strange switch behavior in mstp simple load sharing scenario,

From: sirus MOGHADASIAN (cyrus.mgh@gmail.com)
Date: Thu Aug 16 2007 - 15:04:25 ART


I have 2950 (port5,6)-----3550 (port 7,8) scenario, and I want to implement
mstp and load sharing between them.
I want to configure 2950 as a root for instance 1 and 3550 as a root for
instance 2

2950 config:
spanning-tree mst configuration
 name MSTREG
 revision 2
 instance 1 vlan 3-5
 instance 2 vlan 20-23
!
spanning-tree mst 1 priority 24576

C2950# sh spanning-tree

MST00
  Spanning tree enabled protocol mstp
  Root ID Priority 32768
             Address 000d.651e.fd80
             Cost 200000
             Port 5 (FastEthernet0/5)
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

  Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
             Address 000d.bdfa.6680
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/5 Root FWD 200000 128.5 P2p Bound(RSTP)
Fa0/6 Altn BLK 200000 128.6 P2p Bound(RSTP)

MST01
  Spanning tree enabled protocol mstp
  Root ID Priority 24577
             Address 000d.bdfa.6680
             This bridge is the root
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

  Bridge ID Priority 24577 (priority 24576 sys-id-ext 1)
             Address 000d.bdfa.6680
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/5 Boun FWD 200000 128.5 P2p Bound(RSTP)
Fa0/6 Boun BLK 200000 128.6 P2p Bound(RSTP)

MST02
  Spanning tree enabled protocol mstp
  Root ID Priority 32770
             Address 000d.bdfa.6680
             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 000d.bdfa.6680
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/5 Boun FWD 200000 128.5 P2p Bound(RSTP)
Fa0/6 Boun BLK 200000 128.6 P2p Bound(RSTP)

and 3550 config:

spanning-tree mode mst
spanning-tree extend system-id
!
spanning-tree mst configuration
 name MSTREG
 revision 2
 instance 1 vlan 3-5
 instance 2 vlan 20-23
!
3550#sh spanning-tree

MST0
  Spanning tree enabled protocol mstp
  Root ID Priority 32768
             Address 000d.651e.fd80
             This bridge is the root
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

  Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
             Address 000d.651e.fd80
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/1 Desg FWD 200000 128.1 Edge P2p
Fa0/7 Desg FWD 200000 128.7 P2p
Fa0/8 Desg FWD 200000 128.8 P2p
Fa0/13 Desg FWD 200000 128.13 P2p

MST1
  Spanning tree enabled protocol mstp
  Root ID Priority 32769
             Address 000d.651e.fd80
             This bridge is the root
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

  Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
             Address 000d.651e.fd80
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/7 Desg FWD 200000 128.7 P2p
Fa0/8 Desg FWD 200000 128.8 P2p

MST2
  Spanning tree enabled protocol mstp
  Root ID Priority 32770
             Address 000d.651e.fd80
             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 000d.651e.fd80
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/7 Desg FWD 200000 128.7 P2p
Fa0/8 Desg FWD 200000 128.8 P2p

the problem is as u see 2950 see 3550 bridge id but 3550 not see 29500
bridge id ,even more strange 2950 see itself as a root bridge for instance 1
but it's not put its port in forwarding state.

Why 3550 is not see 2950 as a root bridge for instance 1 ??? and why 2950
see itself as a root bridge but it's not put it's ports into forwarding
state???

any idea???

Thanks



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