portfast sending BPDU

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Feb 21st, 2008
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the results i got from debugging stp transmit / receive shows that the edge ports on my access switches are transmitting STP BPDU. I was under the impression that edge port (stp portfast) can receive but do not part in STP in other wards transmitting!!!. the edge ports are setup with spanning-tree portfast / spanning-tree bpduguard enable but not spanning-tree bpdu filter. so when is my edge ports tranmitting stp bpdu?


50w4d: RSTP(161): sending BPDU out Fa1/0/48

50w4d: RSTP(160): sending BPDU out Fa1/0/4


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Kevin Dorrell Thu, 02/21/2008 - 05:54
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Your edge ports will transmit BPDUs even with portpast. And so they should, for it is that which protects your network against users who decide it would be fun to connect two ports back-to-back.


If a user does that you will get a broadcast storm for up to 2 seconds. But the next time either port sends a BPDU, the other one will see it and will put the port in blocking (depending on the port priorities). If you have bpduguard, it will go one stage further and errdisable the port. That is good. It protects your network.


If you really want to stop sending the BPDUs on the access port, then you need to apply bpdufilter. But I would most strongly advise against do so. You would be inviting your users to bring down the network.


Kevin Dorrell

Luxembourg

Jon Marshall Thu, 02/21/2008 - 05:57
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Hi


Just to add to Kevin's excellent post. Portfast means that the port does not send out TCN's (Topology Change Notifications) when it changes from up to down and down to up. TCN's are responsible for creating a STP reconvergence. But as Kevin says they still send BPDU's.


Jon

Francois Tallet Thu, 02/21/2008 - 13:49
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You can still enable bpdufilter globally. The port will send out few bpdus at link up, to detect cabling error, and then will stay silent. As soon as a bpdu is received (if any is received), portfast and thus bpdufilter is disabled. Well, in your case, with BPDU guard, the port is also shut down, so it does not really matter;-)

Regards,

Francois

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