[c-nsp] Cisco MSTP - PVSTP question

[Mihai Tanasescu]

Hello,



I saw some previous topics in the cisco-nsp archive regarding MSTP and 
PVSTP interaction but I still have some uncertainties.
Also I've read the "understanding MSTP article" from cisco's website.


I've setup a test scenario with the following configuration:


C1 Cisco PVST  ---- C2 MST ---- C3 MST non-cisco switch (doesn't know PVSTP)

C1 has vlans 1-10 for which it must necessarily be root bridge

C2 must be root bridge for all other vlans

C3 will transport 1-10 + the other vlans

C1 can not be migrated to MST while C2 if possible should interoperate  
with the non-cisco MST enabled switch.

What I have done:

Setup 1:

C2 mst root bridge for all vlans
C3 learnt of C2 being the root bridge
C1 the PVST also learnt this (as far as I've read all  communication 
with the PVST is done via the IST-CST instance)

Although this worked just fine, unfortunately it wasn't what I was 
searching for.


Setup 2:

C1 -- C2

C1 lower priority for vlans 1-10 (disabled spanning tree on the other 
vlans / or removed them from the trunk to C2 if not required to be 
present there)

C2 reported:

SPANTREE-2-ROOTGUARD_UNBLOCK: Root guard blocking port ...

And the port was shown as blocked in both the IST0 and the other MSTIs.

I've also tried the alternate configuration (not recommended) from:
http://www.cisco.com/warp/public/473/147.html#alternate_configuration
without any luck.

Disabling PVSTP on the C1 interface to C2 of course made the C2 port to be removed from blocking as expected.

I have several questions in regard to this:

a) Why are both IST0 and the boundary ports for the MSTIs placed in Blocking ?

b) According to that article shouldn't there be a way to have the PVST be root bridge for all the instances present on it ?

c) What alternate setup could there be possible to achieve the redundancy desired while maintaining 
C1 root bridge for vlans 1-10
C2 root bridge for the others
C3 interoperability with C2 (C3 only knows MST and RSTP)



Any advice would be appreciated.


Thanks,
Mihai