STP doubt

[sww231222u]

Hello all,

Inthis documentation i have read the following sentence . The designated port transmits BPDUs, and the non-designated port receives BPDUs. https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10596-84.html#loop_guard

This means that BPDUs go only downstream, because the root bridge has only designated ports.

Even so, other official documentation says: When a switch detects a link status change, it send a TCN BPDU toward the root bridge, out its RP

As far as it I understand, these two sentences are totally contradictory. 

If anyone can help me I would be very grateful.

I hope I have explained myself well, sorry for any inconvenience.

Thanks in advance

[Martin L]

Yes, BPDUs go only downstream, from Root Sw down to non-root switches after Root election and during normal operation. 

once there is a topology change, all switches must be notified via Root sw.  First, I -where topo change happened- will send TCN BPDU up to the Root sw via Root Port.  Then, Root sw passes info down to others so that they can adopt and flush MACs.  Some books describe (summarize) TCN as direct, indirect failures, and non-significant topo changes.

so, those sentences are not contradictory.  

you can find some videos on youtube; it helped me to learning stp along with reading books.

Regards, ML
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[Martin L]

Configuration BPDUs .. In steady state, the BPDU flow is unidirectional: root ports and blocking ports only receive configuration BPDUs, while designated ports only send configuration BPDUs.
For every BPDU received by a switch from the root, a new one is processed by the Catalyst central NMP and sent out containing the root information. In other words, if the root bridge is lost or all paths to the root bridge are lost, BPDUs stop being received (until the maxage timer starts re-election).

TCN BPDUs are sourced from leaf switches and flow towards the root bridge when a topology change is detected in the spanning tree. Root ports only send TCNs, and designated ports only receive TCNs.
The TCN BPDU travels toward the root ridge and is acknowledged at each step, so this is a reliable mechanism. Once it arrives at the root bridge, the root bridge alerts the entire domain that a change has occurred by sourcing Configuration BPDUs with the TCN flag set for maxage + fwddelay time (35 seconds by default). This causes all switches to change their normal CAM aging time from five minutes (by default) to the interval specified by fwddelay (15 seconds by default). Refer to Understanding Spanning Tree Protocol Topology Changes for more details
source: https://www.cisco.com/c/en/us/support/docs/switches/catalyst-4500-series-switches/13414-103.html