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So I've had a question that I haven't had a real answer to, I'm not even sure if there is an answer other than "That's just how the algorithm does it."

So why does STP only block one end of a line on a switch? So you'll see one port on one end in a BLK(ing) status, and the other one as FWD. Why aren't they both blocked? You can't use that line now, so what's the point?

Is there any real reason why? Have I overlooked something?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 16 '17 at 22:02
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You need to think about ethernet. Ethernet is a protocol originally designed to connect multiple devices on a link. STP must assume that there could be more than just the two switch interfaces on the link. If STP blocked both ends of the link, any other devices on the link would be disconnected from the rest of the network.

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when STP is used in a switch topology it allows 3 ports: root port: It is the port which connects the switch to the root port (I am assuming you know how the root port selection works) Blocked and forwarding port: The basic property of forwarding port is that it can receive BPDU's but not send them.

Now consider an example where the one of the switch is down STP needs to be converged.

If both the ports of a link are down, both ports will wait for the convergence information(that is the information needed for the selection of the new root port) but none will be able to send it.

thus both links cannot be blocked. I encourage you to read about STP time outs and how convergence process works

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