uplink and designated port

Answered Question
Jul 23rd, 2008

hi everybody!

i would really appreciate if anyone can help me with my questions

1)a switch (sw2) is connected to root switch by f0/1 port.The sw2 designated port is connected to sw3 blocked port.

If uplink is configured only on sw2, and sw2 loses f0/1 rootport, would sw2 designated port be put in forwarding state immediately?

if yes, whats the point even if designated port put in forwarding state directly, sw3 formerly blocked port which has become desingnated port will go through stp states i.e listening,learning and forwarding.

Am i right?

sw3 in that case become transit switch for sw2 to reach root switch and uplink should not be configured on transit switch.That means in order to configure uplink feature on switch, it should not have designated port connected to bocked port f another switch.Am i correct ?

another question is if switch loses root port,assuming switch has only two ports root port and blocked port, can it transmit bpdu claiming to be root, on blocked port connected to another switch designated port?

thanks a lot and have a nice day !

I have this problem too.
0 votes
Correct Answer by izackvail about 8 years 4 months ago

Yes. Once that switch loses connection to the root it will transition its blocked port to the forwarding state and begin sending inferior BPDUs. Remember the port is blocked because it has the highest port cost to get to the root. Once connectivity to the root has been lost that Superior BPDU information is flushed and it begins announcing itself as the root until it hears otherwise.

Correct Answer by izackvail about 8 years 4 months ago

SW2 has the designated port it is already in forwarding mode. When its connection to the root goes down it will begin to announce itself as the root. SW3 will wait the maxage and then begin transitioning through the STP states eventually becoming the new designated port for that segment. Once it starts forwarding the superior BPDUs to SW2, SW2 will realize it is not the root.

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Correct Answer
izackvail Wed, 07/23/2008 - 09:17

SW2 has the designated port it is already in forwarding mode. When its connection to the root goes down it will begin to announce itself as the root. SW3 will wait the maxage and then begin transitioning through the STP states eventually becoming the new designated port for that segment. Once it starts forwarding the superior BPDUs to SW2, SW2 will realize it is not the root.

sarahr202 Wed, 07/23/2008 - 11:55

thanks for your reply

can a switch which has lost rootport tranmit bpdu claiming to be root on its blocked port, given that there are only two ports , one is root port which is down the other is blocked connected to designated port of neighboring switch.

thanks a lot!

Correct Answer
izackvail Wed, 07/23/2008 - 12:17

Yes. Once that switch loses connection to the root it will transition its blocked port to the forwarding state and begin sending inferior BPDUs. Remember the port is blocked because it has the highest port cost to get to the root. Once connectivity to the root has been lost that Superior BPDU information is flushed and it begins announcing itself as the root until it hears otherwise.

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