Transmission of MSTP's BPDU

[taolei.cn]

Hello.

In a MSTP region, only IST (instance 0) sends BPDU. These BPDUs carry all information of other instances. (right?)
However, if a port is blocked in IST but is designated in another instance, doesn't it send BPDU any more?

Take an example here, there are 3 MST-enabled switches in a region:
1A2--1B2
|               |
1C2--1D2
Suppose there are 2 instances, IST and MSTI1.
In IST, A is root, port 2 of D is blocked and port 2 of B is the designated port on the segment.
In MSTI1, C is root, port 2 of B is blocked and port 2 of D is the designated port on the segment.

I'm puzzled here:
Because IST is the only instance that sends/receives BPDU, D doesn't send BPDU on its port 2 because it is in blocking state. (right?)
The contradictory thing is that since port 2 of D is a designated port in MSTI1, it should send BPDU (right?), but actually it does NOT, because in IST it is blocked.

Have I misunderstood something ?

Thanks.

[phiharri · Accepted solution]

Greetings Lei,

I don't think it's entirely accurate to say that "only IST sends BPDU" within an MST region - each bridge sends MST BPDUs which can be seen as RSTP BPDUs for the IST plus Mrecords for any applicable MSTIs.

Check the following document which I believe will answer your queries fully:

Understanding Multiple Spanning Tree Protocol (802.1s)

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml

Specifically: "Unlike regular converged spanning tree topology,  both ends of a link can send and receive BPDUs simultaneously. This is  because, as shown in this diagram, each bridge can be designated for one  or more instances and needs to transmit BPDUs. As soon as a single MST  instance is designated on a port, a BPDU that contains the information  for all instances (IST+ MSTIs) is to be sent."

In your scenario above, both switches B and D will originate BPDUs onto the segment connecting them.

Hope this helps!

/Phil

[taolei.cn]

Thank you very much, Phillip.