What are Inferior and Superior BPDUs of STP

Today i want to have a very brief talk about what the Spanning-Tree Inferior and Superior BPDUs are and what is the difference between them.

Inferior BPDU of STP
The BPDU is considered inferior, if it carries information about the root bridge that is worse than the one currently stored for the port, or the BPDU has longer distance to reach the current root bridge. Inferior BPDUs may appear when a neighboring switch suddenly loses its uplink and claims itself the new root of the topology. By default, every switch should ignore inferior BPDUs, until the currently stored BPDU expires (time=Max_Age – Message_Age). This feature intends to stabilize STP topology in situations where an uplink on some switch flaps (goes down and up frequently for any malfunction), causing the switch to start sending inferior information

Superior BPDU of STP
A superior BPDU is one that has a lower Bridge ID. An inferior BPDU would have a higher Bridge ID. This can’t be judged on a single BPDU basis. It’s only in comparison that one can be considered superior or inferior. Receiving a superior BPDU typically means that a switch received a BPDU with a lower Bridge ID than the Bridge ID of the currently elected root bridge.
If a switch receives an inferior BPDU, nothing changes. Receiving a superior BPDU will kick off a reconvergence of the STP topology.


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