STP convergence

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Oct 12th, 2009
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Wud overall stp convergence take place if access port flaps having pc connected to it.If port fast is enabled then it should transisition immediately to forwarding and overall stp convergence should not take place.Any suggestion???

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Giuseppe Larosa Mon, 10/12/2009 - 09:53
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Hello Mohammed,

your understanding is correct: if you use STP portfast no topology change notification BPDU is sent from switch upwards in direction of root bridge and STP doesn't need to recalculate.


The usage of STP portfast = to define ports as edge ports is also important when using Rapid PVST or MST 802.1s: it helps in achieving fast convergence by reducing dramatically the number of links where synchronization process occurs (the process of port state negotiation using BPDUs handshaking)


Hope to help

Giuseppe


tahseen12 Mon, 10/12/2009 - 10:26
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u mean if we dont put port fast on access port stp convergence will be triggered overall and TCN will be sent towards root bridge.I don't think so that will happen, rather port will take 50 sec to be in forwarding state if there is no portfast on access port connected to end station.

Jon Marshall Mon, 10/12/2009 - 10:36
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Mohammed


"u mean if we dont put port fast on access port stp convergence will be triggered overall and TCN will be sent towards root bridge. I don't think so that will happen, rather port will take 50 sec to be in forwarding state if there is no portfast on access port connected to end station."


Both will happen. The port will indeed take approx 50 seconds with standard 802.1d timers.


But more importantly without portfast a TCN will be generated each time the port transitions from up to down or down to up which you certainly do not want within your switched network. So portfast really is an important part of your configuration for edge ports.


Jon

tahseen12 Mon, 10/12/2009 - 10:45
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i still doubt on stp convergence trigger if access port flaps to which end station is connected as it will not recieve BPDU on that port.Do you have document supporting this?

Jon Marshall Mon, 10/12/2009 - 11:32
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Mohammed


If an access port does not have portfast enabled then any change in it's status will by defintion be a change in the STP topology of your L2 network.


A topology change automatically creates a TCN which is sent to the root bridge. To prevent this you need to enable portfast. Refer to this document for more details -


http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094797.shtml


Jon

tahseen12 Mon, 10/12/2009 - 12:05
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The same documents states below also

"An important point to consider here is that a TCN does not start a STP recalculation".Thats what I was saying its not neccessary STP convergence will happen if access port flaps.

Jon Marshall Mon, 10/12/2009 - 13:21
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Mohammed


Think we might be talking at cross purposes here.


A TCN is the not the cause of a change in the STP topology, rather it is a result of a change in the STP topology.


So a TCN does not trigger an STP recalculation but it does trigger the flushing of the CAM tables on the switches. There is no need for this flushing every time an end device such as a host transitions from up to down and down to up.


Jon

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