[c-nsp] What is spanning tree interface "St1"

[john douglas]

Hi All,

I am investigating some ongoing spanning tree root changes and I am
seeing this interface "St1" appearing in the STP root debug

016536: Jul  7 16:10:01.680 AEST: STP: VLAN0148 heard root
32916-0013.60a9.0f00 on St1
016537: Jul  7 16:10:01.680 AEST: STP: VLAN0149 Topology Change rcvd on St1
016538: Jul  7 16:10:01.680 AEST: STP: VLAN0148 heard root
32916-0013.60a9.0f00 on St1
016544: Jul  7 16:10:01.722 AEST: STP: VLAN0148 we are the spanning tree root
016545: Jul  7 16:10:01.722 AEST: STP: VLAN0148 heard root
8340-0008.e379.d980 on Gi1/0/28
016546: Jul  7 16:10:01.722 AEST:     supersedes 32916-0013.60a9.0f00
016547: Jul  7 16:10:01.722 AEST: STP: VLAN0148 new root is 8340,
0008.e379.d980 on port Gi1/0/28, cost 39
016548: Jul  7 16:10:01.722 AEST: STP: VLAN0148 sent Topology Change
Notice on Gi1/0/28
016549: Jul  7 16:10:02.678 AEST: STP: VLAN0148 Topology Change rcvd on St1

My google fu must be bad today because I cannot seem to find any
reference to spanning tree and "St1" at all .. I see similar output in
the sh spanning tree details for this vlan. I only have 2 interfaces
on the switch that live in Vlan 148, however I now have this magical
interface "St1" which has appeared with cost 100 ?

#sh vlan id 148

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
148  foofoofoobar1                   active    Gi1/0/28, Gi2/0/23

#sh spanning-tree vlan 148
VLAN0148
  Spanning tree enabled protocol ieee
  Root ID    Priority    8340
             Address     0008.e379.d980
             Cost        38
             Port        28 (GigabitEthernet1/0/28)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32916  (priority 32768 sys-id-ext 148)
             Address     0013.60a9.0f00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/28         Root FWD 29        128.28   P2p
St1              Desg FWD 100       128.1000 P2p
Gi2/0/23         Desg FWD 9         128.75   P2p

Ideas anyone?