06-12-2016 12:44 AM - edited 03-08-2019 06:09 AM
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
06-12-2016 10:05 AM
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
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