Unanswered Question


I have two 3750 switches and provided two back to back uplinks from SW1gig1/0/1 -SW2gig1/0/1 and SW1gig1/0/2 - SW2gig1/0/2 and both ports are in trunk mode and i have allowed few vlan per my understanding one port should be blocked by STP but in my case both ports are in active state and have the reachability between two. just confused.. can anyone clear me ????


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glen.grant Sun, 10/18/2009 - 02:27
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Spanning tree will only block a link if there is a built in loop in your design . Unless you have a crosslink between the 2 upper switches you have attached the 3750 to neither side will show in blocked state because there is no inherent loop in the network . If you put in crosslink with the vlans you are using allowed across that trunk I will guarantee 1 of your links will go into a blocked state. The way it is now there is no loop is no thus nothing is blocked.

pkurdziel Sun, 10/18/2009 - 15:42
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What does show spanning {int trunk port or vlan} show?

vazquez.jorge Sun, 10/18/2009 - 19:40
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You didnt provide which protocol you are using for trunking, dot1q or isl. I will assume dot1q for now. I am suspecting that your two trunks are not configured exactly the same, cause if they were, then yes, you just created a loop which stp should take care of (by shutting down one port). Check to see if the trunks are configured exactly the same:

conf t

sho int gig1/0/1 trunk

Do this command for each of the ports, and notice any differences. My guess is something is different and stp sees it as two seperate trunks not passing the same traffic over the two links. Otherwise, its a loop for sure.

milan.kulik Mon, 10/19/2009 - 13:06
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as you are running per-VLAN STP probably, STP will not block the whole trunk port.

It will block particular VLANs on that port instead based on a position of the STP root in each VLAN.

Try using "show spanning-tree vlan X blockedports" or similar command to display blocked ports in particular VLANs.




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