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MST and PVST+ Interoperability

MST (Multiple Spanning Tree) and PVST+ (Per VLAN Spanning Tree) both offer loop-free layer two topologies but they each use a different approach:

  • MST maps multiple VLANs to an instance, reducing the number of spanning tree instances.
  • PVST+ calculates an instance for each spanning tree instance.

The two versions are compatible, but how should the two behave on the link that connects the two spanning tree protocols?

PVST+ sends BPDUs for each instance/VLAN, so you could let MST process each BPDU separately with the instance that is configured for the VLAN. MST, however, doesn’t have an instance for each VLAN, so what kind of BPDUs should it send?

This is an issue. Two VLANs may share the same MST instance but use different root bridges, root ports, and other attributes in the PVST+ domain. Which information should MST send in its BPDUs? Creating a 1:1 mapping between the MST and PVST+ information is impossible.

Instead, when an MST region is connected to a PVST+ topology, MST will simulate PVST+ with a feature called the PVST simulation mechanism. What it does is that the MST region will send PVST+ BPDUs (one for each VLAN) on the interfaces that are connected to PVST+ switches. These BPDUs all carry the same information and advertise the same root bridge. The interfaces that connect to the PVST+ topology are called boundary interfaces/ports.

I’m talking about PVST+, but PVRST+ is also supported. However, MST only uses PVST+ features, so there is no performance difference.

Since PVST+ switches now receive BPDUs for each VLAN from MST carrying the same information, they will all make the same decisions when selecting a root bridge, root port, etc.

Where does the information MST advertises in the simulated PVST+ BPDUs come from?  All MST switches have to agree on the same information, so we use the information from the IST for this. The root bridge in the MST region, elected as the root bridge for the IST, will also be used in the PVST+ BPDUs that the PVST simulation uses.

What about the other way around? From PVST+ to MST?

MST decides the port role (root, designated, non-designated) for the boundary interface for all VLANs only by looking at the information in the VLAN 1 BPDU.

Deciding the port role on the boundary interface for all VLANs is risky…after all, it means that we assume that all VLANs use the same root bridge, root ports, etc. We don’t know if the PVST+ domain agrees on the port role for all VLANs we select on our MST boundary interface. However, there are some simple checks that MST can use to determine if PVST+ agrees on the port roles. Let’s take a look at each possible port role.

Designated Port

Suppose MST wants to use the boundary as a designated port. It uses the following check:

MST BPDU from IST must be superior compared to each PVST+ BPDU from every VLAN that you receive on the boundary interface.

So, MST compares its IST BPDU to all PVST+ BPDUs (one for each VLAN). If its own BPDU is superior, it knows that PVST+ agrees on the port role, and we can safely turn the boundary interface into a designated port. If not, we know that PVST+ doesn’t agree on our port role, and we block our boundary interface. The status of the interface shows up as broken.

Root Port

The boundary interface can become a root port if the VLAN 1 BPDU it receives is the best BPDU that it gets on any of its boundary interfaces. This means that the root bridge for VLAN 1 is located in the PVST+ domain. When this happens, the interface will go into the forwarding state for all VLANs. This is a tricky situation. MST acts as if all VLANs have their root bridge in the PVST+ domain and turns its boundary interface into the root port for all VLANs, but that might not be the case. To counter this, MST uses the following check:

PVST+ BPDUs for all VLANs except VLAN 1 must be identical or superior compared to the PVST+ VLAN 1 BPDU.

This means that MST listens to all PVST+ BPDUs that it receives on its boundary interface and checks if those BPDUs are superior to the VLAN 1 BPDU from PVST+. If so, MST can safely turn its boundary interface into a root port. If not, the switch reports an inconsistency error; the interface will be blocked and show the broken state.

One caveat is that PVST+ uses the system ID extension, which uses the VLAN number as part of the bridge ID. This makes it impossible to have PVST+ BPDUs from different VLANs with identical values. Because of this, PVST+ BPDUs require a priority of at least 4096 lower than the PVST+ VLAN 1 BPDU.

Non-designated Port

Last but not least, the boundary interface could be a non-designated port. It becomes a non-designated port if the boundary interface receives a VLAN 1 PVST+ BPDU superior to its own MST IST BPDU but not good enough to become a root port. In this case, the boundary interface becomes a non-designated port for all VLANs. MST has to check if the PVST+ domain also thinks this should be a non-designated port, and it can quickly check this by listening to all PVST+ BPDUs. When the PVST+ BPDUs are superior to its own BPDU, it can become a non-designated port. Cisco switches don’t do any additional checks. Even if a superior BPDU were received, it would report a PVST inconsistency error, and the port would go into broken mode, but since it’s blocked anyway, it doesn’t matter.

It is easiest to configure your network so that the MST region is the root bridge in your network. If your PVST+ domain has the root bridge, then MST will use the same root port for all VLANs. If the root bridge is in your MST region, then you change the cost per VLAN on your PVST+ switches to use different root ports and a bit of load balancing.

Configuration

Let’s take a look at all of this in action. There are three switches I use:

Mst Pvst Topology Sw1 Sw2 Sw3

SW1 will use rapid spanning tree.SW2 and SW3 will use MST. We will use three MST instances and six VLANs. Let’s start by configuring the interfaces as trunks and adding some VLANs:

SW1, SW2, & SW3
(config)#interface range GigabitEthernet 0/0 - 1
(config-if-range)#switchport trunk encapsulation dot1q
(config-if-range)#switchport mode trunk
SW1, SW2 & SW3
(config)#vlan 10
(config-vlan)#vlan 20
(config-vlan)#exit

We now have a basic configuration that we can use.

MST Root Bridge

In the first scenario, I will use the MST region as the root bridge.




The VLANs are mapped like this:

  • VLAN 10 is mapped to instance 1.
  • VLAN 20 is mapped to instance 2.

SW2 is the root bridge for instances 0 and 1:

SW2(config)#spanning-tree mode mst

SW2(config)#spanning-tree mst configuration
SW2(config-mst)#name REGION1
SW2(config-mst)#instance 1 vlan 10
SW2(config-mst)#instance 2 vlan 20
SW2(config)#spanning-tree mst 0-1 priority 8192

SW3 will become the root bridge for instance 2:

SW3(config)#spanning-tree mode mst

SW3(config)#spanning-tree mst configuration
SW3(config-mst)#name REGION1
SW3(config-mst)#instance 1 vlan 10
SW3(config-mst)#instance 2 vlan 20
SW3(config-mst)#spanning-tree mst 2 priority 8192

SW1 is running rapid spanning tree with its default priority.

Configurations

Want to take a look for yourself? Here you will find the startup configuration of each device.

SW1

hostname SW1
!
spanning-tree mode pvst
spanning-tree vlan 1 priority 4096
spanning-tree vlan 10,20 priority 0
!
interface GigabitEthernet0/0
 switchport trunk encapsulation dot1q
 switchport mode trunk
!
interface GigabitEthernet0/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
!
end

SW2

hostname SW2
!
ip cef
!
spanning-tree mode mst
!
spanning-tree mst configuration
 name REGION1
 instance 1 vlan 10
 instance 2 vlan 20
!
spanning-tree mst 0-1 priority 8192
!
interface GigabitEthernet0/0
 switchport trunk encapsulation dot1q
 switchport mode trunk
!
interface GigabitEthernet0/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
!
end

SW3

hostname SW3
!
ip cef
!
spanning-tree mode mst
!
spanning-tree mst configuration
 name REGION1
 instance 1 vlan 10
 instance 2 vlan 20
!
spanning-tree mst 2 priority 8192
!
interface GigabitEthernet0/0
 switchport trunk encapsulation dot1q
 switchport mode trunk
!
interface GigabitEthernet0/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
!
end

Here’s what all switches think of the topology:

SW1#show spanning-tree vlan 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    8192
             Address     5254.0016.8d33
             Cost        4
             Port        1 (GigabitEthernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     5254.000d.0b6c
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Root FWD 4         128.1    P2p 
Gi0/1               Altn BLK 4         128.2    P2p
SW1#show spanning-tree vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    8192
             Address     5254.0016.8d33
             Cost        4
             Port        1 (GigabitEthernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     5254.000d.0b6c
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Root FWD 4         128.1    P2p 
Gi0/1               Altn BLK 4         128.2    P2p
SW1#show spanning-tree vlan 20

VLAN0020
  Spanning tree enabled protocol ieee
  Root ID    Priority    8192
             Address     5254.0016.8d33
             Cost        4
             Port        1 (GigabitEthernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32788  (priority 32768 sys-id-ext 20)
             Address     5254.000d.0b6c
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Root FWD 4         128.1    P2p 
Gi0/1               Altn BLK 4         128.2    P2p

The output above is interesting to see. As explained earlier, MST maps all VLANs to the IST, so even VLAN 20 shares the same information, even though SW3 is the root bridge for VLAN 20. SW1 sees SW2 as the root bridge for all VLANs.

Here is SW2:

SW2#show spanning-tree mst 0

##### MST0    vlans mapped:   1-9,11-19,21-4094
Bridge        address 5254.0016.8d33  priority      8192  (8192 sysid 0)
Root          this switch for the CIST
Operational   hello time 2 , forward delay 15, max age 20, txholdcount 6 
Configured    hello time 2 , forward delay 15, max age 20, max hops    20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg FWD 20000     128.1    P2p Bound(PVST) 
Gi0/1            Desg FWD 20000     128.2    P2p
SW2#show spanning-tree mst 1

##### MST1    vlans mapped:   10
Bridge        address 5254.0016.8d33  priority      8193  (8192 sysid 1)
Root          this switch for MST1

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg FWD 20000     128.1    P2p Bound(PVST) 
Gi0/1            Desg FWD 20000     128.2    P2p
SW2#show spanning-tree mst 2

##### MST2    vlans mapped:   20
Bridge        address 5254.0016.8d33  priority      32770 (32768 sysid 2)
Root          address 5254.001d.738f  priority      8194  (8192 sysid 2)
              port    Gi0/1           cost          20000     rem hops 19

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg FWD 20000     128.1    P2p Bound(PVST) 
Gi0/1            Root FWD 20000     128.2    P2p

SW2 is the root bridge for the CIST (instance 0) and instance 1. Here is SW3:

SW3#show spanning-tree mst 0

##### MST0    vlans mapped:   1-9,11-19,21-4094
Bridge        address 5254.001d.738f  priority      32768 (32768 sysid 0)
Root          address 5254.0016.8d33  priority      8192  (8192 sysid 0)
              port    Gi0/1           path cost     0        
Regional Root address 5254.0016.8d33  priority      8192  (8192 sysid 0)
                                      internal cost 20000     rem hops 19
Operational   hello time 2 , forward delay 15, max age 20, txholdcount 6 
Configured    hello time 2 , forward delay 15, max age 20, max hops    20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg FWD 20000     128.1    P2p Bound(PVST) 
Gi0/1            Root FWD 20000     128.2    P2p
SW3#show spanning-tree mst 1

##### MST1    vlans mapped:   10
Bridge        address 5254.001d.738f  priority      32769 (32768 sysid 1)
Root          address 5254.0016.8d33  priority      8193  (8192 sysid 1)
              port    Gi0/1           cost          20000     rem hops 19

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg FWD 20000     128.1    P2p Bound(PVST) 
Gi0/1            Root FWD 20000     128.2    P2p 
SW3#show spanning-tree mst 2

##### MST2    vlans mapped:   20
Bridge        address 5254.001d.738f  priority      8194  (8192 sysid 2)
Root          this switch for MST2

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg FWD 20000     128.1    P2p Bound(PVST) 
Gi0/1            Desg FWD 20000     128.2    P2p

SW3 sees SW2 as the root bridge and regional root bridge for instance 0. SW3 also sees SW2 as the root bridge for instance 1. SW3 is the root bridge for instance 2.

Let’s see what happens now when I try to make SW1 the new root bridge for one VLAN:

SW1(config)#spanning-tree vlan 20 priority 4096

As soon as you do this, this message will show up on SW2 and SW3:

SW2#
%SPANTREE-2-PVSTSIM_FAIL: Blocking designated port Gi0/0: Inconsistent superior PVST BPDU received on VLAN 20, claiming root 4116:5254.000d.0b6c
SW3#
%SPANTREE-2-PVSTSIM_FAIL: Blocking designated port Gi0/0: Inconsistent superior PVST BPDU received on VLAN 20, claiming root 4116:5254.000d.0b6c

The spanning tree PVST simulation fails. For the boundary interface to be a designated port, the MST IST BPDU must be superior compared to all PVST+ BPDUs, which is not the case now.  As a result, the boundary interface is now in the broken state on SW2:

SW2#show spanning-tree mst 0

##### MST0    vlans mapped:   1-9,11-19,21-29,31-39,41-49,51-59,61-4094
Bridge        address 5254.0016.8d33  priority      8192  (8192 sysid 0)
Root          this switch for the CIST
Operational   hello time 2 , forward delay 15, max age 20, txholdcount 6 
Configured    hello time 2 , forward delay 15, max age 20, max hops    20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg BKN*20000     128.1    P2p Bound(PVST) *PVST_Inc 
Gi0/1            Desg FWD 20000     128.2    P2p
SW2#show spanning-tree mst 1

##### MST1    vlans mapped:   10,20,30
Bridge        address 5254.0016.8d33  priority      8193  (8192 sysid 1)
Root          this switch for MST1

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg BKN*20000     128.1    P2p Bound(PVST) *PVST_Inc 
Gi0/1            Desg FWD 20000     128.2    P2p 
SW2#show spanning-tree mst 2

##### MST2    vlans mapped:   40,50,60
Bridge        address 5254.0016.8d33  priority      32770 (32768 sysid 2)
Root          address 5254.001d.738f  priority      8194  (8192 sysid 2)
              port    Gi0/1           cost          20000     rem hops 19

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg BKN*20000     128.1    P2p Bound(PVST) *PVST_Inc 
Gi0/1            Root FWD 20000     128.2    P2p

And also on SW3:

SW3#show spanning-tree mst 0

##### MST0    vlans mapped:   1-9,11-19,21-4094
Bridge        address 5254.001d.738f  priority      32768 (32768 sysid 0)
Root          address 5254.0016.8d33  priority      8192  (8192 sysid 0)
              port    Gi0/1           path cost     0        
Regional Root address 5254.0016.8d33  priority      8192  (8192 sysid 0)
                                      internal cost 20000     rem hops 19
Operational   hello time 2 , forward delay 15, max age 20, txholdcount 6 
Configured    hello time 2 , forward delay 15, max age 20, max hops    20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg BKN*20000     128.1    P2p Bound(PVST) *PVST_Inc 
Gi0/1            Root FWD 20000     128.2    P2p
SW3#show spanning-tree mst 1

##### MST1    vlans mapped:   10
Bridge        address 5254.001d.738f  priority      32769 (32768 sysid 1)
Root          address 5254.0016.8d33  priority      8193  (8192 sysid 1)
              port    Gi0/1           cost          20000     rem hops 19

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg BKN*20000     128.1    P2p Bound(PVST) *PVST_Inc 
Gi0/1            Root FWD 20000     128.2    P2p
SW3#show spanning-tree mst 2

##### MST2    vlans mapped:   20
Bridge        address 5254.001d.738f  priority      8194  (8192 sysid 2)
Root          this switch for MST2

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/0            Desg BKN*20000     128.1    P2p Bound(PVST) *PVST_Inc 
Gi0/1            Desg FWD 20000     128.2    P2p

The GigabitEthernet 0/0 interface enters the BKN (broken) status for all instances/VLANs. In the last output, you can see that even though GigabitEthernet 0/0 is in the broken state, SW2 does recognize SW1 as the root bridge.

Let’s increase the priority of SW1 so that SW2 and SW3 become the root bridges in this topology.

SW1(config)#spanning-tree vlan 20 priority 32768

This solves our issues with the broken interfaces:

SW2#
%SPANTREE-2-PVSTSIM_OK: PVST Simulation inconsistency cleared on port GigabitEthernet0/0.
SW3#
%SPANTREE-2-PVSTSIM_OK: PVST Simulation inconsistency cleared on port GigabitEthernet0/0.

SW1 now sees the MST region as the root bridge. The inconsistency is cleared since all PVST+ BPDUs that SW2 and SW3 receive are inferior to the MST IST BPDU.

Having the MST region as the root bridge is a best practice. It’s simple to maintain, and it allows load balancing. For example, let’s say we want the PVST+ domain to use another root port for VLAN 20. We can achieve this by playing with the cost:

SW1(config)#interface GigabitEthernet 0/1
SW1(config-if)#spanning-tree vlan 20 cost 1

SW1 now uses GigabitEthernet 0/1 as its root port to reach VLAN 60:

SW1#show spanning-tree vlan 20

VLAN0020
  Spanning tree enabled protocol ieee
  Root ID    Priority    8192
             Address     5254.0016.8d33
             Cost        1
             Port        2 (GigabitEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32788  (priority 32768 sys-id-ext 20)
             Address     5254.000d.0b6c
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Altn BLK 4         128.1    P2p 
Gi0/1               Root FWD 1         128.2    P2p

GigabitEthernet 0/1 is now the root port for VLAN 20. All other VLANs still use GigabitEthernet 0/1 as their root port:

SW1#show spanning-tree vlan 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    8192
             Address     5254.0016.8d33
             Cost        4
             Port        1 (GigabitEthernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     5254.000d.0b6c
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Root FWD 4         128.1    P2p 
Gi0/1               Altn BLK 4         128.2    P2p
SW1#show spanning-tree vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    8192
             Address     5254.0016.8d33
             Cost        4
             Port        1 (GigabitEthernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     5254.000d.0b6c
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Root FWD 4         128.1    P2p 
Gi0/1               Altn BLK 4         128.2    P2p

PVST+ Root Bridge

It is possible to have the root bridge in the PVST+ domain.




Let’s see how we can make this work. I’ll make SW1 the root bridge for VLAN 1:

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Forum Replies

  1. Hello Stefano.

    PVST+ as its name suggests creates an spanning tree instance for EACH VLAN. MST on the other hand can have many VLANs participate in a single spanning tree instance thus reducing the number of spanning tree instances and thus consuming fewer system resources (CPU, Memory etc).

    I hope this has been helpful!

    Laz

  2. Hello Ehsan

    Essentially, PVRST uses multiple instances of IEEE 802.1w. Most non-cisco devices that support RSTP will support one instance of PVRST. In most cases, because PVRST is backward compatible with RSTP, the PVRST implementation will revert to RSTP. So you are essentially correct in your descsription.

    I hope this has been helpful!

    Laz

  4. Hi Rene,

    In the video example, why did SW3 choose to have SW2 as its root where SW1 in the PVST+ is root ? Did it have to follow MST root port ?

    Thanks,
    M

  5. Hello Madhu

    Can you please clarify at which point in the video this situation takes place? Also, if the same is found in the lesson text, please specify the location so we can look more closely at it and so that we can respond more appropriately to your question.

    Thanks!

    Laz

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