MST regions and interaction with other region or PVST

[SJ K]

Hi all,

I got some clarifications about MST region that I hope gurus here can advise.

q1) if I have 2 switch (S1 and S2). Both are running MST and are connected via a trunk link.

They have the same region name, same revision number, but different instance to vlan mapping as below

SW1
------

Region name - Apple

Revision - 1

Instance  0 - vlan 1,5-4096

Instance  1 - vlan 2,3

Instance 2 - vlan 4

SW2
------

Region name - Apple

Revision - 1

Instance  0 - vlan 1,4-4096

Instance  1 - vlan 2,3

Q1) Are they consider to be the in same "Apple" region ?

Q2) What the difference if they are consider as 2 different region as compared to if they are in the same region ?

------------------

Q3) If I add a 3rd Switch (SW3) running PVST and add a trunk link between SW3 and SW1;

SW3 has the same VLANs setup as SW1
SW3 has a lower bridge mac address then SW1

a) Does MST0 on SW1 view SW3 as its root bridge ?

b) Will Instance1or2 of SW1 send any BPDUs to SW3 through the trunk link ?
Will the trunk link be available for usage (FWD state) for MST instance 1 and 2 in SW1 ?

Regards,
Noob

[Francesco Molino]

Hi

I will try to reply very simply as there are a lot of great documents made by Cisco.

Within MSTP, to consider a region as identical (same region), all 3 values must be exactly the same:

 - instance name

 - revision number

 - instances 1

In your example, instances are not the same that means MSTP will consider those 2 MSTP configurations as 2 different regions. Links between Regions are called boundary port. There will be 1 CIST Region root elected within each region and 1 CIST Root elected between the regions by using boundary ports. The standard STP election process take place to elect the CIST Root.

For a sTP design between PVST and MSTP, you need to follow some rules in order to avoid issues (connectivity issues).

I will not write all things right here as this has been discussed a lot and some good documentation are talking about that:

http://blog.ine.com/2010/02/22/understanding-mstp/#PVSTP_INTEROP

http://blog.internetworkexpert.com/2008/07/27/mstp-tutorial-part-i-inside-a-region/

http://blog.internetworkexpert.com/2008/09/24/mstp-tutorial-part-ii-outside-a-region/

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24248-147.html

https://supportforums.cisco.com/discussion/10834356/mstp-and-pvst-cohabitation

Hope you will have a better understanding after reading this docs.

Thanks