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Help with Spanning Tree | Migrate from PVST+ to MST

Dear all,

I am facing some difficulty into migrating switches to Multiple Spanning Tree.

I am planning to perform a migration as follows:

I have two distribution switches A and B, linked by fibre, which are running pvst spanning tree. Each switch has other access switches under it, linked through trunk ports. I am now adding two more switch C and D - distribution switches.

switches.png

I want normal VLANS (current ones) between A and B, and I am using a new VLAN (Private) especially between C and D.

I have successfully configured a lab between 4 four switches and my MST is working fine with the following conditions:

- Normal VLAN is on MST Instance 1. MST 1 is blocked between C and D.

- Private VLAN is on MST Instance 2. MST 2 is blocked between A and B.

- If C talks with D with normal VLAN, it HAS TO go through C--A--B--D.

- If C talks with D with new Private VLAN it goes directly C--D

- If A talks with B on normal VLAN, it goes directly.

- If A talks with B on private VLAN, it goes A--C--D--B.

- If any fibre fails, all traffic (normal + Private VLAN) go through the other fibre.

No problem with the configuration. I do not have any issue with the MST, as I have made extensive lab testing on physical hardware and this seems to satisfy all conditions.

I have already connected switches C and D, interconnecting them, but not to A or B (A and B are in production network)

Today, I have changed config on switch A from PVST to MST and connected switch C to A. I can see MST goes up successfully. Traffic flows well between A, C and D which are now connected.

Issue

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My issue starts when I see one access switch, connected to Switch A, through a trunk port. This port on switch A is blocked by spanning-tree.

The port shows "Bound(PVST) *PVST_Inc " under show spanning-tree mst command. See code below

switches.png

Interface             Role Sts Cost      Prio.Nbr Type

----------------      ---- --- --------- -------- --------------------------------

Gi0/14           Root BKN*100       128.15   P2p Bound(PVST) *PVST_Inc

PVST is configured on the access switch.

1. Do I need to configure MST to ALL switches connected to my 4-switch setup ?

2. How can I best achieve connectivity, since I have other access switches like this connected to the distribution switches over trunk ?

Any help and comments please ?

Thanks

Message was edited by: Shrawan Kumar Ramnohur

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3 REPLIES
Cisco Employee

Help with Spanning Tree | Migrate from PVST+ to MST

Hi Shrawan,

My issue starts when I see one access switch, connected to Switch A,  through a trunk port. This port on switch A is blocked by spanning-tree.

The port shows "Bound(PVST) *PVST_Inc " under show spanning-tree mst command. See code below

This output refers to a so-called PVST Simulation inconsistency. PVST Simulation is a Cisco mechanism run on MST boundary ports towards PVST+ switches that tries to provide cooperation between the MST and the PVST+ region. This mechanism tries to make sure that despite only VLAN1 information from PVST+ region is processed by MST region, and only MSTI 0 (i.e. IST) information is processed by PVST+ region, they both arrive at the same consistent conclusion about who is the root switch for all VLANs. This requires that

  • Either the added PVST+ switch has a higher priority in each VLAN than the MSTI0 root switch, meaning that the MSTI0 root switch will win over the added PVST+ switch
  • Or the added PVST+ switch has a lower priority in VLAN1 than the current MSTI0 root switch, and in addition, its priority in any other VLAN is even lower than its priority in VLAN1, making the PVST+ switch become the root for all VLANs including those in the MST region

If either of these requirements is not met, PVST Simulation inconsistency is declared, and the port is blocked.

In your case, if you wanted to remain with PVST+ on the added switch, the easiest way of solving the inconsistency would be to configure it with the priority of 61440 in all VLANs, assuming that the MSTI0 root switch has a lower priority.

1. Do I need to configure MST to ALL switches connected to my 4-switch setup ?

Technically, you don't if you follow the rules above. However, mixing STP versions is always a bad idea. It is always better to use a single STP version throughout your network.

2. How can I best achieve connectivity, since I have other access  switches like this connected to the distribution switches over trunk ?

If you follow the advice above, you will be able to run the switches even if the network core is MST and the access switches are PVST+. However, I really recommend moving to a single STP version, and if you are fine with MST, so the better.

Best regards,

Peter

Re:Help with Spanning Tree | Migrate from PVST+ to MST

Hi Peter,

Thanks for the response. This sure makes things clearer.

The added switch belongs to another customer, and he has additional subsequent devices connected to it.

Now, what i also know is that the customer has not modified the default PVST values for priorities on his devices.


Also i do not / can not manage all his switches.

With the above conditions, i will want to increase priority of my mst switches on mst0, right ?

All i have is increase priority of my mst 0 on all my mst switches ?
Hope i am going in your direction
Thanks.

Nitish




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Cisco Employee

Help with Spanning Tree | Migrate from PVST+ to MST

Hi Shrawan,

Now, what i also know is that the customer has not modified the default PVST values for priorities on his devices.

You are saying your customer uses default PVST priorities - okay. What about your priorities? Are you using default MST priority settings?

With the above conditions, i will want to increase priority of my mst switches on mst0, right ?

Well, you want to select a single switch inside your MST region which will become the MST0 root and its priority will be numerically lower than priority of any PVST switch in any VLAN. This will make it win over any PVST switch in any VLAN, and thus meet the first consistency criterion.

All i have is increase priority of my mst 0 on all my mst switches ?

No - just on a single MST switch, and you want to decrease the priority (i.e. set it to 4096 or 0), not increase it. You want your MST region to be the root, not the customer's switches.

Best regards,

Peter

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