre from our supplier for internet. 

Just to confirm, this higher priority is to make sure this standalone switch is least preferred for all VLAN's in MST domain and also by doing this way (by configuring higher priority) wont affect the existing stack switches and cause re-election? 

The reason I ask is, i got another scenario where I have a VLAN in 2 DC's connected via L2 switch. Its VLAN 810 in DC1 which is MST and VLAN810 in DC2 in PVST. Its working OK at the moment as I found a way to avoid PVST simulation failure. But i want to change this to DC2 switch to MST as well. The DC1 MST config is same as I mentioned in previous post. In this second scenario, i need to change the DC2 switch to MST using the same priority as DC1 MST?  

[paul driver]

Hello

---

@Psmurali89 wrote:

Just to confirm, this higher priority is to make sure this standalone switch is least preferred for all VLAN's in MST domain and also by doing this way (by configuring higher priority) wont affect the existing stack switches and cause re-election? 

---

Yes correct...

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@Psmurali89 wrote:

i got another scenario where I have a VLAN in 2 DC's connected via L2 switch. Its VLAN 810 in DC1 which is MST and VLAN810 in DC2 in PVST. Its working OK at the moment as I found a way to avoid PVST simulation failure. But i want to change this to DC2 switch to MST as well. The DC1 MST config is same as I mentioned in previous post. In this second scenario, i need to change the DC2 switch to MST using the same priority as DC1 MST?  

---

It depends on how they are connected, if the DC2 switch is the CIST root then I would say you do need to be mindful of STP recalculation, and it is good to know your aware of PVST simulation, as it can be a problem if you are not

Can you elaborate further on the DC stp estate. 

[Psmurali89]

Hi, 

Below is the logical connectivity for VLAN810:

DC1: Router ->Existing Stack1 (MST) -> New Stack (MST) ---Supplier Cloud--- DC2: Standalone Switch (PVST) -> Stack1 (MST) ->Router 

Below is the STP state in each DC switch. 

DC2#sh spanning-tree vlan 810

VLAN0810
Spanning tree enabled protocol ieee
Root ID Priority 8192
Address xxxx.xxxx.xxx
Cost 2
Port 29 (TenGigabitEthernet1/1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 33690 (priority 32768 sys-id-ext 922)
Address xxxx.xxxx.xxx
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 350 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/3 Desg FWD 4 128.3 P2p <<<<< to Stack
Twe1/0/10 Root FWD 2 128.29 P2p <<<<< To DC1

================================================================================================

DC1#sh spanning-tree vlan 810

MST1
Spanning tree enabled protocol mstp
Root ID Priority 8193
Address xxxx.xxxx.xxx
Cost 10000
Port 2094 (Port-channel6)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 8193 (priority 8192 sys-id-ext 1)
Address xxxx.xxxx.xxx
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Twe2/0/22 Desg FWD 2000 128.406 P2p Bound(PVST) <<<<<< to DC2
Po6 Root FWD 10000 128.2094 P2p <<<<<< to Stack

In the above scenario, its working ok for VLAN810 between both DC's after fixing the PVST Simulation failure. But I want to change the DC2 switch to MST. 

[Psmurali89]

Hi

Based on the above scenario, will there be any issues if I change DC2 switch from PVST to MST using same priority as DC1 switch? 

[MHM Cisco World]

Hi friend, 
I config three SW with MST 
IOU1 is root for MST 
and config one SW IOU2 with PVST 
then I change the mode from PVST to MST, 
what exactly want from me to check ??
NOTE:- I prefer keep one of MST SW as root. 

[Psmurali89]

Hi, 

Below is the logical connectivity for VLAN810:

DC1: Router ->Existing Stack1 (MST) -> New Stack (MST) ---Supplier Cloud--- DC2: Standalone Switch (PVST) -> Stack1 (MST) ->Router 

Below is the STP state in each DC switch. 

DC2#sh spanning-tree vlan 810

VLAN0810
Spanning tree enabled protocol ieee
Root ID Priority 8192
Address xxxx.xxxx.xxx
Cost 2
Port 29 (TenGigabitEthernet1/1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 33690 (priority 32768 sys-id-ext 922)
Address xxxx.xxxx.xxx
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 350 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/3 Desg FWD 4 128.3 P2p <<<<< to Stack
Twe1/0/10 Root FWD 2 128.29 P2p <<<<< To DC1

================================================================================================

DC1#sh spanning-tree vlan 810

MST1
Spanning tree enabled protocol mstp
Root ID Priority 8193
Address xxxx.xxxx.xxx
Cost 10000
Port 2094 (Port-channel6)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 8193 (priority 8192 sys-id-ext 1)
Address xxxx.xxxx.xxx
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Twe2/0/22 Desg FWD 2000 128.406 P2p Bound(PVST) <<<<<< to DC2
Po6 Root FWD 10000 128.2094 P2p <<<<<< to Stack

In the above scenario, its working ok for VLAN810 between both DC's after fixing the PVST Simulation failure. But I want to change the DC2 switch to MST. Will this be an issue? I got MST priority as 8192 in DC1 and am looking to have same priority in DC2 as well. 

[paul driver]

Helllo

---

@Psmurali89 wrote:

DC2
sh spanning-tree vlan 810
VLAN0810
Spanning tree enabled protocol ieee
Root ID Priority 8192 Address xxxx.xxxx.xxx
Cost 2 Port 29 (TenGigabitEthernet1/1/1)
Bridge ID Priority 33690 (priority 32768 sys-id-ext 922)

Gi1/0/3 Desg FWD 4 128.3 P2p <<<<< to Stack
Twe1/0/10 Root FWD 2 128.29 P2p <<<<< To DC1

DC1
sh spanning-tree vlan 810

Spanning tree enabled protocol mstp
Root ID Priority 8193
Cost 10000
Port 2094 (Port-channel6)newBridge ID Priority 8193 (priority 8192 sys-id-ext 1)

Twe2/0/22 Desg FWD 2000 128.406 P2p Bound(PVST) <<<<<< to DC2
Po6 Root FWD 10000 128.2094 P2p <<<<<< to Stack

---

Based on the stp readout above, the "new stack" is the current stp root for vlan810, not DC1 or DC2.

In fact DC1 root port is PC6 towards "new stack" and DC2 root port is twe1/0/10 via DC1, which means instead of DC2 going direct the "new stack" is switch path is via DC1 <> new stack

This is due to DC1 & New stack having the same set mst priority  8192 and Dc2 set to 32768 + 922
so the next calculation will be lower stp link cost 
DC1
Twe2/0/22 = cost 2000 (20GB link)
Po6 =cost 1000 (40GB link PC) root

Dc2
G10/3  =cost 4  (1GB link)
Twe1/0/10 =cost 2  (20GB link) root

So if you change DC2 to MST with the same priority as DC1/New stack then the stp should calculate on received path cost as such DC2 should still elect DC1 again for Vl810, be it the stp port cost will change to more recent 8012.D standard, however this only for Vlan810 you do not show any thing in relation to the other pvst vlans on DC2 but MST is per instance for ALL vlans as such this change will relate to all vlans on MST2 if you are only using the single MST instance

Calculated Post Change stp cost of DC2 after change to MST

Dc2     stp cost to root switch - 2000
Twe1/0/10 =cost 1000  (20GB link) root via DC1

DC1
Po6 =cost 1000 (40GB link PC) root