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AR500, AR510, and AR530 V200R007 CLI-based Configuration Guide - Ethernet Switching

This document describes the configuration of Ethernet services, including configuring transparent bridge, MAC table, link aggregation, VLANs, STP/RSTP/MSTP, and so on.The document provides the configuration procedures and configuration examples to illustrate the service configuration methods and application scenario.
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BPDU Format

BPDU Format

A BPDU carries the BID, root path cost, and PID. There are two types of STP BPDUs:

  • Configuration BPDUs are heartbeat packets. STP-enabled designated ports send configuration BPDUs at Hello intervals.
  • Topology Change Notification (TCN) BPDUs are sent only after a device detects a network topology change.

A BPDU is encapsulated in an Ethernet frame. Its destination MAC address is a multicast MAC address 01-80-C2-00-00-00.

Configuration BPDU

Configuration BPDUs are used most commonly and are sent to exchange topology information among STP devices.

Each bridge actively sends configuration BPDUs during initialization. After the network topology becomes stable, only the root bridge actively sends configuration BPDUs. Other bridges send configuration BPDUs only after receiving configuration BPDUs from upstream devices. A configuration BPDU is at least 35 bytes long, including the parameters such as the BID, root path cost, and PID. A bridge processes a received configuration BPDU only when it finds that at least one of the sender BID and PID is different from that on the local receive port. If both fields are the same as those on the receive port, the bridge drops the configuration BPDU. In this way, the bridge does not need to process BPDUs with the same information as the local port.

A configuration BPDU is sent in one of the following scenarios:
  • After STP is enabled on ports of a device, the designated port on the device sends configuration BPDUs at Hello intervals.
  • When the root port on a device receives a configuration BPDU, the device sends a copy of the configuration BPDU to each of its designated ports.
  • When a designated port receives an inferior configuration BPDU, the designated port immediately sends its own configuration BPDU to the downstream device.

Table 11-7 describes fields in a BPDU.

Table 11-7  Fields in a BPDU




Protocol Identifier


The value is fixed at 0, representing a spanning tree protocol.

Protocol Version Identifier


The value is fixed at 0, representing the STP protocol



Indicates the type of a BPDU. The value is one of the following:
  • 0x00: configuration BPDU
  • 0x80: TCN BPDU



Indicates whether the network topology has changed.

  • The rightmost bit is the Topology Change (TC) flag.
  • The leftmost bit is the Topology Change Acknowledgment (TCA) flag.

Root Identifier


Indicates the BID of the current root bridge.

Root Path Cost


Indicates the accumulated path cost from a port to the root bridge.

Bridge Identifier


Indicates the BID of the bridge that sends the BPDU.

Port Identifier


Indicates the ID of the port that sends the BPDU.

Message Age


Records the time that has elapsed since the original BPDU was generated on the root bridge.

If the configuration BPDU is sent from the root bridge, the value of Message Age is 0. Otherwise, the value of Message Age is the total time spent to transmit the BPDU from the root bridge to the local bridge, including the transmission delay. In real world situations, the Message Age value of a configuration BPDU increases by 1 each time the configuration BPDU passes through a bridge.

Max Age


Indicates the aging time of a BPDU.

Hello Time


Indicates the interval at which BPDUs are sent.

Forward Delay


Indicates the period during which a port stays in the Listening and Learning states.

Figure 11-6 shows the Flags field. Only the leftmost and rightmost bits are used in STP.

Figure 11-6  Format of the Flags field


A TCN BPDU contains only three fields: Protocol Identifier, Version, and Type, as shown in Table 11-7. The Type field is four bytes long and is fixed at 0x80.

When the network topology changes, TCN BPDUs are transmitted upstream until they reach the root bridge. A TCN BPDU is sent in either of the following scenarios:
  • A port transitions to the Forwarding state.
  • A designated port receives a TCN BPDU and sends a copy to the root bridge.
Updated: 2019-05-25

Document ID: EDOC1000097279

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