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Tuesday, April 12, 2011

STP: when switch will not send BPDU?

Qsolved Question : We have STP enabled for all vlans on all ports. In what port role/state the switch will NOT send PBDU ?

In the ciscopress article we found sentence:

"By default, the spanning tree does not send BPDUs on root or alternative ports." On several other pages on some of them i found that port in BLK state send BPDUs, and on other that it will not send BPDUs.

What is true ? And is there any difference between forwarding BPDUs in STP and creating BPDUs in RSTP (regarding situations in which switch will not send BPDUs) ?

Qsolved Answers:

Switches will exchange initially BPDU to Elect the Root Switch. As long as the root switch is elected the Root will send BPDUs to the downstream switches and there will be no BPDU sent from Downstream switches to the Root unless there is TCN. This means that BPDUs will be not send from blocked port because the BPDUs are sent from the connected on the opposite side Designated Port. When the port in Role Root or Blocked no BPDUs should be send out of that interface only receive.In nexus was introduced feature Bridge Assurance and the feature is getting introduced in 6500 and other IOS switches. If that feature is enabled the ports are going to exchange BPDUs between each other as hello packets and that will break the Rule above but it will not affect the STP functionality.There is one more situation where BPDUs will be not send out of the interface. This is when PortFast is enabled on the interface.

Actually, portfast on a 2950/3560 is only telling the switch not to react to changes on the configured interface. For example, on a switch running the default ieee-compatible pvst , if you configure f0/5 with switchport mode access and spanning-tree portfast and run wireshark on the pc on that port, you will still see BPDUs. Portfast tells the switch not to send a TCN (topology change notification) BPDUS based on changes to the portfast-configured interface. If a BPDU is RECEIVED on a portfast interface, it loses it's portfast status and begins taking part in the normal spanning tree operations. The command to completely eliminate BPDUs being sent/received on an interface is the spanning-tree bpdufilter.Please click here for the configuration guide.

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