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Now.

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How do switches learn about each other?

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As mentioned, bridge protocol data units or BPD use are sent out of all ports on switches by default

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every 2 seconds when running spanning tree.

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So switches will learn about each other when they receive BPD use from other switches on their ports.

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As an example, in this topology switch, one will know that there's a loop because it's receiving a

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BPD due from switch two on port two as well as port three.

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So BPD use from a switch with the same bridge ID are received on multiple ports and thus switch.

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One knows that there's a loop between itself and switch two in the same way.

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Switch two knows that there's a loop because it receives BPD use from switch one on both Port one and

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port three.

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In other words, it's receiving BPD use from the same switch on multiple ports.

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Hence there must be a loop.

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Now, BP does contain lots of information, and I'll demonstrate that in a moment.

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One of the pieces of information that a BPU contains is the bridge ID of the sending switch.

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A spending tree bridge ID is an eight byte of value unique to the switch.

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It consists once again of a two byte priority field and a six byte system ID, with the system ID being

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based on the burnt in Mac address of the switch.

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So in other words, the bridge ID consists of the property 32768 by default in decimal or 8000 in hexadecimal.

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And the MAC address of the switch.

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That means that the bridge ID is going to be unique in spending tree so bpd use contain information

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about spending tree switches send BPD use using their unique mac address to a destination multicast

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address of zero one ATC 2000000 or if using per VLAN spanning tree 01000c cc cc cd they have three kinds

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of BPD use a configuration BPU which is used by spending tree to provide information to switches, a

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topology change BPU to tell switches of a change and an acknowledgement BPU which is used to confirm

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the receipt of a topology change notification beads are sent by default every 2 seconds once again.