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What is the meaning of "transmit BPDUs" for xSTP ports?

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Level 1


I am trying to understand how the different flavours of STP process BPDUs during port state transitions and I have two questions. 


In STP, only the Learning State transmits BPDUs that are received from the system module.

In Rapid PVST+, both the Blocking State and the Learning State transmits BPDUs received from the system module.

1. Does this imply that for the other flavours, PVST+, only the Learning State transmits, and that in RSTP, both Blocking and Learning states transmit?

2. What does transmit mean exactly? In a topology sw1---sw2---sw3, does transmit mean that during the Learning state sw2 will transmit sw1 BPDUs to sw3? I created this topology in EVE-NG, but saw no sw1 BPDUs at any moment on the sw2-sw3 link. 


Any clarification would be greatly appreciated. 





4 Replies 4


Hello @akitirija 

  1. In PVST+ and MST, only the Learning State transmits BPDUs that are received from the system module. In RSTP, both the Blocking and Learning states transmit BPDUs received from the system module.

  2. When we say "transmit" in the context of STP, it means that the switch will forward the received BPDUs out of all of its designated ports, including the port in the Learning state. This is necessary to ensure that the entire network is aware of the switch's presence and the current state of its ports.

In your example topology of sw1---sw2---sw3, if sw2 is in the Learning state, it should forward BPDUs received from sw1 out of its designated ports, including the port connecting to sw3. However, it's possible that the BPDUs are being filtered or suppressed somewhere along the path. You may want to check the configuration of all switches involved to ensure that BPDUs are not being blocked or dropped. Additionally, you can use a protocol analyzer tool to capture and analyze the traffic on the sw2-sw3 link to see if any BPDUs are being transmitted.

Best regards
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Thank you for your reply. There is no filtering going on, because it's just a lab with switches containing no configuration at all. However, I did the same lab with another vendor using RSTP, and it seems that BPDUs are never forwarded, at least in the topology of sw1---sw2---sw3, I never see sw1 BPDUs on the sw2-sw3 link using Wireshark and looking at the source MAC addresses of the BPDU frames. 

it seem to me that both send/receive via BLK but I will make double check run some lab to be sure

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