STP BPDU Format
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 9-6 shows the Ethernet frame format.
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, 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, 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.
- 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 a low-priority configuration BPDU, the designated port immediately sends its own configuration BPDU to the downstream device.
Table 9-7 describes fields in a BPDU.
Field |
Bytes |
Description |
|---|---|---|
Protocol Identifier |
2 |
The value is fixed at 0. |
Protocol Version Identifier |
1 |
The value is fixed at 0. |
BPDU Type |
1 |
Indicates the type of a BPDU:
|
Flags |
1 |
Indicates whether the network topology has changed.
|
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 9-7 shows the Flags field. Only the leftmost and rightmost bits are used in STP.
TCN BPDU
A TCN BPDU contains only three fields: Protocol Identifier, Version, and Type, as described in Table 9-7. The Type field is four bytes long and is fixed at 0x80.
- A port transitions to the Forwarding state.
- A designated port receives a TCN BPDU and sends a copy to the root bridge.