mst region and pvst+ network

Answered Question
Sep 24th, 2008
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hi every body!

I need some help with following:

My book (cisco press) says" There is no seperate bpu for each msti.IST bpdu is used to carry information such as root priority ,root cost etc for each instance in MR"

then at the end of the chapter, i find this question:

When an mst region meets a pvst+ domain,how is each mst instance propagated into pvst+?

The answer in book as follows:

"The bpdu from each instance are replicated and sent into all appropriate vlans in the pvst+ switch"

My point is one hand , the book says there is no seperate bpdu for each instance, on the other hand i find the above as answer, saying bpdu from each instance is replicated.

i will really appreciate any help !

thanks a lot!

Correct Answer by Giuseppe Larosa about 8 years 7 months ago

Hello Sarah,

my understanding is different I think that at the boundary the MST switch can use the CST instance:


Backward Compatibility: MST is backward compatible with PVST+, Rapid-PVST+, and Prestandard MST (MISTP). MST switch is connected to the other STP (PVST+ and Rapid-PVST+) switches by the Common Spanning Tree (CST). Other STP (PVST+ and Rapid-PVST+) switches see the entire MST region as a single switch. When you connect the prestandard MST switch with the standard MST switch, you need to configure spanning-tree mst pre-standard in the interface of the standard MST switch, .


http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b075f.shtml#PVST


and:


MST can interact with legacy bridges that run PVST+ on a per-port basis, so it is not a problem to mix both types of bridges if interactions are clearly understood. Always try to keep the root of the CST and IST inside the region. If you interact with a PVST+ bridge through a trunk, ensure that the MST bridge is the root for all VLANs that are allowed on that trunk. Do not use PVST bridges as the root of CST.


My understanding is that the MST switch will send the CST BPDus out a trunk to a PVST+ switch but the BPDUs sent out each vlan are actually all the same and that of the CST.


if it was true that it could be able to send out each vlan the appropriate instance for that vlan MST would be seen as capable of supporting PVST+.

One thing is to able to interact another is to be able to emulate a PVST+ switch !


Hope to help

Giuseppe


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Istvan_Rabai Wed, 09/24/2008 - 20:46
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Hi Sarah,


On the MST region boundary there is no reason for the MST switch to send IST bpdus, as the next switch will not understand it (it is configured for pvst+).


This is why the MST switch will send pvst+ bpdus for each vlan into the pvst+ switch.


IST bpdus are only sent within the MST region.


Cheers:

Istvan

Correct Answer
Giuseppe Larosa Wed, 09/24/2008 - 23:12
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Hello Sarah,

my understanding is different I think that at the boundary the MST switch can use the CST instance:


Backward Compatibility: MST is backward compatible with PVST+, Rapid-PVST+, and Prestandard MST (MISTP). MST switch is connected to the other STP (PVST+ and Rapid-PVST+) switches by the Common Spanning Tree (CST). Other STP (PVST+ and Rapid-PVST+) switches see the entire MST region as a single switch. When you connect the prestandard MST switch with the standard MST switch, you need to configure spanning-tree mst pre-standard in the interface of the standard MST switch, .


http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b075f.shtml#PVST


and:


MST can interact with legacy bridges that run PVST+ on a per-port basis, so it is not a problem to mix both types of bridges if interactions are clearly understood. Always try to keep the root of the CST and IST inside the region. If you interact with a PVST+ bridge through a trunk, ensure that the MST bridge is the root for all VLANs that are allowed on that trunk. Do not use PVST bridges as the root of CST.


My understanding is that the MST switch will send the CST BPDus out a trunk to a PVST+ switch but the BPDUs sent out each vlan are actually all the same and that of the CST.


if it was true that it could be able to send out each vlan the appropriate instance for that vlan MST would be seen as capable of supporting PVST+.

One thing is to able to interact another is to be able to emulate a PVST+ switch !


Hope to help

Giuseppe


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