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NE40E-M2 V800R010C10SPC500 Feature Description - LAN Access and MAN Access 01

This is NE40E-M2 V800R010C10SPC500 Feature Description - LAN Access and MAN Access

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BPDU Format

BPDU Format

The BID, path cost, and PID that are described in the previous sections are all carried in Bridge Protocol Data Units (BPDUs).

  • Configuration BPDUs are heartbeat packets. STP-enabled designated ports send BPDUs at intervals specified by the Hello timer.
  • Topology Change Notification (TCN) BPDUs are sent only after the router detects network topology changes.

A BPDU is encapsulated into an Ethernet frame. In an Ethernet frame, the destination MAC address is the multicast MAC address 01-80-C2-00-00-00; the value of the Length/Type field is the length of MAC data; in the LLC header, as defined in the IEEE standard, the values of DSAP and SSAP are 0x42 and the value of UI is 0x03; the BPDU header follows the LLC header. Figure 9-6 shows the format of an Ethernet frame.

Figure 9-6 Format of an Ethernet frame

Configuration BPDU

Configuration BPDUs are most commonly used.

During initialization, each bridge actively sends configuration BPDUs. 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 routers. 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.

Table 9-6 shows the format of a BPDU.

Table 9-6 BPDU format




Protocol Identifier


Always 0

Protocol Version Identifier


Always 0



Indicates the type of a BPDU. The value can be:
  • 0x00: configuration BPDU
  • 0x80: TCN BPDU



Indicates whether the network topology is changed.

  • The right most bit is the Topology Change (TC) flag.
  • The left most bit is the Topology Change Acknowledgement (TCA) flag.

Root Identifier


Indicates the BID of the current root bridge.

Root Path Cost


Indicates the cumulative cost of all links to the root bridge.

Bridge Identifier


Indicates the BID of the bridge sending a BPDU.

Port Identifier


Indicates the ID of the port sending a BPDU.

Message Age


Records the time since the root bridge originally generated the information that a BPDU is derived from.

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

Max Age


Indicates the maximum time that a BPDU is saved.

Hello Time


Indicates the interval at which BPDUs are sent.

Forward Delay


Indicates the time spent in the Listening and Learning states.

Figure 9-7 shows the Flags field. Only the left most and right most bits are used in STP.

Figure 9-7 Format of the Flags field

A configuration BPDU is generated in one of the following scenarios:
  • Once the ports are enabled with STP, the designated ports send configuration BPDUs at intervals specified by the Hello timer.
  • When a root port receives configuration BPDUs, the router where the root port resides sends a copy of the configuration BPDUs to the specified ports on itself.
  • When receiving a configuration BPDU with a lower priority, a designated port immediately sends its own configuration BPDUs to the downstream router.


The contents of TCN BPDUs are simple, including only three fields: Protocol ID, Version, and Type, as shown in Table 9-6. The value of the Type field is 0x80, four bytes in length.

TCN BPDUs are transmitted by each router to its upstream router to notify the upstream router of changes in the downstream topology, until they reach the root bridge. A TCN BPDU is generated in one of the following scenarios:
  • Where the port is in the Forwarding state and at least one designated port resides on the router
  • Where a designated port receives TCN BPDUs and sends a copy to the root bridge
Updated: 2019-01-02

Document ID: EDOC1100058405

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