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AR100-S, AR110-S, AR120-S, AR150-S, AR160-S, AR200-S, AR1200-S, AR2200-S, and AR3200-S V200R009 CLI-based Configuration Guide - Ethernet Switching

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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. The Length field specifies the MAC data length, and is followed by the LLC header. Figure 11-6 shows the Ethernet frame format.

Figure 11-6  Format of an Ethernet frame

Configuration BPDU

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

During initialization, each bridge actively sends configuration BPDUs. 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, including the parameters such as the BID, path cost, and PID. A BPDU is discarded if both the sender BID and Port ID field values are the same as those of the local port. Otherwise, the BPDU is processed. In this manner, BPDUs containing the same information as that of the local port are not processed.

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 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 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

Field

Bytes

Description

Protocol Identifier

2

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

Protocol Version Identifier

1

The value is fixed at 0, representing the STP protocol

BPDU Type

1

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

Flags

1

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

8

Indicates the BID of the current root bridge.

Root Path Cost

4

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

Bridge Identifier

8

Indicates the BID of the bridge that sends the BPDU.

Port Identifier

2

Indicates the ID of the port that sends the BPDU.

Message Age

2

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

2

Indicates the aging time of a BPDU.

Hello Time

2

Indicates the interval at which BPDUs are sent.

Forward Delay

2

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

Figure 11-7 shows the Flags field. Only the leftmost and rightmost bits are used in STP.

Figure 11-7  Format of the Flags field

TCN BPDU

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.
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Updated: 2019-05-17

Document ID: EDOC1000174108

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