When creating a loop-free logical
topology, Spanning Tree always uses the same four-step decision
sequence:
- Lowest root BID
- Lowest path cost to root bridge
- Lowest sender BID
- Lowest port ID
Bridges pass Spanning-Tree
information between themselves using special frames known as bridge
protocol data units (BPDUs). A bridge uses this four-step decision
sequence to save a copy of the "best" BPDU seen on every
port. When making this evaluation, it considers all the BPDUs
received on the port as well as the BPDU that would be sent on that
port. As every BPDU arrives, it is checked against this four-step
sequence to see if it is more attractive (that is, lower in value)
than the existing BPDU saved for that port. If the new BPDU (or the
locally generated BPDU) is more attractive, the old value is
replaced.
Additionally, this
"saving-the-best-BPDU" process also controls the sending
of BPDUs. When a bridge first becomes active, all of its ports are
sending BPDUs every 2 seconds (when using the default timer values).
However, if a port hears a BPDU from another bridge that is more
attractive than the BPDU it has been sending, the local port stops
sending BPDUs. If the more attractive BPDU stops arriving from a
neighbor for a period of time (20 seconds by default), the local
port can once again resume sending BPDUs.
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