I have two Cisco switches connected with two trunk links.
On the first switch spanning tree is switched off.
On the second switch, MST is turned on.
Ports configuration is as follows:
interface Ethernet0/2 switchport trunk allowed vlan 2-4 switchport trunk encapsulation dot1q switchport trunk native vlan 4 switchport mode trunk ! interface Ethernet0/3 switchport trunk allowed vlan 2-4 switchport trunk encapsulation dot1q switchport trunk native vlan 4 switchport mode trunk
(the configuration is the same on both switches)
I expect switch #2 to block one of the trunk ports: however, both of them are in FORWARDING.
I dumped traffic between two switches and I saw no BPDUs are send back to switch #2. Switch #1 receives them but instead of sending them back to switch #2 does nothing.
When I change native vlan to 1 (and allow it on trunk link), everything works correctly and one of the ports is being blocked.
Why it happens?