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For last few days I've been reading about Spanning Tree Protocol ,L2 protocol and understood how it prevents loop in network ,various steps in STP but one thing i wanted to know how STP actually detects the loops in network so that it can prevent it.Somewhere I read STP uses BPDU as probe and detects loops I mean how it happen is when switch send a BPDU with Destination Address as multicast and receive same BPDU again mean there is loop in network .

But is it how STP detects loops in network?

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For every bridge, it computes the least cost paths to the root bridge, and disable all other root paths by blocking the other ports. So obviously, any path including a loop wouldn't have the least cost, and would thus be discarded.

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so, somehow, STP doesn't bother detecting loops: it just always prefer loopless paths because they're shorter... – sylvainulg Sep 6 '12 at 10:05
Absolutely, that was my point :) – m4573r Sep 6 '12 at 10:14

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