Confused about MST spanning-tree output

[Patrick McHenry]

I'm just labbing some MST for some CCNP practice and I noticed that my 2950 switches always think they are the root for both MST instances, when they shouldn't be the root for any. The output of the 3550s seem to make sense. One is the root for one instance and the other is the root for the other instance. I even configured pre standard on the links that go to the 2950s. Any ideas? Thanks, Pat.

2950

MST01
  Spanning tree enabled protocol mstp
  Root ID    Priority    32769
             Address     0016.c87e.2240
             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     0016.c87e.2240
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/3            Boun BLK 200000    128.3    P2p Bound(RSTP)
Fa0/4            Boun FWD 200000    128.4    P2p Bound(RSTP)

MST02
  Spanning tree enabled protocol mstp
  Root ID    Priority    32770
             Address     0016.c87e.2240
             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     0016.c87e.2240
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/3            Boun BLK 200000    128.3    P2p Bound(RSTP)
Fa0/4            Boun FWD 200000    128.4    P2p Bound(RSTP)

ALS1#

3550

MST1
  Spanning tree enabled protocol mstp
  Root ID    Priority    1
             Address     000d.65eb.3800
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    1      (priority 0 sys-id-ext 1)
             Address     000d.65eb.3800
             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    P2p
Fa0/3               Desg FWD 200000    128.3    P2p Pre-STD
Fa0/5               Desg FWD 200000    128.5    P2p Pre-STD-Cf

MST2
  Spanning tree enabled protocol mstp
  Root ID    Priority    2
             Address     000d.65b3.ec00
             Cost        200000
             Port        1 (FastEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    4098   (priority 4096 sys-id-ext 2)
             Address     000d.65eb.3800
             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
Fa0/3               Desg FWD 200000    128.3    P2p Pre-STD
Fa0/5               Desg FWD 200000    128.5    P2p Pre-STD-Cf

DLS1#

[Peter Paluch]

Patrick,

Your 2950 believes that both Fa0/3 and Fa0/4 are boundary ports, and therefore it considers itself to be a standalone region. Quite logically, it considers itself to be a root switch in that region (a single switch in a region is always its root). Can you confirm that the configuration of the MSTP region is identical on both switches? Please use the show spanning-tree mst config digest command to verify the MD5 digests along with the region name and revision number to make sure that the switches are configured correctly for a single region operation.

Best regards,

Peter

[Patrick McHenry]

Thanks, Peter. The revision number has only local significance, correct?

[Peter Paluch]

Hello Patrick,

The revision number has only local significance, correct?

No, the region revision number is a value that is carried in MSTP BPDUs and compared among MSTP switches. It must be set to an identical value on all switches in the same region. By definition, two switches are in the same MSTP region if and only if all of the following requirements are met:

• The region name must be identical (set with the name command)
• The configuration revision must be identical (set with the revision command)
• The mapping of VLANs to instances must be identical (set with the instance command)

Best regards,

Peter