Spanning-Tree & Trunk Issue

Unanswered Question
Sep 3rd, 2009
User Badges:

You can see diagram the attached file.

Diagram has three device with triangle BB#1, BB#2 and L2_Switch.

BB#1 is a root, BB#2 is a root secondary.

L2_Switch is not enabled Spanning-tree protocol.

BB#1 and BB#2 is connected to Port-channel 1 and trunk.

I changed Access port connected L2 Switch to trunk port on BB#2.

I would like to know why this trunk port change stp status block to forwarding for Vlan10?

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Francois Tallet Thu, 09/03/2009 - 21:23
User Badges:
  • Gold, 750 points or more


Is the L2_switch a cisco device? It should complain about the trunk inconsistency then.

If not, then it depends what the L2 switch does. I assume that the native vlan on your newly created trunk is 1. That means that vlan 10 will have a 1q tag (I'm assuming 1q trunk here). If BB1 receives this 1q tagged frame, depending on the platform, it will silently drop it or create a trunk inconsistency. In either way, you don't have connectivity any more between BB1 and BB2 through L2_switch any more, so there is no reason to block a port.



y-yoon Thu, 09/03/2009 - 21:53
User Badges:

Hi, Thank you for your reply.

L2 switch is cisco product(Cat2960).

If L2 switch enable STP, occur trunk inconsistency and L2 switch is blocked(BKN) STP statu. but L2 switch is STP disable.

Native vlan configuration is not effected STP status on BB.



Francois Tallet Thu, 09/03/2009 - 22:01
User Badges:
  • Gold, 750 points or more

Well, seems you found a way to beat the protection features included in the IOS:-(




This Discussion