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Beginner

Switch is not sending BPDUs back in native VLAN (different from vlan 1)

I have two 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 second switch 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 valn to 1 (and allow it on trunk link), everything works correctly and one of the port is being blocked.

Why it happens?

Everyone's tags (6)
3 REPLIES 3
VIP Expert

Can you change the config on

Can you change the config on both switches to look like below and test with vlan 4 again?

interface Ethernet0/2
switchport trunk allowed vlan 2-3
switchport trunk encapsulation dot1q
switchport trunk native vlan 4
switchport mode trunk
!
interface Ethernet0/3
switchport trunk allowed vlan 2-3
switchport trunk encapsulation dot1q
switchport trunk native vlan 4
switchport mode trunk

HTH

Beginner

Thank you, but it didn't. How

Thank you, but it didn't help. How exactly should it help? I mean, MST sends its messages untagged, and, if I understand right, your configuration restricts any untagged traffic at all.

Beginner

Could you issue the following

Could you issue the following command "show interface trunk”. This command will show your trunk ports and show you clearly the native VLAN ID. The command’s output will go on to tell you about what VLANs are allowed to traverse the various trunks you have configured, the VLANs allowed and active in the management domain, and the VLANs that are in spanning tree forwarding state but not pruned. This is a useful command to detect any inconsistencies on detect any inconsistencies on undesirable VLAN trunking configuration.

Why did you turn off STP on switch #1?

Could you turn STP back on and make sure you are running same STP on both ends,perhaps Switch 1 still sees on its ingress path from Switch 2 the Native VLAN 1,even though you clearly have VLAN 4 as Native VLAN.HTH

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