Spanning Tree Protocol questions

[zafar_118]

Hello,

I have couple of basic questions regarding STP

1) Lets see a switch is brought online with STP enable. All the ports will first go into listening state and per documentation, during this period switch will only send and receive BPDUs. Does the switch only send its BPDU or it also relays the ones recieved from other swithces i.e. let say switch has two ports enabled and in listening state. It receives BPDU on port1 from an upstream switch, will the switch relay that BPDU (with bridge id being changed to itself) to the port 2?

2) When port is in blocking state, it can still recieve BPDUs. Does switch relay the BPDUs that its receivnig on blocked ports to the ones that are in forwarding state?

Thank you       

[Peter Paluch]

Hello,

1) Lets see a switch is brought online with STP enable. All the ports  will first go into listening state and per documentation, during this  period switch will only send and receive BPDUs. Does the switch only  send its BPDU or it also relays the ones recieved from other swithces

The key fact to remember here is that a switch relays only BPDUs received on its root port (after updating a series of fields). Therefore, the answer to this question depends on whether the new switch becomes the root or not. At the very beginning, the switch indeed considers itself to be the root but a single received BPDU claiming a root switch with a lower BID will cause this switch to cease considering itself as the root and start relaying BPDUs received on its root port.

2) When port is in blocking state, it can still recieve BPDUs. Does  switch relay the BPDUs that its receivnig on blocked ports to the ones  that are in forwarding state?

No. Only BPDUs received on the root port are relayed further.

Best regards,

Peter

[zafar_118]

Hi Peter,

Thanks for the information. Another quick question:

If a switch BPDUs from two switches both advertising equal cost path to root, which will be used to make root port?

I know for DP, it uses lowest root bridge id, does it hold true for root port as well?

Also while STP is going through learning state, does it only learn source MAC addresses from receiving BPDU or also the Sender Bridge ID inside the BPDU?

Thanks

[chandra_rc16]

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If a switch BPDUs from two switches both advertising equal cost path to root, which will be used to make root port?

I know for DP, it uses lowest root bridge id, does it hold true for root port as well?

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     Yes the switch with the lowest BID will win and the port connected to it will be the ROOT port. And if a switch is receiving BPDU's from the same switch on two of its ports then the tie breaker is the Lowest Port ID.

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Also while STP is going through learning state, does it only learn  source MAC addresses from receiving BPDU or also the Sender Bridge ID  inside the BPDU?

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     No it listening it can forward and receive BPDU's but i won't learn the MAC addresses.

Regards,

Chandu

[zafar_118]

Hi Chandu,

In my second question, i wanted to check for "learning" state. During this state, does swtich

only learn  source MAC addresses from receiving BPDU or also the Sender Bridge ID  inside the BPDU?

Another question

If a switch receives TCN BPDU on blocking port, will it relay onto its root port?