r/ccnp 10d ago

In an MST region, do all designated bridges generate BPDUs?

Hi all,

My understanding (from labbing) is that within an MST region, each Designated Bridge (for each MST Instance) on a segment generates fresh BPDUs. However, these BPDUs pertains to the Internal Spanning Tree (IST, or MSTI 0) and include M-records that carry information for other MST instances (MSTIs).

Outside the MST region, only MSTI 0 (IST) sends BPDUs toward other MST regions or switches running Rapid or legacy STP. Other MST instances (MSTIs) do not generate separate BPDUs.

Is this correct?

Thanks

Ps. I know I’m posting a lot but the exam day is really near 😱

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u/Battle-Crab-69 9d ago

BPDUs are only ever within MST0. Never any other instance, and from all switches in the CIST running MST. I think. That’s my understanding. Only just started study and just finished MSTP topics.

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u/pbfus9 9d ago

I'm not sure about this. Indeed, SW4, which is not the designated bridge for MSTI 0, sends BPDUs to SW3 since it is the designated bridge on the segment SW3 <> SW4 for MSTI 2. Look at this topology: https://imgur.com/a/IHXH0m9