In one of the articles i read the following regarding Bridge assurance:

"When enabled, BDPU’s are sent out all ports including the backup and alternate for each Hello Period. How this feature works is that it monitors the receipt of BPDUs on point to point links. If it does not hear a reply it changes the port state to an inconsistent state. This prevents any frames forwarding and a loop is avoided. The same is true of UDLD aggressive mode where a port err-disables."

Q: When bridge assurance is enabled on a port, does that mean that it will start sending out BPDUs(irrespective of it's STP state)? If Bridge Assurance is enabled on a BLOCKED port, will the Blocked port also start sending BPDUs? If so, why? From what i know, the BPDUs are sent out ONLY from Designated ports once the STP is converged. Please correct me if i am wrong.

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 14 '17 at 23:06

BPDUs are sent out all STP ports. This is because you need to detect a topology change. If the topology changes at a distant switch, the current designated or root ports may need to start blocking, and a blocking interface may need to change to a designated or root port. A switch cannot detect this on a port where it is not sending or receiving BPDUs. STP is dynamic, and it needs to detect and respond to topology changes, even from the other side of a large STP domain.

In practice, you really don't want to design a large or complex STP domain, but people do, and STP needs to be able to deal with that.

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