CST runs with vlan id/sys ext id =0

[cartesius]

Hello,

I have a test lab with 2 switches, both installed from scratch, all default.

Switch A  - MAC 0026.xxxx.xxxx, switch prio 32768, unchanged

Switch B  - MAC 0022.xxxx.xxxx, switch prio 32768, unchanged

"sh span" shows me that the Root bridge is B, which is correct.

SwitchB#sh span

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0022.xxxx.xxxx
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 0022.xxxx.xxxx
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/7 Desg FWD 19 128.8 P2p

After activating mst ONLY on Switch A(no other change in between), Switch A became Root bridge.

SwitchA#sh span

MST0
Spanning tree enabled protocol mstp
Root ID Priority 32768
Address 0026.xxxx.xxxx
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 0026.xxxx.xxxx
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/7 Desg FWD 200000 128.8 P2p Bound(STP)

SwitchA#

SwitchB#sh span

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32768
Address 0026.xxxx.xxxx
Cost 19
Port 8 (FastEthernet0/7)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0022.xxxx.xxxx
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/7 Root FWD 19 128.8 P2p

SwitchB#

I was surprised until I saw in the pcap traces(see attachments) that on switch A the vlan id is 0, not 1 like on Switch B; this means that the bridge priority on Switch B was indeed 32768 and on A was 32769. In this case seems logical that Switch A becomes root.

I just wanted to know if this is a normal behavior and to have a confirmation that CST always runs with vlan id/sys ext id =0.

thanks

Lucien

[AnthonySylvester]

Hey Lucien,

Initially when both switches were using PVST (acting like CST by default), the bridge ID is bridge priority + vlan (sys ext id). So both switches will use 32769 (32768 + 1) for the PVST instance for VLAN 1.

Now when you converted SwitchA to MSTP, MSTI 0 is created. MSTI 0 is a special STP instance called Internal Spanning Tree and is used to provide info about the other MSTIs via BPDUs. MSTI 0 is what is shown in the pcap. MSTP bridge ID is the bridge priority + MST instance. So, SwitchA will have bridge ID 32768 (32768 + 0) once configured for MST.

Hope this helps clarify what you're seeing in the pcap.