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In the STP root bridge election process, all of the switches advertise themselves as the root bridge until they receive a superior BPDU. Every source about STP says that after initialisation all of the ports start in the Blocking state. If the ports are in blocking state, which means they cant forward any frames, how can the switches forward their bridge id in order for the root bridge election process to happen?

dor
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Because Blocked means blocked to user frames. From wikipedia:

A port that would cause a switching loop if it were active. To prevent the use of looped paths, no user data is sent or received over a blocking port. BPDU data is still received in blocking state. A blocked port may go into forwarding mode if the other links in use fail and the spanning tree algorithm determines the port may transition to the forwarding state.

The purpose of STP is to avoid loops in the network, by blocking links that would form a loop. This means blocking frames destined for hosts.

BPDU frames can still be received by a port in the block state.

vidarlo
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  • i understand BPDU frames can be recieved, but lets say that we have connected 2 switches in a loop. We turn them on for the first time, if all the ports are only listening for BPDUs and no one can sends any BPDUs, how can a root bridge be elected/ – dor Jul 06 '21 at 07:53
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    One of the switches enters forwarding state as the timer runs down. It starts sending bdpu. The other switch learns about it. Done. – vidarlo Jul 06 '21 at 07:54
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The root bridge election process occurs when the ports are in the listening state. after the default time the ports will transition from blocking to listening, then they would be able to send and receive BPDUs, in order to advertise themselves as the root bridge

dor
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