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MSTIs and MST Regions

Hi,

An MST region is defined by bridges sharing the same config name, config rev and vlan to instance mapping digest. If that is the case, how do you define MSTIs within a region since by definition, an MST region will have only a single MST instance..?

Thanks!

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MSTIs and MST Regions

You are correct, in that all switched within a MST region must share the following.

- Region Name

- Revision Number

- VLAN-to-Instance Mapping table.

You define MSTI's within an MST region by doing the following

instance x vlan vlan-value

So for instance

spanning-tree mst configuration

instance 1 vlan 10,20,30,40,50

instance 2 vlan 60,70,80,90,100

MSTI, will have a CST (or Common Spanning Tree) and, you will have two MSTIs that will have the vlans specified above.

I do know that the MSTI, helps when MST, comes in contact with another switch running PVST+, that whole spanning-tree domain (if you will) will be like one big switch.

I'm sure the MSTI, can be defined better, but I hope that helps.

Cisco Employee

MSTIs and MST Regions

Hello,

John has provided a very good introduction - please allow me to add some details.

by definition, an MST region will have only a single MST instance

No, this would be a misunderstanding. A region can internally contain multiple MST instances, and that is why you may want to have multiple regions - in order to allow each of them to have their own set of instances, their own mappings of VLANs to these instances, their own root bridges in individual instances etc. What is true, however, is that in each region, there is a special instance MSTI 0, also labeled as IST (Internal Spanning Tree), which is the only one that interacts with the world outside a particular region. Because the region uses only a single instance 0 when talking to another regions, it appears as a single huge switch to another regions and to non-MST parts of the network that may run older STP versions. Perhaps this was the source of confusion.

The CST (Common Spanning Tree) mentioned by John is a somewhat different concept not related to the internal region operation - the CST is the spanning tree that interconnects the regions together, i.e. a spanning tree you could visualize by considering each region to be represented by a large single virtual switch, having as many virtual switches as there are regions in your network, interconnected appropriately, and drawing a classic spanning tree between these virtual switches. This spanning tree that interconnects the regions in a loop-free fashion would be the CST. However, the CST refers to the operation of MST between regions, and it is not that much related to your original question.

Please feel welcome to ask further.

Best regards,

Peter

MSTIs and MST Regions

Thanks for the in depth explantion.

So from my understanding now, the better way to describe MSTI 0 from my point of view woudl be. MSTI 0 is a special instance, within a MST region, that interacts with the world outside of its local region, and it appears to be a single huge switch whe ntaling with other regions and to non-MST parts.

So.

(MST Region RegionA)<------->

So MST RegionA would communicate with the PVST+ region via, MSTI0, and would see the PVST+ (domain if you will) as one big switch?

Cisco Employee

MSTIs and MST Regions

Hi John,

You are heartily welcome.

MSTI 0 is a special instance, within a MST region, that interacts with  the world outside of its local region, and it appears to be a single  huge switch when talking with other regions and to non-MST parts.

Yes, that is correct.

(MST Region RegionA)<------->

So  MST RegionA would communicate with the PVST+ region via, MSTI0, and  would see the PVST+ (domain if you will) as one big switch?

Let's first talk about normal STP or RSTP instead of PVST+. PVST+ complicates things by an order of magnitude.

If you connected an MST region to an STP/RSTP domain, this domain would indeed interact only with MSTI 0, thinking it is talking to a single switch. The STP/RSTP domain would have no clue there are several other instances inside the MST region. From the viewpoint of the MST region, the STP/RSTP domain would not be considered a single switch but the MSTI 0 and STP/RSTP interaction would be performed in a completely normal and usual way, just as a single switch (in this case, the entire MST region) interacts with multiple neighboring switches.

With PVST+, the situation is more complex. MST tries to make the region appear as a single identically configured switch in all VLANs to the PVST+ domain. This is accomplished by taking the MSTI 0 information and replicating it into PVST+ BPDUs for all VLANs on the boundary ports between the MST region and the PVST+ domain. This is called PVST+ Simulation. Because the BPDUs for individual VLANs sent out from MST region boundary ports will contain exactly the same information derived from MSTI 0, the PVST+ domain will consider the MST region to be a single switch identically configured for all VLANs.

From the viewpoint of MST region, the PVST+ domain is more difficult to represent. Although the MSTI0-to-PVST+ replication takes place on the boundary ports, it is again only MSTI0 that talks to the outside PVST+ domain. In a reciprocal fashion, from all VLANs in the PVST+ domain, only a single VLAN will be chosen to represent the entire PVST+ domain. Cisco decided that it is going to be VLAN1 from the PVST+ domain that will be interacting with the MSTI 0 at the region boundary. Therefore, while MST region knows there is a PVST+ domain outside, it sees it only via VLAN1's eyes.

Best regards,

Peter

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