05-20-2015 10:51 AM - edited 03-08-2019 12:05 AM
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.
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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
Solved! Go to Solution.
05-21-2015 05:54 AM
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
05-21-2015 05:54 AM
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
05-22-2015 11:00 AM
Hi Jon,
Thanks for replying.
am posting too much , i miss out this thread ! Thanks!
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