MST and PVST+ Interoperability

We can also have the extended system id disabled to have the priorities on vlan 10, 20, 30, 40, 50, and 60 identical to the priority of vlan 1 in the PVST+ domain (SW1) by using this global configuration command

no spanning-tree extended system-id

Hello Bassam

Yes, this is the case, however, this command is not available on all platforms. It is an older command. Most new switching platforms have it present in the config, but you cannot remove it. You can find out more information about why this command exists and why it no longer is used in new platforms at the following Cisco Community post:

Can you tell us a little bit more about your reasoning behind using this command? Maybe we can help you out in finding a more suitable way to achieve what you are trying to do.

I hope this has been helpful!

Laz

I just want to check my understanding:

MST region sends BPDU for each and every VLAN with the same bridge ID and bridge priority values from the root bridge from IST, but different System Ext IDs.

PVST switch sends BPDU for each and every VLAN with the root bridge ID and bridge priority values from the root bridge from VLAN 1, but different System Ext IDs.

Bridge priority must be consistent accross all VLANs. In other words, there must not be a root switch for one VLAN in PVST, and root switch for other VLANs in MST region.

Is this correct?

Hello Quirik

The way you describe it sounds good! It matches up well with what is described in the lesson. Good job!!

Laz

Hi,

I am confused whit this statement about MST ports becoming non-designated:

It becomes a non-designated port if the boundary interface receives a VLAN 1 PVST+ BPDU that is superior to its own MST IST BPDU but not good enough to become a root port.

So MST receives superior VLAN PVST BPDU but what does it mean “not good enough”? If it’s superior, it’s superior, right?

Hello Quirik

To put it another way, a port becomes a non-designated port if the boundary interface receives a VLAN1 PVST+ BPDU that is superior to its own MST IST BPDU, but not superior to the best BPDU that it received compared to all of its ports.

If it were the best BPDU, then it would become a root port. So in summary:

  1. If the BPDU is the best, the port becomes a root port
  2. If it is not the best, but better than that of the port in question, it becomes a non-designated port.

I hope this has been helpful!

Laz

I still don’t get it. If MST boundary switch receives superior VLAN 1 BPDU with priority from the root bridge of VLAN 1 in the PVST+ region, how can other VLANs BPDUs from PVST+ region be inferior? Wouldn’t it throw inconsitency error?

Hello Quirik

We must keep in mind what is being compared here.

For the root port:

  • MST listens to all PVST+ BPDUs from all VLANs and compares them to the VLAN 1 BPDU received from PVST+.
    • If a received BPDU is superior, the port becomes a root port.
    • if it is not superior, the port goes into inconsistency error.

For the non-designated port

  • MST compares an incoming VLAN 1 PVST+ to its MST IST BPDU. We are comparing something different here.
    • If it is superior, but not superior to a BPDU received on another boundary interface, then it becomes a non-designated port. But, it goes through one more check:
      • MST checks if the PVST+ domain also thinks it would be a non-designated port, and here it listens in to PVST+ BPDUs. If the PVST+ BPDUs are superior r to its own BPDU, it can become a non-designated port.

In the lesson, Rene goes on to say that:

Cisco switches don’t do these additional checks. Even if a superior BPDU was 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.

I hope this has been helpful!

Laz

I will refer to the lab example from the lesson, specifically the second case where the root bridge is located in PVST region:

  1. SW1 is the root
  2. SW2 G0/1 becomes the Root Port
  3. SW3 G0/1 becomes the Blocking Port/Non-Designated

If I have understood correctly, every MST boundary interfaces (SW2 G0/1 and SW3 G0/1) receives PVST BPDU and compares them to its own MST0 BPDUs.

The reason why SW2 G0/1 becomes the Root Port is because all received PVST BPDUs are equal or superior to SW1 MST0 BPDU-s (priority 8192).

The reason why SW3 G0/1 becomes the Blocking Port is because all received PVST BPDUs are superior to SW3 MST0 BPDUs (priority 32768) but this is not good enough to become the Root Port (32768 > 8192). SW3 knows it is not good enough because it also knows the SW1 is the root bridge for MST0 region with a priority lower that its own.

Would this be correct?

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Hello Quirik

Yes, your detailed explanation describes it well, good job! :sunglasses:

Laz

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Great, thanks! I think I have two typos in my previous post:

PVST BPDUs are equal or superior to SW1 MST0 BPDU-s

should read “… to SW2 MST0 BPDU-s”, and

…it also knows the SW1 is the root bridge for MST0

should read “…knows the SW2 is the root bridge for MST0”.

Just in case in case someone will have the same doubts.

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Hallo Rene,

What VLAN BPDU will be used by SW2 and SW3 to select their root port for all VLANs, if the VLANS in MST were just 10,20.30.40.50 and 60? (Around 7th minute).

Thank you

Hello Joyce

The particular scenario you are describing is one where SW1, using PVST, sends BPDUs to SW2 and SW3 using PVST BPDUs, one for each VLAN. These BPDUs indicate that SW1 has a priority of 4096 (as shown in the related section in the lesson). What happens in this case?

MST will perform a check that must be passed in order for these BPDUs to be accepted. THe check states that "in order to become a root port, all PVST BPDUs except those from VLAN 1, must be the same or superior compared to the PVST VLAN 1 BPDU. HOwever, here, this is not the case since VLANs 10 to 60 have a priority of 32768, while the PVST BPDUs are sending a superior priority.

So SW2 and SW3 will use the VLAN 1 BPDUs received from SW1 to determine their port states (root or not) only if this check is fulfilled. If it is not, there is an error and the ports are put into blocking state.

Other than the video, this is more thoroughly explained in the PVST+ Root Bridge section of the lesson.

I hope this has been helpful!

Laz

Hi Laz,

Thanks so much for that. It’s clear now :+1:t4:

Joyce

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21:48 in the video. I have the same question. I expected that the SW3 should have SW1 as a root. Otherwise, all the earlier explanations are wrong.
Priorities of all the VLANs were set to 0.
SW1(config)#spanning-tree vlan 10,20,30,40,50,60 priority 0
BPDUs from SW1 must be superior to SW2 and SW3.

Hello Vadim

I apologize for the late reply. I’ve gone over this one with Rene and the behavior is indeed what is expected. Briefly stated we have to think about the whole MST topology as a single switch. PVST+ perceives SW2 and SW3 as a single switch, and as we know, each switch can have only one root port. If SW3 perceived SW1 as the root, it would have to create a root port on Gi0/0. But that means that the “MST switch” would have two root ports (the other one being Gi0/0 on SW1) and this is not allowed.

Rene has actually gone in and revised the lesson to include this explanation in detail.

I hope this has been helpful!

Laz