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Configuration Guide - Ethernet Switching

S1720, S2700, S5700, and S6720 V200R012(C00 and C20)

This document describes the configuration of Ethernet services, including configuring MAC address table, link aggregation, VLANs, VLAN aggregation, MUX VLAN, VLAN termination, Voice VLAN, VLAN mapping, QinQ, GVRP, VCMP, STP/RSTP/MSTP, VBST, SEP, RRPP, ERPS, LBDT, and Layer 2 protocol transparent transmission.
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A BPDU is encapsulated in an Ethernet frame. Its destination MAC address is a multicast MAC address 01-80-C2-00-00-00. The Length field specifies the number of bytes of the Data field (excluding the CRC field). The LLC header and BPDU packet header are appended to the Length field in sequence. Figure 13-6 shows the Ethernet frame format.

Figure 13-6  Format of an Ethernet frame

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.

Configuration BPDU

Configuration BPDUs are used most commonly and are used for exchanging topology information among STP devices.

Each bridge actively sends configuration BPDUs during initialization. After the network topology becomes stable, the designated port of each device periodically sends configuration BPDUs. A configuration BPDU is at least 35 bytes long, and includes 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. This reduces the number of BPDUs that a bridge needs to process.

A configuration BPDU is sent in 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 a root port receives a configuration BPDU with a priority higher than that of its own configuration BPDU, the device where the root port resides updates the configuration BPDU information stored on its STP ports based on the information in the received configuration BPDU and sends the information to a downstream device through a designated port. In contrast, if the root port receives a configuration BPDU with a priority lower than that of its own configuration BPDU, the root port discards the received configuration BPDU.
  • When a designated port receives a low-priority configuration BPDU, the designated port immediately sends its own configuration BPDU to the downstream device.

Table 13-6 describes fields in a BPDU.

Table 13-6  Fields in a BPDU




Protocol Identifier


The value is fixed at 0.

Protocol Version Identifier


The value is fixed at 0.



Indicates the type of a BPDU. The options are as follows:
  • 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 13-7 shows the Flags field. Only the leftmost and rightmost bits are used in STP.

Figure 13-7  Format of the Flags field


A TCN BPDU contains only three fields: Protocol Identifier, Version, and Type, as described in Table 13-6. 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 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: 2018-12-24

Document ID: EDOC1100038339

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