A few questions regarding the MSTP BPDU

[Mitrixsen]

Hello, everyone.

I've just started with MSTP so I started on viewing and reinforcing topics that I already know from the previous STP/RSTP versions by examining the MSTP BPDU.

However, the structure of it confuses me a little. I have this simple MSTP topology here:

Here are the MSTP BPDUs

There are a few things that don't make much sense to me. Classic STP/RSTP include fields in their BPDUs that show the BID of the root bridge and the BID of the local switch. In MSTP, where exactly is this located? Is it correct to say that the "Regional Root Identifier" identifies who the root bridge is for each instance?

If so, where is the BID of the local switch, then? And why is the Proposal and Agreement bit set at the same time?

Thank you.

David

[Martin L]

When multiple regions connect together, every region needs to construct its own IST and all regions should build one common CIST spanning across the regions.

one of the Best sources for MST is INE this blog /paper https://ine.com/blog/2010-02-22-understanding-mstp#CIST

Regards, ML
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[MHM Cisco World]

in your case there are three Root here 
root for instance 1 
root for instance 2 
root for regional 

the last regional is fact that MST have additional election and STP domain this called CST, 

will I try to close the idea of CST and find the way 
in OSPF there is intra and inter AS, same in MST there is BPDU intra regional (here root for instance 1 and instance 2 take place) and there is inter regional here the regional root take place.

http://www.bscottrandall.com/1.4.4.5

MHM