Spanning Tree Protocol - why both bridges are root bridges same VLANs

[Ivan Mamka]

Hello,

I'm using CML2.0, and have two IOSv2 switches. Both connected with one cable on switch ports G0/0. Ran below commands on same switches.

Question - when running command "show spanning-tree" - The root bridge for vlan1 is selected switch1, but why for vlan100 & vlan200 each switch thinks that it is a root itself (port g0/0 for vlan100 & vlan200 are designated on both switches).

conf term

vtp domain cisco

vtp version 2

vtp password CCNP

int gig0/0

switchport mode dynamic desirable

end

vlan 100

name user-vlan

vlan 200

name file-vlan

end

Thanks!

[MHM Cisco World]

this meaning that the each SW not see BPDU from other SW

SW#show spanning-tree interface gx/x detail
....
   BPDU: sent 3, received 54

 check is each interface in both SW send receive BPDU 

[Ivan Mamka]

It is true, both switches do not receive BPDUs for vlan100&vlan200. But why? Both Vlans are enabled on the trunk cable/interfaces.

[MHM Cisco World]

Yes 
show interface switchport (share output)
there are 
admin mode = trunk 
operational mode = trunk 

I think the operational mode is access not trunk 
check this point 
also check the native and vlan allow in trunk. 

[Ivan Mamka]

Seems set to trunk. Also if port would be not trunk, I've noticed when creating vlan on switch1, vtp will not create that same vlan on switch2.

[MHM Cisco World · Accepted solution]

what I notice and I think it issue but not so sure 
why the encap is ISL not dot1 ?
can you change to dot1

also for VTP config both SW as transparent mode.

[Ivan Mamka]

I have tried changing vtp to transparent - same issue.

When changed to dot1q - resolved! All switches selected one root switch for vlan100 & vlan200.

Will need to understand why is that....But thank you!