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Webcast-Catalyst9k
New Member

spanning tree and blocked ports

Hello

I have a network built with 5 3560 switches. They are linked together over 6 fiber gigabit links. Two of them are for redundancy. I set up STP and all works fine. STP root is on the same switch for all VLANs.

But I'm wondering why blocked links are only show state blocked on one of the two connected switches. I've read the docu but didn't found a hint.

Thanks for any comment.

Thomas

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Accepted Solutions
New Member

Re: spanning tree and blocked ports

Only Ports which are not root ports AND not designated ports are in blocking state.

Watch the flash animation, it's helpful:

http://www.cisco.com/warp/public/473/spanning_tree1.swf

2 REPLIES
New Member

Re: spanning tree and blocked ports

Only Ports which are not root ports AND not designated ports are in blocking state.

Watch the flash animation, it's helpful:

http://www.cisco.com/warp/public/473/spanning_tree1.swf

Re: spanning tree and blocked ports

I guess your question in fact translates to: why is there only one side of my redundant link that is blocking instead of both ends. There are several possible answers to that:

First, because blocking one side is enough;-)

But the an explanation I prefer is to remind that STP cannot know that this link is a fiber going to a single neighbor bridge. This link could be connected to a hub, where on the top of the neighboring bridge there would be some hosts (PCs, routers etc...). To put it short, STP must provide connectivity to this link. That's why *every* link has a designated port that connects it to the root bridge.

Hope this helps;-)

Francois

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