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 ânot good enoughâ? If itâs superior, itâs 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:
- If the BPDU is the best, the port becomes a root port
- 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 donât 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? Wouldnât 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.
- 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:
In the lesson, Rene goes on to say that:
Cisco switches donât 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 itâs blocked anyway, it doesnât 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:
- SW1 is the root
- SW2 G0/1 becomes the Root Port
- 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! 
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 â⊠to SW2 MST0 BPDU-sâ, and
âŠit also knows the SW1 is the root bridge for MST0
should read ââŠknows the SW2 is the root bridge for MST0â.
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. Itâs clear now 
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. Iâve 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 âMST switchâ 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
Hi ,
When a boundary PVST+ switch send a bpdu for VLAN 20 for example. And the MST region that Boundary switch sent the BPDU has VLAN 20 in its instance 2. And there is no configuraton for MST on the Boundary switch at all, it is using only PVST+
Since that boundary switch have no attributes for MST therefore willl not match to a MST region right even VLAN 20 is binded to an instance within MST ?
- I didnt understand this, OK From MST to PVST+ They are based on IST BPDU of MST, From PVST+ to MST it is based on VLAN 1 BPDU of PVST+
But in that case root bridge should be the same switch for PVST+ Domain and MST Domain. How they are negotiating that? Does PVST+ âSyncronizesâ itself according to MST IST 0 For VLAN 1?
In the following sentence from the lesson- âThis 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+.â
it means that all the BPDUs needs to be superiors for the port to be a root port, or only on of them is enough?
and regarding another sentence related to Non desiganed- âMST 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.â
it means that all the BPDUs needs to be superiors for the port to be a Non desiganted port, or only on of them is enough?
thanks
Hello Roi
BPDUs are sent periodically. They are not just checked once and then not examined again. They are examined every time they arrive. So every BPDU is examined, and as long as those BPDUs are superior or equal to the BDPUs received on VLAN 1 of the PVST+ domain, then that boundary port continues to be a root port. If the next BPDU that is received doesnât fulfill this requirement, then that boundary port cannot remain a root port.
Again, the same thing applies. As long as the received BPDUs fulfill the requirements, the status remains the same. As soon as the next BPDU does not, the status of the interface changes. So it is a continuous check for all received BPDUs. Does that make sense?
I hope this has been helpful!
Laz
Hello Görgen
First of all, we have to define a boundary switch as far as MST goes. A boundary switch by definition is the switch within the MST topology that has at least one port connected to a non-MST region. That means that by definition, the boundary switch must be running MST as well. So there is no such thing as a boundary switch that does not have any MST configuration.
So for your first question, a boundary switch will always âknowâ to match the appropriate VLANs.
Your question dives deep into how the root bridge negotiation and BPDU handling occur between MST and PVST+, particularly across a boundary switch. Letâs see how the root bridge negotiation takes place:
- Root Bridge Consistency: Ideally, for optimal path selection and to prevent loops, the root bridge should be consistent across both domains (MST and PVST+). However, because each domain calculates the root bridge based on its own BPDUs, mismatches can occur unless carefully managed.
- MSTâs Function: When MST receives a PVST+ BPDU for VLAN 1, it evaluates whether the sender of the BPDU has a better path cost or bridge ID than its current root for the CST. If so, it might adapt by treating that PVST+ switch as the root for interoperability purposes.
- PVST+'s Reaction: Conversely, when a PVST+ switch receives an IST BPDU from an MST region, it can potentially adjust its root bridge selection for VLAN 1 if the MST IST presents a superior root bridge claim. However, this adjustment doesnât affect other VLANs directly in PVST+.
Now when it comes to the practical implementation, you must keep the following in mind:
- Network administrators need to carefully configure both MST and PVST+ settings to ensure that root bridge selections align or at least donât create suboptimal paths or loops. This may involve adjusting bridge priorities and path costs to steer root bridge election in a desired direction.
- Some network designs might implement BPDU filtering at the boundary to manage or mitigate the impacts of root bridge mismatches between MST and PVST+.
The negotiation between MST and PVST+ regarding root bridge status hinges significantly on how VLAN 1âs BPDUs are treated by both protocols. Synchronization isnât automatic and requires careful network design and configuration to ensure both MST and PVST+ domains behave predictably and efficiently. This is crucial to maintaining network stability and ensuring optimal performance across interconnected spanning tree protocol environments.
I know itâs not a simple answer, but a full understanding of this process is necessary in order to comprehend the operation of this interaction. If you have more clarification questions, feel free to let us know!
I hope this has been helpful!
Laz