inconsistent native vlan - how does the STP block

[SJ K]

Hi all,

In a lab, i have purposely create different native vlan across 2 trunk ports and this result in 1 of the switch showing

Switch(config-if)#do show interface trunk

Port Mode Encapsulation Status Native vlan
Fa0/1 on 802.1q trunking 20

Port Vlans allowed on trunk
Fa0/1 1-1005

Port Vlans allowed and active in management domain
Fa0/1 1,20,30

Port Vlans in spanning tree forwarding state and not pruned
Fa0/1 1

And when I do a packet tracer,  frames that reached the switch above , is block by STP and dropped.

================================================================================

From the statement above

Port Vlans in spanning tree forwarding state and not pruned
Fa0/1 1

I have understood that vlan 20,30 is pruned from the trunk, hence outgoing frames to vlan 20,30 will not be forwarded through the trunk

q1) But how does STP block incoming frames from the trunk to the switch ? via VLAN tag ? mac address ? any show command to see what is being blocked ? It would be good if anyone could point to me read on how this will work.

q2) I have thought STP is to prevent loops, why is it also blocking inconsistent native vlan ? will inconsistent native vlan cause loop ?

Thanks.

Regards,

Noob

[Jon Marshall · Accepted solution]

q1) see this thread for explanation -

https://supportforums.cisco.com/discussion/11884396/cdp-and-native-vlans

You can view what is being blocked etc. by the "sh spanning-tree vlan <x>" command.

q2) it can do because you have in effect joined two vlans together and this could cause a potential loop.

Jon

[SJ K]

Hi Jon,

Thanks for replying.

am posting too much , i miss out this thread ! Thanks!