Hi Michael,
IST0 is called IST (Internal Spanning-Tree) and these are the VLANs that haven’t been mapped to another instance.
PVST uses 1 STP for each VLAN, MST uses 1 STP for multiple VLANs so we have a mismatch here. To make the two compatible, here’s what happens:
- Within the MST region we run multiple STPs (one for each instance).
- On the border links (switches that connect to PVST) our MST switch will detect PVST BPDUs (or another MST region) and marks these interfaces as “boundary”.
- The MST switch won’t send any BPDUs from the instances that we configured.
- The MST switch will copy its BPDU from the IST and sends this BPDU on all VLANs towards the “outside” switch (this is called PVST Simulation mode).
This is what allows switches outside of the MST region to see it as “1 big switch”.
If you need some more detail, let me know. I can always do another write-up on MST…
Rene