Spanning tree root bridge (I am baffled)

[Colin Scales]

Hey all,

I am utterly baffled by what I encountered today on my network.

The below output is the results of a show spanning-tree for a VLAN on my network. The totally confusing part of this is that the switch that the command was run from is the root bridge for the VLAN but also has a root port and a cost of 54 to the root. I was finding it difficult to locate the root bridge within the network and finally saw that this switch has the same MAC output for the Root ID to Bridge ID.

show spanning-tree vlan 2100

VLAN2100
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0c68.03d1.8080
Cost 54
Port 24 (GigabitEthernet1/0/24)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 34868 (priority 32768 sys-id-ext 2100)
Address 0c68.03d1.8080
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/17 Desg FWD 4 128.17 P2p
Gi1/0/24 Root FWD 4 128.24 P2p

Has anyone ever encountered this before? I thought it was a interesting one.

Many thanks,

LeoChan

[Rolf Fischer]

Hi,

Root ID Priority 32769

looks like the VLAN-2100 STP-instance receives BPDUs from the VLAN-1 STP-instance (32768 + 1) of the same switch. You should start on the switch connected to Gi1/0/24 and trace the path back to the root bridge.

HTH
Rolf

[ahmedshoaib]

Hi;

Can you double check is there any unmanageable switch is connected to your root switch?

It's seems to be a network loop due to unmanageable switch.

Thanks & Best regards;