Problem with MST

[per.nielsen]

I have enabled MST on a access layer switch

here is the MST config

spanning-tree mode mst

spanning-tree portfast bpduguard default

no spanning-tree optimize bpdu transmission

spanning-tree extend system-id

!

spanning-tree mst configuration

name building1

revision 1

instance 2 vlan 12

instance 6 vlan 16

Now look at this output fro show sho span tree

How come the switch thinks it's the root switch for MST02 & MST06 and at the same time be blocking ports

ASW12#show spanning-tree

MST00

Spanning tree enabled protocol mstp

Root ID Priority 32768

Address 000a.f49a.b080

Cost 200000

Port 1 (FastEthernet0/1)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)

Address 0013.8038.5fc0

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/1 Root FWD 200000 128.1 P2p Bound(RSTP)

Fa0/2 Altn BLK 200000 128.2 P2p Bound(RSTP)

Fa0/3 Desg FWD 2000000 128.3 Edge Shr

MST02

Spanning tree enabled protocol mstp

Root ID Priority 32770

Address 0013.8038.5fc0

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)

Address 0013.8038.5fc0

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/1 Boun FWD 200000 128.1 P2p Bound(RSTP)

Fa0/2 Boun BLK 200000 128.2 P2p Bound(RSTP)

MST06

Spanning tree enabled protocol mstp

Root ID Priority 32774

Address 0013.8038.5fc0

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32774 (priority 32768 sys-id-ext 6)

Address 0013.8038.5fc0

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/1 Boun FWD 200000 128.1 P2p Bound(RSTP)

Fa0/2 Boun BLK 200000 128.2 P2p Bound(RSTP)

Also I have an upstream switch with this mst config

spanning-tree mode mst

spanning-tree extend system-id

spanning-tree vlan 1,12-13 priority 8192

!

spanning-tree mst configuration

name building1

revision 1

instance 1 vlan 11

instance 2 vlan 12

instance 3 vlan 13

instance 5 vlan 15

instance 6 vlan 16

instance 7 vlan 17

!

spanning-tree mst 1,3,6 priority 8192

spanning-tree mst 2,5,7 priority 4096

We cant even control which which port becomes the root port on out acces layer switch

In fact is doing the total opposite that we have requested

cheers

[smahbub]

Multiple Spanning Tree (MST) is an IEEE standard inspired from the Cisco proprietary Multiple Instances Spanning Tree Protocol (MISTP) implementation. This document assumes that the reader is familiar with Rapid STP (RSTP) (802.1w), as MST heavily relies on this other IEEE standard.

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

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml

[Francois Tallet]

You need to have a little bit more understanding of what MST is, which is not obvious for engineers very used to PVST.

In MST, there is a dependency between instances: basically, all MST Instances (id > 0) depend on the CIST (id 0). MST Instances have a scope restricted to an MST region, ie, a group of switch sharing the exact same MST configuration. At the boundary of a region, all MST Instances follow the state of the CIST.

In your example, the port you mention is at the boundary of the switch's region (its neighbor is not in the same region). As a result, all the MST Instances follow the state of the CIST for this port: as F0/2 is blocking for instance 0, it will be blocking for all the instances.

The bridge is root for the MST Instances, because it has the best root ID for these instances in the region (again, the instances only exist within the region).

Regards,

Francois