STP - BPDU generation and port roles

Spanning Tree Protocol (STP) uses what are known as BPDUs of various types to communicate and maintain the condition of STP within a topology. BPDUs are sent out of specific ports that have particular roles. This behavior differs for standard STP and for RSTP since their port states are different and they function differently.

The following indicates what port roles send out BPDUs and under what conditions:

Standard STP (IEEE 802.1D)

  • BPDUs are sent and received on Designated ports
  • Root ports receive BPDUs but don't send them
  • Blocked ports receive BPDUs but don't send them
  • Disabled ports don't send or receive BPDUs.

If STP is disabled ports with no role will forward BPDUs but will not process them.

Rapid STP (IEEE 802.1w)

In RSTP, BPDUs are sent and received on all operational ports regardless of their role, but the way they process these BPDUs will depend on their role:

  1. Root Ports receive BPDUs from the root bridge. They keep track of these BPDUs to monitor the health of the spanning tree and adjust if necessary.
  2. Designated Ports send BPDUs to the root port on another switch. The designated port's function is to share information about the topology with other switches.
  3. Alternate Ports receive BPDUs. This port has an alternative path to the root bridge, different from the one used by the root port.
  4. Backup Ports also receive BPDUs. This port provides a redundant (backup) path to a segment where another switch port already connects.

The process of sending and receiving BPDUs allows switches to communicate their configuration information with each other, allowing the network to construct and maintain the loop-free tree topology.


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